This HTML document is automatically generated from the Help file that accompanies Bracmat, using Bracmat to do the conversion.
starting with Bracmat
pattern matching
grammar
binary operators
flags/unary operators
strings or atoms
symbols
the four evaluation contexts
programming in Bracmat
functions
data structures
objects
hash tables
how Bracmat evolved
how to obtain Bracmat
Bracmat is an interpreted programming language that will evaluate instructions that you type at the keyboard. For example, Bracmat can
In Bracmat, data and instructions have the same syntax: a mix of parentheses, binary operators, operands and flags preceding those operands or parentheses. In fact, in many cases you can't tell the difference between the data and instructions. Bracmat offers a simple environment for input of both data and code. After the prompt {?} you can write. When you hit Enter Bracmat evaluates your input and writes the result to the screen following a {!} sequence (unless there was no visible result). Under the result follows a line that tells whether the evaluation was successful (S) or not (F) (in rare cases you may see a "I", which, for the time being, you may interpret as failure). In the same line the machine shows how much processor time it needed. Intermediary results may also appear on the screen. Bracmat and calculators have an important common feature: as long as the input data are simple enough, the user need (and can) not specify how these data have to be processed. Calculators handle basic calculations in a predictable, unchangeable way, e.g. "4+7" will always result in "11", and not in, say, "10" or "23-12". That is because the manufacturer had good reasons to think that "11" is what the user expects and nothing else. In the same way, Bracmat handles basic "calculations" with a much wider variety of data: rational numbers, symbols, words and collections thereof. For example, "a+b+a" becomes "2*a+b" and not, say, "x.a" or "b+2*a". Again, its creator has taken decisions that you can't easily circumvent. However, the more complex the data are, the better are the chances that not all results, although defendable, have an appearance that suits you. It is here that programming comes in: Bracmat leaves certain kinds of data unchanged, but opens the possibility to dissect data, to perform calculations on the parts, and to assemble an answer from the resulting sub-answers. First and foremost Bracmat was developed with symbolic algebra in mind. You may add, multiply, take powers and logarithms and differentiate. Oddly enough, Bracmat has no operators for subtraction and division. This reduces the amount of arbitrariness in the presentation of formulae. "a-b" must be written as "a+-1*b" and "a/b" must be written as "a*b^-1". Calculations are always exact. Number expressions for which no rational representation exists are not further evaluated. Bracmat knows how to handle the special symbols i, e and pi, but it offers no numerical representation for e and pi. Examples:
{?} a+b+-1*a { this is how you subtract in Bracmat }
{?} 1+(1+i)*(1+-1*i)+-1 { the leading and trailing terms force
Bracmat to expand the product }
{?} -12345/54321 ^ 1/2 { the square root of -12345/54321 }
{?} x^(a+x\L2*1000)
{?} 5/2 \L 987654
{?} y\D(x\D((x+y)^-2))
The only numbers that Bracmat can calculate with are integer and rational numbers. There is virtually no limit to their size, but the program is not optimised for number crunching. Bracmat handles a non-integer power of an integer if the absolute value of this integer is less than 2^32. Likewise, both the numerator and the denominator of a rational number should be less than 2^32. Bracmat happily adds and multiplies numbers of, say, a few hundred digits. There is a pre-defined function, flt$, that represents rational numbers in a "scientific" floating point notation, but Bracmat cannot do calculations with these "reals", unless you write a function to convert them back to rational numbers.
The binary operators have an ordering of precedence. Of the mathematical operators, + has the lowest precedence and \D the highest (+ * ^ \L \D). You may use parentheses to overrule this ordering:
{?} (a+b)*(a+c)+a^(-1*d^2+(d+1)*(d+-1))
When instructions are entered from the keyboard, the program waits until all of the conditions below are fulfilled:
You can write a multiple-line instruction by putting the instruction inside an extra pair of parentheses. After each non-terminating <return>, Bracmat shows in the next line how many closing parentheses are needed for the completion of the instruction. If you are in the middle of a string or a comment when pressing <return>, the next line starts with {str} or {com}, respectively. If you want to enter several instructions on the same line, you should write a semicolon ";" between the instructions. These instructions are executed in the same order. If you do not want to see the result of a calculation, you may write a semicolon after the last instruction. Instructions in a text file must be separated by semicolons. You may freely surround binary operators with white-space characters (e.g. space, tab, line feed). Take care not to put spaces between the characters that make up a rational or negative number, - 1234 / 5678 is not the same as -1234/5678.
Bracmat can open and read a text file and execute the instructions as they are read. After the last instruction has been executed, the file is closed. The program that currently produces this information, is read from the file named "help". The instruction for reading a file "myprog" and executing the instructions therein is "get$myprog". The "$" is a binary operator that has to do with function evaluation. "get" is the function name of one of the few built-in functions. The result of the last executed instruction in a file is written to the screen. For better control over screen output one may use the built-in function "put", which writes from the current cursor position to the right, or the pre-defined (but changeable) function "out", which writes an extra line feed after its argument has been written to the screen.
{?} put$(x*x);put$(y+y)
Often you will need the result of the last evaluated instruction in the next one. You can use the exclamation mark "!" instead of re-entering the result. (Because Bracmat is still evaluating the instruction "get$help", this trick is of no use in the context of this program.) Example (try later):
{?} 1+1
{?} !^!^!^!
When you write programs in the Bracmat language you will normally use an external text editor. You can enter small programs directly at the Bracmat prompt {?}, but a small change in an instruction can only be done by re- entering the whole instruction. You may save instructions that are still held in memory (which is only possible if they are bound to variables), by the built-in "lst" function. This function takes a number of optional parameters that tell the system whether the instruction has to be written in a compact and barely readable form, or in a more pretty form, with lots of indentations. Comments are never written, as they are ignored at input time. As an example, you can write the definition of the pre-defined factorisation function "fct" to a file "factorise" by entering the following instruction:
{?} lst$(fct,factorise,NEW) {NEW:replace old file with same name}
At first sight, a Bracmat program doesn't look like programs written in other languages. This may even become a permanent impression. So here are some recommendations about programming habits:
⇑ BRACMAT .
The single most outstanding feature of Bracmat is how it can recognise patterns in data. The data can be an algebraic expression, a directory listing in table form, a thesaurus structured like a tree, a text or whatever data that can be expressed as a string of characters or as a tree of such strings. Bracmat's patterns are much more advanced than regular expressions. Regular expressions are fixed patterns once the matching operation is started. Bracmat implements not only propositional rules comparable to regular expressions, but also first order predicate rules, backtracking to search the space of combinations of data that make its predicates come true. In this way it is very easy to implement a relational database. But it doesn't stop here, because Bracmat supports recursive invocations of pattern matching and other operations. Examples:
{?} S=(|0 !S|1 !T);T=(0 !T|1 !S); { Regular grammar }
{?} 0 1 0 1 0:!S { Check whether subject contains an even number of 1's. }
{?} P=(|0 ?x 1 & !x:!P); { context free grammar }
{?} 0 0 0 1 1 1:!P { Check whether subject consist of a row of 0's
followed by a row of 1's of the same length. }
{?} connected=("South America"."North America") (Africa.Asia) (Asia.Europe);
{?} (reachable = a b f
. !arg:(?a.?b.?f)
& !f:? ((!a.!b)|(!b.!a)) ?
| !f:?A ((!a.?c)|(?c.!a)) ?Z
& reachable$(!c.!b.!A !Z)
); {Remove used fact from fact base.}
{?} ( Antarctic Europe Australia Africa Asia "North America" "South America"
: ?
%@?x {Pick a continent.}
?
( %@?y {Pick another continent}
& reachable$(!x.!y.!connected) {Are they reachable?}
& out$(!x "is reachable from" !y)
& ~ {Force backtracking to collect all answers.}
)
?
); { pattern using second order logic }
Europe is reachable from Africa
Europe is reachable from Asia
Africa is reachable from Asia
North America is reachable from South America
Pattern matching in character strings
Pattern matching in tree structures
⇑ BRACMAT .
@(<string> : <pattern>) "match <string> with <pattern>"
Pattern matching in a string of characters (a single atom) is like pattern matching in a string of atoms. Use the '@' to instruct the program to look inside the atom and use space operators to combine subpatterns. The space operator does not itself match any characters. To match a space in an atom, use a space in an atom!
You cannot negate the result of string pattern matching by adding the ~ flag, unless there are other flags than @ and ~ present.
{?} a b:(~@(? b:a %)) {succeeds, ~@ means:'not an atom'}
{?} ~@(a b:a ?) {succeeds, ~@ means: not a string match'}
{?} ~@(a:a) {succeeds, same reason}
{?} 12/34:@(?x:#?a ~#%@ #?b) {succeeds, ?x matches the atom 12/34,
while #?a (~#%@:?y) #?b matches 12/34 as a string.}
{?} 12:~/@(?x:#%?a #%?b) {succeeds, ~ negates /, not @, so we have a string
match.}
⇑ Pattern matching.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇐ Binary operators in pattern matching.
<subject> : <pattern> "match <subject> with <pattern>"
A match succeeds if <subject> succeeds and <pattern> is successfully matched with <subject>. The returned value is the left operand, <subject>. Patterns may be built up from sub-patterns and may also include actions that are triggered if a sub-pattern successfully matches (part of) the subject. As with the evaluation of other binary operators, the left operand of the ":" operator is evaluated first. The other operand, <pattern>, is not evaluated, but in the process of pattern matching (parts of) <pattern> may be evaluated several times. This is the case with function calls, atoms with a "!" flag and all right hand sides of the "&" operator. The use of the involved binary operators ($ ' &) and flags (! !!) as "meta operators" does not restrict the range of matchable expressions in a serious way, as these operators and flags normally do not occur in evaluated subject expressions. The same is true for some other operators (: | _ and =). These operators, too, have a special meaning within patterns. All other binary operators occurring in a pattern are searched for in the subject expression as part of the pattern matching.
Especially the & | and : operators are helpful in formulating complex patterns with alternatives, conjunctions and side effects in the form of actions. In the following examples, !s stands for the subject expression, the expressions in parentheses are patterns and !p, !pa, !pb, etc. are sub-patterns therein. !a, !aa, etc. stands for an action (a part of the pattern that is conditionally evaluated).
!s:(!p&!a)
If !p matches successfully with !s, then !a is evaluated. If !a fails, the whole match fails. In more complex patterns, only part of the match might fail, resulting in backtracking and retry.
!s:(!pa|!pb)
If pattern !pa does not match with subject !s, then !pb is tried.
!s:(!pa:!pb)
If pattern !pa matches with !s, then pattern !pb is also tried. The next example combines these operators in a grammar-like expression:
!s:( !pa & !aa If either !pa, or !pb or both of !pc1 and !pc2
| !pb & !ab "fire", actions !aa, !ab and !ac, respectively
| (!pc1:!pc2) & !ac are triggered.
)
Take care for the grouping of the : & and | operators : (!s:!pa):!pb and !s:(!pa:!pb) have, accidentally, the same effect, but the following expressions are very different :
(!s:!p)&!e
or !s:!p&!e
If !s matches with !p, !e is returned.
!s:(!p&!a)
If !s matches with !p, !a is evaluated, but the expression as a whole returns !s.
(!s:!p)|!e
or !s:!p|!e
If !s matches with !p, !s is returned. Otherwise, !e is returned.
!s:(!pa|!pb)
If !s matches with either !pa or !pb (in that order), !s is returned.
The possibility that !s might fail further complicates the above examples.
⇑ Pattern matching.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇑ Programming advice.
⇐ Program flow.
⇐ Assignment to variables.
⇒ Pattern matching in strings.
⇒ Flags and pattern matching.
Note: in long lists the "|" is left out.
<input> ::= [<expression>] [;<input>]
<expression> ::= <white space> <expression> <white space>
| [<flags>] ( <expression> )
| <leaf>
| <expression> <binop> <expression>
<leaf> ::= [<flags>] <atom or nil>
<atom or nil> ::= <atom> | <nil>
<atom> ::= "<string>" | <string>
<string> ::= <character> [<string>]
<character> ::= any printable character except \ and " | <spec>
<spec> ::= \a \b \t \n \v \f \r \" \\
<nil> ::= "" (or nothing at all, such as in "()")
<binop> ::= = . , | & : <white space> + * ^ \L \D ' $ _
<flags> ::= <flag> [<flags>]
<flag> ::= ~ / # < > % @ ` ? ! !!
<white space> ::= spaces, tabs, new line and form feed characters
White space (operator/cosmetic measure) almost never leads to confusion. It does in (some) cases where a <nil> leaf without flags is adjacent to the white space operator. For example : "get' out$now". Bracmat interprets this as : "get'(out$now)". "" or () fixes the problem : "get'() out$now". Quotation marks are not part of the string they surround. They should be used if necessary, e.g. "in this case" or "he{this is not a comment}re". Comments can be written everywhere, except in the middle of a string in Quotation marks. Comments are enclosed in {} and may be nested.
⇑ BRACMAT .
choose a specific binary operator
algebraic operations
program flow
pattern matching
data structures
assignment
functions and macros
the dummy operator
⇑ BRACMAT .
These are, in order of growing priority, the 15 binary Bracmat operators.
= operator (assignment)
. operator (lists)
, operator (lists with "unwrinkling")
| operator (OR ELSE)
& operator (AND THEN)
: operator (match subject with pattern)
[blank] operator (word lists)
+ operator (addition)
* operator (multiplication)
^ operator (exponentiation)
L operator (logarithm)
D operator (differentiation)
' operator (function, macro)
$ operator (function, macro)
_ operator (dummy)
⇑ Binary operators.
This operator assures that the right hand operator stays unevaluated. It is
mainly used in the definition of pieces of code (e.g. functions). The code
on the right is bound to the name on the left.
<atom>=<expression>
Each time when the value of <atom> is asked for, a fresh copy of <expression>
is made available. <expression> itself is unchangeable and can only be wiped
out by removing the binding between <expression> and its name, <atom>. This
has, in turn, no influence on the copies made earlier.
{?} a=2 { create binding }
{?} !a:?b { bind copy to b }
{?} !b { show b's value }
{?} a=3 { remove a's binding to 2}
{?} !b { show b's value }
There is a second way of using the = operator, with a slightly different
syntax :
<nil>=<expression>
The = operator serves as a shock proof container for <expression>.
The effect of evaluating this type of expression is almost the same as that
of the macro instruction ()'<expression>. Indeed, after evaluating a macro
instruction we have an expression with the <nil>=<expression> syntax.
{?} out$(b+a)
{?} out$(=b+a)
{?} out$('(b+a))
{?} c=3
{?} out$(=b+a+$c)
{?} out$('(b+a+$c))
⇑ =.,|&: +*^LD'$_.
⇒ Objects.
With the = and . operators you can construct and dereference conventional data structures and even objects with methods. In an expression, each subexpression with a = operator in the top node and an atom in the lhs of the top node indicates a field or object method that can be accessed and changed independently of other fields and methods, i.e. without the need to dissect and reassemble the whole expression. Such expressions are objects. An object member (a field or method) is addressed by using the lhs of the = operator as the member's name, preceded by the objects name. The name of the object and the name of the member must be separated by a dot operator. In the example below an object named "John" is created with the members "length", "age" and "name". The "name" member has two sub-members "first" and "family":
{?} John = (length = 180),(age = 30),(name = (first=John) (family=Bull)) There is no prescribed way in which the members should be glued together to form an object. Here, the comma operator and blank operator are used, but any operator but the = operator can be used to separate field names. John's length can be changed to 185 in the following ways:
{?} John.length = 185 or
{?} 185 : ?(John.length)
The same object can be assigned to another variable, creating an alias, but we have to take care not to evaluate John, because that would create or overwrite the variables "length", "age" and "name"):
{?} !John:?alias {Wrong}
{?} '$John : (=?alias) {Right} Bracmat replaces the expression '$John by the value of John, protected against evaluation by a = operator. For that reason, the pattern on the rhs of the match operator : contains a = operator as well. Now we can change John's age by operating on the variable "alias":
{?} alias.age = 31 To see that the above expression indeed has the wanted (side-)effect, we can inspect John:
{?} lst$John
(John=
(length=180)
, (age=30)
, (name=(first=John) (family=Bull))); Alternatively, we can also just show the field "age" in John:
{?} !(John.age)
{!} 31
It is also possible to create an alias for a sub-object. Taking the previous example, we could create an alias for the name member:
{?} '$(John.name):(=?nm) Now assign a new family name:
{?} Flinter:?(nm.family)
{?} lst$John
(John=
(length=180)
, (age=30)
, (name=(first=John) (family=Flinter))); Using an alias for a sub-object can save some code and processing time if the sub-object is accessed many times. Without the alias for John's name, we can change his family name in this way:
{?} Flinter:?(John.name.family)
It is valid to have an empty name for a member:
{?} x=(header=blabla) (=(a=1) (b=2)) Here, a and b are fields in a "nameless" sub-object of x. We can ask for the value of b:
{?} !(x..b)
{!} 2 To retrieve the whole sub-object:
{?} '$(x.):(=?sub-object)
{?} lst$sub-object
(sub-object=
(a=1) (b=2));
An alias can also be created for part of an object:
{?} x=(a=) (b=) (c=) (d=)
{?} '$x:(=(a=) ?alias (d=)) Now 'alias' only shares the members x.b and x.c with x. The same result follows from
{?} '$x:(=? ((b=) (c=):?alias) ?)
Objects can be composed to form new objects containing the union of the members of the contributing objects:
{?} x=(a=) (b=)
{?} '((p=) ($x) (q=)):(=?r)
Evaluation of an expression that contains '=' operators can have unexpected side effects, as the following example shows. First suppose that x (containing one record with one anonymous field) is unevaluated (case A) and assigned to two other variables:
{?} x=(=)
{?} !x:?y
{?} !x:?z In this case, x, y and z are different objects. For example does
{?} 2:?(y.) not affect x and z. Do the assignment again, but this time evaluating x only once:
{?} !x:?y:?z Now y and z are the same object, but still different from x. A change made to y affects z but does not affect x.
Suppose that x IS evaluated (case B):
{?} (=):?x
{?} !x:?y
{?} !x:?z Now x, y and z are the same object. Explanation: in (A) the value of x is not evaluated, especially the lhs of the '=' operator. Therefore, a new '=' node is created each time x is evaluated. In (B), the value of x IS evaluated, so no new copies of the '=' node are made.
⇑ =.,|&: +*^LD'$_.
⇑ Programming advice.
⇑ BRACMAT .
⇐ The = operator.
⇐ Construction of data structures.
⇒ Assignment to variables.
There are two forms of assignment to a variable:
<variable> = <expression>
<variable> must evaluate to an atom. <expression> is not evaluated.
<expression> : ?<variable>
<expression> is evaluated before assignment takes place.
<variable> must be an atom.
The = operator is mainly used to bind (still) unevaluated expressions to
variables (patterns, but above all functions and routines).
Assignment with the ":" makes use of pattern matching with a universally
unifying pattern. This way of assignment is very powerful and can even be
used to assign unevaluated expressions, by preceding the subject with an
= or an ' operator.
{?} car=.!arg:(?%arg ?)&!arg { one may freely reuse arg ! }
{?} (=(.!arg:(?%arg ?)&!arg)):?car { another way to define car }
{?} car$(one two three)
{?} (four five six):(?`%first ?rem) {"`": 0 of 1, "%": 1 or more, together 1}
{?} The first element is !first and the remainder is !rem.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇐ Objects.
⇒ Binary operators in pattern matching.
In Bracmat linear lists can be made by separating the elements with any binary operator, but most usual are the comma, plus sign, asterisk and space. These four operators do not lend themselves to forming other structures than linear lists. In addition, the plus sign and the asterisk don't preserve the order of the elements under addition of new elements. With the dot any tree structure can be built. Which operator one should use depends on the following considerations :
Examples :
{?} x=a.b.c
{?} y=p.q
{?} !x.!y
{?} x=a b c
{?} y=p q
{?} !x !y
{?} rotate=car,cdr.!arg:(?car,?cdr) & (!cdr,!car)
{?} rotate$(one,two,three,four)
{?} rotate$((one,two),(three,four)) { stop! }
By combining dots, commas and spaces, one may build any tree-like data structure that, thanks to the backtracking mechanism on space-separated lists, make the formulation of queries (goals) almost as easy as in Prolog. This is an example of a simple database, in which each row starts with a descriptor field, followed by a varying number of similar fields.
{?} M=((odd ,1 3 5 7 9)
(even ,0 2 4 6 8)
(prime ,2 3 5 7)
) We choose the space operator to form the backbone of the lists of numbers, because we want to access these numbers associatively, by using the back- tracking mechanism.
Let us formulate a query that searches for all numbers that occur in two or more categories (odd, even, prime). The findings are to be printed to the screen.
{?} (!M:(? { skip 0 or more rows --- }
(?c1,?row) { ---fetch (number type, number row)--- }
? { ---skip 0 or more rows --- }
(?c2, { ---fetch another number type,... }
? { ...skip 0 or more numbers... }
(?`%el { ...fetch a number... }
& !row : ? !el ? { does number occur in earlier row ? }
& out$(!el is both !c1 and !c2){ yes? show }
& ~ { not satisfied yet: fail and backtrack }
)
? { ...skip rest of numbers--- }
)
? { ---skip rest of rows }
)
)
Experimentation with the implementation of matrices in Bracmat has learned that lists (of lists (of lists..)) lead to smaller and faster programs than arrays, artificially made multidimensional by playing with the index. A drawback of the list approach is its unconventionality. Much time has to be spend in reformulating existing algorithms based on indices. On the other hand, the list approach is essentially insensitive to the dimensionality of the matrix at hand, and may even be indifferent to the number of indices.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇑ Programming advice.
⇑ BRACMAT .
⇒ Objects.
<exprA> & <exprB> ("<exprA> and then <exprB>")
<exprB> is only evaluated if <exprA> succeeds,
<exprA> | <exprB> ("<exprA> or else <exprB>")
<exprB> is only evaluated if <exprA> does not succeed.
In both cases <exprA> is always evaluated and <exprB> conditionally.
If <exprB> is to be evaluated, <exprA> and the & or | operator have served
their purpose. Therefore, they are eliminated before <exprB> is evaluated.
In this way, the program stack doesn't grow indefinitely when recursive
calls are made from the right hand side of any & or | operator occurring
in an expression . Even a conventional sequence of instructions (where the
success or failure of the evaluations of each instruction do not matter) can
make use of this tail recursion optimisation. In that case one uses the
pacifier (short cut flag) ` .
(`!a & !b) !b is always evaluated. (sequence) (`!a | !b) !b is not evaluated. (useless in this form)
The pacifier or shortcut flag is inherited by higher levels, it percolates towards operators that are closer to the root of the tree, until it is subsumed in situations like the above ones.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇒ Program flow.
⇒ Some often used control structures.
Most binary operators are used in expressions that "flow on their own" or "flow not at all". In the first group are the arithmetic operators, in the second is the dot-operator. In between are the two other structuring operators, comma and white space. Branching to a function is done with the $ and ' operators :
a $ b (or a'b) "evaluate function a with argument b"
Branching without argument passing and local variables is done with the unary operator (flag) ! :
!X "do subroutine X"
but often this flag and its cousin !! are used for the purpose of variable expansion, it just depends on whether a variable is bound to an unevaluated or to an evaluated expression :
!X "expand X" !!Y "expand expansion of Y" (two !'s is the maximum)
Conditional evaluation is decided by the success or failure of subexpressions. Every (sub)expression has two kinds of value: a visible value and a success(S)/failure(F)/ignore(I) value. Success and failure are primarily decided by the low level functions in the interpreter. The ignore value is generated if a failing expression is back-quoted. The & and | operators are sensitive to the S/F/I value of the left operand (where I counts as S). Often this left operand is a matching expression.
!a & !b "if !a succeeds do !b" !a | !b "if !a fails do !b" !subject : !pattern "try to prove that !pattern describes !subject"
The back quote ` can be used to overrule the failure of a subexpression. The tilde ~ negates failure and success.
`!p & !q "do !p and then do !q" !a:!p & `!b | !c "if !a matches !p do !b else do !c" ~!a "succeed if !a fails and fail if !a succeeds"
⇑ =.,|&: +*^LD'$_.
⇑ Programming advice.
⇐ Binary operators in program flow.
⇒ Binary operators in pattern matching.
⇒ Some often used control structures.
<term> + <term> addition <factor> * <factor> multiplication <base> ^ <exponent> exponentiation <base> \L <expr> logarithm <variable> \D <expr> differentiation
Subtraction and division are treated as special forms of addition and multiplication. Therefore there are no binary operators for subtraction and division. (The minus sign - and the slash / can be used in numbers, however.) If one operand of an algebraic operator is evaluated then the other one is normally evaluated as well, even if this may seem unnecessary (multiplication by 0). This is done to ensure that all side effects take place as intended. However, if an operand fails to evaluate then the algebraic expression fails too and if the failing operand is the left hand side of the expression, then the right hand side is not evaluated. In this sense algebraic operators behave like the logical & operator.
Bracmat gives the user practically NO control over the format of evaluated algebraic expressions, such as the order of terms or factors. Bracmat tries to present algebraic objects in a unique (canonical) form. This is in many cases an unattainable goal : the forms (a+b)*(c+d) and a*c+a*d+b*c+b*d are both stable expressions. On the other hand, (a+b)*(c+d)+e becomes e+a*c+a*d+b*c+b*d. Bracmat keeps "expensive" completely factorised expressions, but does not automatically factorise factorisable expressions. Another domain of duality are expressions with logarithms. Sums and products start with rational numbers, followed by pi, i and e (if present, that is). Then follow other terms and factors. It is recommended not to assume anything about the ordering of these terms and factors, as this may change in later versions of the program.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇐ Differentiation.
<variable> \D <expr>
Bracmat knows how to differentiate expressions in which no other binary
operators occur but + * ^ and \L.
Example:
{?} y\Dx\D(a^(x^2+y^2))
{!} 4*a^(x^2+y^2)*x*y*e\La^2
{?} y\Dr
{!} 0
The last example gives zero, which in many applications isn't what we want. Often, with y we express the y-component of a vector with length r, and r consequently is a function of y (and the other components). We can solve this as follows:
{?} dep=(r.x) (r.y) (r.z) {'dep' is a special variable}
{?} y\Dr
{!} y\Dr
Now the expression is just left unevaluated. Later, you can substitute an expression for r in terms of its components
{?} y\D(r^-1):?derivative
{!} -1*r^-2*y\Dr
{?} sub$(!derivative.r.(x^2+y^2+z^2)^1/2):?derivative
{!} -1*y*(x^2+y^2+z^2)^-3/2
And, if you like, you can simplify the result by putting r back in:
{?} sub$(!derivative.x^2+y^2+z^2.r^2):?derivative
{!} -1*r^-3*y
⇑ =.,|&: +*^LD'$_.
⇒ Algebraic operations.
The binary operators $ and ' are similar in most respects. In general, the left operand evaluates to the name of a built-in or defined function, whereas the right operand is an expression that is passed as an argument to the function. The $ evaluates the right operand before it is passed over, the ' doesn't. Parameter passing is by value, although the implementation postpones and limits copying of data as much as possible. In the code of the called function, the passed argument is bound to a local variable that is always called arg. Most often, the left operand of the $ and the ' operator evaluates to an alfa- numeric name. There are a few special function names:
Function calls are even effective in patterns, as it is fair to assume that the $ and ' operators seldom occur in subjects and so need not to be matched (the same is, a fortiori, true for the & and | operators). In patterns, the return value of a function is part of the pattern. A function may be called several times during one evaluation of a matching expression, due to backtracking and retrying.
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇑ functions.
⇒ The "nameless" functions $<expression> and '<expression>.
Bracmat has only 1 "variable" that binds to a binary operator, the "_" operator. Worse even, this variable is global. Nevertheless this variable is most useful in definitions of certain types of recursive functions ("tree walkers"). The assignment of a new value to the "_" variable can only take place in a match. A "_" in a pattern is always "receiving", whereas a "_" outside a pattern is either "giving" or left unchanged. Try this :
{?} a_b { This has unpredictable results. }
{?} x^y:?_? & a_b { _ gets bound to ^. Thus a_b evaluates to a^b }
A "_" is evaluated by the expression evaluator, but also by the macro evaluator. The latter is useful if the "_" has matched an operator that is very volatile, such as "&" and "|".
{?} (=!a:!b&!c):(=?left_?right) {match the "&"}
{?} '_
{!} =& { It worked, the "_" is replaced by a "&". }
{?} get$(str$('_),MEM,VAP):"=" ?op & !op { "freeze" and slice }
{!} & { The operator is immobilised in a string. }
The "_" variable is always expanded BEFORE the left and right hand side operands are evaluated. That explains why new assignments in the operands do not result in unwanted side effects in the upper node with the "_".
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇒ Recursion and the "_"-operator.
In Bracmat functions are allowed to call themselves. Often this happens if a the function's argument is split into a left subtree and a right subtree and the function is called with each subtree in turn as its argument. If the operator in between the subtrees is unknown, it is time consuming to try all patterns ?+? ?*? ?$? ?'? etc. The _ operator circumvents this problem. It is a dummy operator that matches any other operator and expands to the operator with which it matched last time. Thereby preceding matches are "forgotten" : the _ operator is a global variable.
{?} reverse=l,r
.
(!arg:?l_?r) { If arg is a compound expression ...}
& (reverse$!r)_(reverse$!l) { ... swap the reversed operands. }
| !arg { Let atoms as they are. }
{?} reverse$(Bill loves sweet Nancy. This is true)
⇑ =.,|&: +*^LD'$_.
⇑ Binary operators.
⇑ functions.
⇐ The dummy operator "_".
⇐ <function name>=<var1> [,<var2>, ...].<function body>.
Sequence :
-----------
(pseudo code) (Bracmat)
a; b; `!a & !b { !b must be evaluated,
even if !a fails }
Repetition : -----------
WHILE a DO b; while = !a & `!b & !while;
~!while;{ if !a fails, then !while
fails, but ~!while succeeds }
DO b WHILE a; do = `!b & !a & !do;
~!do; { see remark above }
DO b UNTIL a; do = `!b & ~!a & !do;
~!do;
FOR i := m TO n DO b; for = !i:>!n | `!b & 1+!i:?i & !for;
!m:?i & !for;
Selection :
-----------
(pseudo code) (Bracmat)
IF a THEN b ELSE c; !a & `!b | !c;
v := IF a THEN b ELSE c; (!a & `!b | `!c) : ?v
{ works even if !c fails }
SELECT a !a : (!c1 & `!b1 | !c2 & `!b2 |? & !bx);
WHEN c1 : b1
WHEN c2 : b2
OTHERWISE bx; { the ? matches anything }
branching :
------------------------------
(pseudo code) (Bracmat) CALL a; !a; CALL b(x,y,z); b$(x,y,z); v := b(x,y,z); b$(x,y,z) : ?v;
⇑ Binary operators.
⇑ Programming advice.
⇐ Binary operators in program flow.
⇐ Program flow.
These functions play complementary roles.
Depending on the context, $<expression> can have two effects :
In the context of a pattern, which is a largely unevaluated expression,
$<expression> forces an evaluation of <expression> before the match goes on.
The evaluated <expression> thereby becomes part of the pattern (without the
$ operator). This is repeated on every retry after backtracking.
In the following example, a list of numbers is searched for two numbers that
are followed by a third number that is the sum of the first two:
{?} 41 2 5 8 1 23 12 11 19 : ? `%?A ? `%?B ? ()$(!A+!B) ? Notice that !A+!B is re-evaluated after each mismatch of the sub-pattern ()$(!A+!B). This can be used to count the number of trials (do the same without the ` flags !!):
{?} 0:?count
{?} 41 2 5 8 1 23 12 11 19 : ? `%?A ? `%?B ? ()$(!count+1:?count & !A+!B) ?
{?} found !A and !B after !count trials
The other context is in a macro. Macro's are mainly used to selectively
replace late binding by early binding and may thereby improve performance.
In Bracmat, a macro has the general form '<expression>. When '<expression> is
evaluated, <expression> is searched for sub-expressions of the form $<atom> or
$(<object>). Such a sub-expression is replaced by the binding of <atom> or
<object> (if it has any).
Replacements and sub-expressions of the form '<sub-expression> are not scanned.
After macro substitution has taken place, what remains is an expression of
the form =<expression>. The = operator is a safeguard against evaluation of
<expression>. The = operator is not seen in pattern-matching.
Macro substitution makes it possible to dynamically create unevaluated code and
bind it to a variable.
{?} '($cat):?my-cat-var
{?} !my-cat-var$(Hello world)
{?} '($cat):(=?my-cat-alias)
{?} my-cat-alias$(Hello world)
Pattern matching can sometimes be made more efficient by using macro substitution, but the resulting code is harder to understand:
{?} 0:?count
{?} 41 3 5 7 6 23 12 11 19 : ? `%?A ? (`%?B {Each number pair [A,B] ... }
{is added only once, giving C. } & !A+!B:?C
{C's value is hard-coded into rem} & '(? ()$(!count+1:?count & $C) ?) : ?rem
{which is the remaining pattern. } ) !rem
{?} found !A and !B after !count trials
In the same way, function code can be pieced together before it is ever executed.
{?} ((!power : two & (=!arg^2)) | (=!arg^3)) : ?abc {If power="two",
abc is bound to !arg^2 (unevaluated). Otherwise, abc is bound to !arg^3.}
{?} '(.!arg + -1*$abc + 2) : ?poly {"poly" is the name of a new function
that will return a value that depends on the current value of arg
and on the value of power at the time when abc got its binding.}
{?} lst$poly { show poly's definition }
The macro construct '<expr> is useful if an expression has to be executed many times while parts of it remain constant, for example in nested loops: Without macro construct (5 X 5 multiplication table) :
{?} 0 : ?m { initialise counter of outer loop }
{?} (outer = 1+!m : <6 : ?m { code for outer loop : }
& put$\n { start output on new line }
& 0 : ?n { initialise counter of inner loop }
& `!inner { execute inner loop }
& !outer) { loop }
{?} (inner = 1+!n : <6 : ?n { code for inner loop : }
& put$(!m X !n "=" !m*!n ", ")
{ the same !m is expanded 10 times }
& !inner) { loop }
{?} !outer
With macro construct :
{?} 0:?m
{?} (outer = 1+!m : <6 : ?m
& '( 1+!n : <6 : ?n
& put$($m X !n "=" ()$m*!n ",")
{ !m is expanded only 2 times }
& !inner
) : ?inner { at each pass through the outer loop
the inner loop "inner" is defined anew }
& put$\n
& 0 : ?n
& `!inner
& !outer)
{?} !outer
⇑ Binary operators.
⇑ functions.
⇐ function evaluation.
program flow
pattern matching
flags combined with expressions
unary operators
⇑ BRACMAT .
Unlike other programming languages, Bracmat does not return the value of a variable or object member if we type its name. In Bracmat, variables and object members have to be told explicitly that we want their value, not their name. This is achieved with the ! and the !! flags in front of the variable name or object member name. !<atom> is replaced by the binding of <atom> !!<atom> is replaced by the binding of the binding (after evaluation) of <atom> Likewise !(<object name>.<member name>) is replaced by the binding of <object name>.<member name> Bindings can be evaluated or unevaluated. In the last case, the next step after expansion is the evaluation of the binding, unless expansion took place within a pattern.
{?} 2+3:?four { bind 5 to "four" }
{?} !four { evaluation has already taken place when four is expanded }
{?} 5=2+!four { numbers are legal names ! 5 is bound to 2 + !four}
{?} !5 { evaluation takes place immediately after expansion }
{?} sum=%+% { define pattern "sum" }
{?} a+b+c:!sum { is a+b+c a sum? After expansion, %+% is not evaluated }
The !! flag is not used as often as the single !, but comes in handy if you want to pass a variable by name instead of by value.
{?} (check=one,two,criterion
. !arg:(?one,?criterion,?two)
& !!criterion
& TRUE
| FALSE
)
{?} is-greater-than = !one:>!two
{?} is-divisor-of = (div$(!two,!one)*!one):!two
{?} check$(3,is-greater-than,15) { pass by name }
{?} check$(3,is-divisor-of,15) Passing by name is used here to postpone the evaluation of the second argument until it has arrived in the function check and the local variables "one" and "two" have been bound to the first and the third arguments, respectively.
Postponement of evaluation can also be achieved with the = and the ' operators.
{?} (chack=one,two,criterion
. !arg:(?one,?criterion,?two)
& !criterion
& TRUE
| FALSE
)
{?} is-greater-than == !one:>!two { an extra = }
{?} is-divisor-of ='((div$(!two,!one)*!one):!two) { an extra ' }
{?} chack$(3,!is-greater-than,15) { pass by value }
{?} chack$(3,!is-divisor-of,15)
⇑ Flags.
In patterns, atoms and expressions within parentheses may be preceded by flags that
control the matching process.
! and !! in front of an non-nil atom or an expression denoting a member of an
object causes expansion of the atom or the member to its direct or
indirect binding. This binding is matched with the subject.
` causes backtracking if the pattern did not successfully unify with
a non-trivial element of the subject-list. A list is an expression
consisting of terms (+ operator), factors (* operator) or words
(white-space operator). Trivial elements are 0 in a sum, 1 in a
product and a word without characters in a list of words. In non-
sophisticated patterns, ` means simply : unify with at most one non-
trivial element. Zero non-trivial elements are allowed, in which
case unification takes place with an implicit trivial element :
Bracmat sees 0's everywhere in a sum, 1's in a product and zerolength
words in a sentence.
? unifies with anything. If ? is followed by a non-nil atom denoting
a variable or an expression denoting a member of an object, then the
matched part of the subject is captured by this variable or member.
In other words, pattern matching can have assignment as a
side-effect.
@ unifies only with atoms.
% causes the match to succeed only with one or more non-trivial elements
of the subject-list.
< and > unifies only with atoms that are "less" or "greater" than the
atom following the < or > flag.
# unifies only with rational numbers.
/ unifies only with non-integer rational numbers.
~ constrains the match to subjects that are not equal to the atom
following the ~ flag.
[ Position flag. Must be followed by an expression that evaluates to a
number (For example [4 or [(!pos+3) ) or by a variable having a
question mark as in [?pos.
In the first case, the pattern cannot succeed unless the element
following the [ element is at the indicated position. The [ element
itself does not occupy a position; it sits in front of the indicated
position. The second form is for querying the current position.
Position 0 is the start of the subject. Positive positions count
from the beginning of the subject, negative positions from the end.
Position -1 is the position following the last element.
The above flags may be combined. The ordering in which they are input by the user is irrelevant; Bracmat keeps flags in this order : ~ / # < > % @ ` ? ! !! Repeating flags in front of the same atom does not convey a new meaning to the pattern, except for the ! and the ~ flags. More than one ! is interpreted as the !! flag. An odd number of ~ is treated as a single ~, an even number thereof is treated as none. A ~ in front of other flags negates the first of them. The most useful combinations are :
?! in front of an atom causes the atom to be expanded to its binding.
This binding is treated as a variable name.
?!! is like ?!, but expands two levels deep (with an evaluation of the
first level expansion), instead of one.
<> is like a solitary ~.
/<>5/6 unifies only with non-integer rational numbers unequal to 5/6.
~< means "greater or equal" ("not less").
~> means "less or equal".
~<> means "not different", i.e. "the same, in some sense". Strings are
compared case insensitive. This applies to ASCII and the upper 128
characters in the ISO8859 (Latin 1) character set.
~# does not unify with rational numbers.
~/ does not unify with non-integer rational numbers.
~/# unifies only with integer numbers.
~/#<9 unifies only with integer numbers less than 9.
~/#<>0 unifies only with non-zero integer numbers.
~@ unifies only with non-atomic expressions.
~` backtrack immediately.
Many of these combinations can be combined further, e.g. ~/#?!! accepts only an integer number and binds it to the indirect binding of the atom following the flags.
If you want to match pattern !pat one or more times (in the literature, this is often written as {pat}+ ), use the complex pattern (? !pat|`). Likewise, if you want to match !pat zero or more times ( {pat}* ), use (|? !pat|`). These patterns should not be the last sub-pattern, because their correct working depends on repeated backtracking from following sub-patterns. In the last resort, you can add a pattern like () or (&) or (|) or (:), which match with an empty list only (assuming that the connecting nodes are spaces, otherwise use 0 in the case of a sum and 1 in the case of a product). Example:
{?} a a a c c:(? a|`) (|? b|`) (? c|`) (&) { {a}+ {b}* {c}+ }
⇑ Flags.
⇑ Programming advice.
⇐ <flags>$<expression> and <flags>'<expression>.
⇐ Binary operators in pattern matching.
Functions that have only flags as their function name can have two effects : Most flags (/ # @ % < > ?) are added to the flags of the function argument. The enriched argument is passed back as the function's return value. The function named ~ negates success and failure of its argument, which is passed back. In the same way, the function named ` (back-quote) changes failure into "failure that is to be ignored and leaves program flow undisturbed, as if there was success" (outside patterns) or into "failure that cuts off further attempts at matching the failing subpattern and forces immediate backtracking to previous matches." (inside patterns). The ! and !! flags cannot occur as function names. A single ! evaluates to the answer to the last typed-in expression. A !! tries to expand this answer one step further.
{?} (!arg:(smaller,?number) & <'!number | >'!number):?pat
{ Either "<!number" or ">!number" is bound to pat, postponing the
evaluation of !number until !pat is evaluated. }
{?} apple banana tomato grape orange : ?fruits { make a list of fruit names }
{?} ~$(!fruits : ? raspberry ?) & raspberry !fruits : ?fruits
{ if "raspberry" is NOT in "fruits" then put it in front of the list.}
{ this has the same effect as : }
{?} condition = !fruits : ? raspberry ? { bind code to atom }
{?} ~!condition & raspberry !fruits : ?fruits { test atom's failure }
As in every computer language, it's bad practice to use negation if there exists a readable and short alternative formulation. The above membership checking example has a shorter alternative without negation:
{?} !fruits : ? raspberry ? | raspberry !fruits : ?fruits
{ "raspberry" is in "fruits" or else it is put in front of the list.}
⇑ Flags.
⇑ functions.
⇒ Flags and pattern matching.
The minus sign "-" has only its normal arithmetic meaning when used as an unary operator in front of a rational number or the imaginary number "i". If a product contains both a rational number and the number "i", the "i" takes precedence in accepting a minus sign:
-7*i*a
is evaluated to 7*-i*a.
The advantage of having both "i" and "-i" becomes clear by considering the following:
(-1*i)^1/3
evaluates to (-i)^1/3,
which is written as -i^1/3. As expected, this is the complex conjugate of
i^1/3.
If Bracmat did not have a separate representation for -i, then
(-1*i)^1/3
would evaluate to i,
(because i^3 is equal to -i),which means that Bracmat would not consider (-1*i)^1/3 and i^1/3 as complex conjugates. The transcendental numbers e and pi do not accept arithmetic minus signs.
⇑ Flags.
A string in Bracmat is the same as an "atom". If you envisage a Bracmat expression as a tree like structure, atoms or strings are to be found in the leafs. In Bracmat terminology, an empty leaf is syntactically represented by <nil>. <nil> is not an <atom> proper, but an <atom or nil>. So not every leaf contains an atom. On the other hand, leafs may contain other things besides <atoms>, such as flags. In Bracmat, atoms are less accessible than trees. Therefore there are some ways to convert atoms to trees and back.
Atoms can be used as names for variables, functions, files, etc.. Often they are used as literals, such as mathematical symbols or text.
Most characters which the computer knows of can be members of an atom. Only the first seven characters in the ASCII character set are forbidden. These characters are used by the system. In most cases you don't need quotation marks in order to get a string of characters into an atom. You do need them if you want parentheses, operators or flags to be part of an atom. Some special characters have to be preceded by a back slash:
\a attention (bell) \b backspace \t tab \n new line \v vertical tab \f form feed \r carriage return \\ back slash \" double quote
If you precede a string with the flag @, then back slashes are treated as normal characters. E.g. sys$@"C:\dos\edit". In stead of the tab and new line characters above, you may enter tabs and new lines by pressing the tab and the return key, respectively.
Examples:
{?} this is a "tree" with\nsix leafs
{?} (this is a "tree" with
seven leafs)
{?} "this" has 4 characters and "" (nil) none
{?} "this is an \"atom\" with 36 characters"
{?} "this string\nno verb"
{?} "this string no verb either"
{?} "if zero equals one, I hang up" = "1:0&get$(\")y\",MEM)"
{?} get$(!"if zero equals one, I hang up",MEM)
⇑ BRACMAT .
literals
variables
⇑ BRACMAT .
In Bracmat, symbols have only literal meaning, unless we explicitly state that we want a symbol to behave like a programming variable. Contrary to most computer languages, Bracmat evaluates an expression with literals not by expanding these literals to their associated values (if they have any) and computing with these values until a result is obtained, but by rearranging and transforming the expression until a stable form is reached.
{?} a + a
{?} i*i
{?} e^(19/2*pi*i)
In Bracmat, the context of a symbol decides whether it is treated as a variable or as a literal. So it is not necessary to "kill" a variable in order to use its symbol as a literal, the two uses live peacefully together.
{?} i=2 { variable i is bound to the literal "2" }
{?} !i^2 { the associated value of i is squared }
{?} i^2 { the literal "i" (a special one, like "pi" and "e") is squared }
{?} 7 = prime { the variable 7 is bound to the literal "prime"}
{?} 7 is !7 { the symbol 7 is used as both a literal and a variable}
⇑ Symbols.
Variables are represented by <atom>'s, but not all <atom>'s are variables. The context of a symbol determines whether it is a variable or not :
⇑ Symbols.
In Bracmat, a binary operator may have four different effects, depending on the context of the operator. For each of these contexts there is one evaluator. Of these four evaluators, the macro evaluator is relatively unimportant. The four evaluators are :
The expression evaluator is the first evaluator that a newly input expression is confronted with. If necessary, it delegates tasks to one of the other three evaluators. The match evaluator can only delegate tasks to the expression evaluator and to the archivist. The macro evaluator can only delegate tasks to the expression evaluator. The archivist doesn't delegate any tasks to other evaluators. The cross link is in most cases a binary operator. The exceptions to this rule are in the context of the match evaluator: some (combinations of) flags involve the expansion of a chain of variable bindings and all but the last subexpansion demand the expression evaluator. In the scheme below, you'll find the "current" evaluator in the left column and the successor evaluators in the top row. A cross link is represented by the relevant operator or flags. If the change of evaluator only applies to the left (right) operand of the cross link operator, the symbol "l" ("r") is used. If the transition depends on the left operand being <nil>, the symbol "n" is used.
expression match macro archivist
expression :r n'r =r 'r
match &r $ 'l ?! !! n'r =r 'r
macro $r
archivist
⇑ BRACMAT .
program flow
pattern matching
data structures
debugging
⇑ BRACMAT .
If a program written in the Bracmat language doesn't work properly, the same debugging protocol applies as with other programming languages :
⇑ Programming advice.
⇒ using out$ as debugging aid.
The best aid in finding out what a program does, is using the out$ function. The following code is part of a function that computes n! .
loop = !k+1 : ?k { increment k }
: <!n { compare (old) k+1 with n; if not less, stop }
& !fac*!k : ?fac { multiply fac with k }
& !loop { repeat until k = n }
Outside patterns out$ is most easily used. Inside patterns, if you want to inspect a variable that has just been assigned a new value, you use the & operator to temporarily escape into the non-pattern world. If you want to add extra text to the output, remember that all of out$'s argument is returned.
loop = out$!k+1 : (?k & out$(k is !k)) {show k before and after increment}
: <!n {but before comparison with n}
& out$("new fac is:" (!fac*!k:?fac)) {show fac after computation}
& out$(still need !n+-1*!k loops) {you don't always need quotation marks}
& !loop
Now an example that is faulty. The purpose is to find two equal words in a sentence. This expression succeeds, but finds nothing:
(He loves her and she loves him : (? ?a ? !a ?) & out$(!a is occurring twice))
Check what is unified with "? ?a". To do so, put a variable after the first "?" and insert an output action after each sub-pattern.
(He loves her and she loves him
: ((?x & out$(x is !x)) {output x after unification}
(?a & out$(a is !a)) {output a after unification}
? !a ?) {the remainder of the pattern}
& out$(!a is occurring twice)
)
The program would have to backtrack several times until ?a was unified with "loves", but the match succeeds with ?a unified with the omnipresent zero length word. A % sign avoids this. A back quote ` helps speeding up, since it avoids multi-word assignments and forces immediate backtracking.
(He loves her and she loves him
: ((?x & out$(x is !x)) { Watch the number of words in ?x grow ... }
(%`?a & out$(a is !a)) { while ?a moves towards "loves". }
? !a ?) { There backtracking stops }
& out$(!a is occurring twice) { and the message is output. }
)
⇑ Programming advice.
⇑ Predefined functions.
⇐ Debugging.
⇐ out$<expression>.
⇒ using dbg' as debugging aid.
Some programming errors may be found with the built-in dbg function. The argument of the dbg function is evaluated with an internal debugging flag set. With this flag set, suspicious code is warned against. It is important that the argument is not evaluated before being passed to the dbg function.
⇑ Programming advice.
⇑ Predefined functions.
⇐ using out$ as debugging aid.
function evaluation
defining functions
nameless functions
flag functions
built-in functions
predefined, changeable functions
⇑ BRACMAT .
Definition of a function without explicit local variables. The function is
called by
<function name>$<argument expression> or
<function name>'<argument expression> ,
depending on whether <argument expression> must be evaluated ($) or not (')
before it is passed to the function in the always present local variable
"arg". The returned value of a function is simply the function body after it
has been evaluated.
{?} square=.!arg^2 {definition}
{?} square$5 {call}
⇑ functions.
⇒ <function name>=<var1> [,<var2>, ...].<function body>.
Definition of a function with local variables <var1>,<var2>, etc. .
{?} swap = a,b {declare local variables a and b}
. (!arg:(?a,?b)) {dissect arg to find the "real" arguments}
& (!b,!a) {swap and return}
{?} swap$(I think,I guess) Local variables in Bracmat are shallowly bound dynamically scoped variables. This means that variables that are used in a function but not locally declared in that function, are inherited from the (function or global) context from which the function is called, which in turn may inherit any undeclared variables from another calling context. This scheme contrasts with most programming languages. It is efficient, but the effect of forgetting to declare a local variable can be unexpected behaviour of conceptually unrelated code. It is possible to declare a function inside another function. Always declare the name of an embedded function as a local variable.
⇑ functions.
⇐ <function name>=.<function body>.
⇒ Recursion and the "_"-operator.
access array element
allocate memory (low level)
convert character to internal representation
convert internal representation to character
convert Unicode codepoint to UTF-8 character
debugging aid
denominator
quotient
file I/O (low-level)
splits expression in flags and flagless expression
opposite of flg: combines flags and expression
return allocated memory (low level)
get input (from file,keyboard or memory)
convert to lower case
list un-evaluated value of variable(s)
list existing variable names
remainder
create new object as a copy of another object
get value from address (peek) (low level)
put value at address (poke) (low level)
write output
rename file or directory or move file
string reverse
similarity between two atoms
stringize expression into atom
software interrupt (low level)
command line shell
establish array size
convert to upper case
convert UTF-8 character to Unicode codepoint
while loop
⇑ functions.
<array name> and <index> should both evaluate to atoms. <array name> may be
preceded by flags, such as ? or !. Indexing starts at 0 and is done
modulo(size-of-array). Negative values count from the upper end of the array.
The chosen index remains in force until a new indexing function is evaluated.
{?} tbl$(array,4) { declare array[0..3] }
{?} a-value : 2 $ ?array { array[2] := a-value }
{?} array = another-value { array[2] := another-value }
{?} !array : -1 $ ?array { array[3] := another-value }
{?} 45 : 1$?array { array[1] := 45 }
{?} 2'!array : 3'!array { are array[2] and array[3] equal?
Notice use of ' instead of $. }
This function allocates memory and returns the starting address of the allocated memory, but crashes the program if not enough memory can be allocated. Access to memory that has been allocated in this way is by means of pee$ and pok$. Any allocated memory should at some time be returned to the memory heap with the function fre$.
{?} alc$1000:?p {allocate 1000 bytes and assign starting address to p}
asc$ returns the integer value that corresponds to the character according to the current table used by the operating system (e.g. an extended ASCII table).
{?} asc$"+" {return "ASCII" value of character +}
chr$ returns the character at location <value> in the current table of characters used by the operating system. (e.g. an extended ASCII table). chr$ fails if <value> equals 0.
{?} chr$255 {return the last character from the current table of characters
(assuming a machine with 8-bit characters)}
{?} tolower=.
!arg:~<A:~>Z {If arg is in the range A-Z,}
& chr$(asc$!arg+-1*asc$A+asc$a) {then return its lower case equivalent,}
| !arg {else return arg unchanged.}
{(Works only correctly if the "distance" between lower and upper case
versions is the same for all characters in the range A-Z.)}
{?} tolower$G
chu$ returns the UTF-8 character at Unicode code point <value>. chu$ fails if <value> equals 0 or if <value> exceeds 2147483647.
Create warnings in situations that probably are programming errors. Currently, a warning is generated when a function definition can not be found.
{?} dbg'(foo$a)
{?} dbg'((myclass.yourfunc)$X)
The denominator of <rational number> is returned. There is no built-in numerator extractor function.
{?} den$22/7
{?} num = .!arg*den$!arg { home-made numerator function }
{?} num$22/7
{?} den$sim$(,monkey) { return length of word "monkey"}
div$ returns the (integral) quotient of its arguments.
{?} div$(123/45,67/890)
fil$ is a multi-purpose low level I/O function.
(1) opening a file
fil$([<file name>],<mode>)
Option <mode> is one of the following :
r open text file for reading
w create text file for writing, or truncate to zero length
a append; open text file or create for writing at eof
rb open binary file for reading
wb create binary file for writing, or truncate to zero length
ab append; open binary file or create for writing at eof
"r+" open text file for update (reading and writing)
"w+" create text file for update, or truncate to zero length
"a+" append; open text file or create for update, writing at eof
"r+b" or "rb+" open binary file for update (reading and writing)
"w+b" or "wb+" create binary file for update, or truncate to zero length
"a+b" or "ab+" append; open binary file or create for update, writing at eof
(2) Prepare for reading or writing fixed sized records
fil$([<file name>],<type>,<size>[,<number>])
Option <type> is one of the following :
CHR character or string I/O
DEC number I/O
Option <size> must be a non-negative integer and determines the number of
bytes that are read or written as one chunk during a future call to fil$.
If <type> is "DEC", only values 1,2 and 4 are valid, corresponding to 1, 2
and 4 byte sized integers, respectively. Notice that 2 and 4 byte integers are
not portable between implementations with different byte order. The optional
<number> tells how many read or write operations of size <size> have to be
performed. If <type> is "DEC", the product of <size> and <number> may not be
greater than 4. If a number is read or written in 2 or more chunks, the least
significant bytes or 16 bit words are read or written first (little endian).
(3) Prepare for reading or writing variable sized record
fil$([<file name>],STR[,<stop>])
Option <stop> is a string of characters. If the read character or the character
to be written is equal to one of the characters in the stop string, reading or
writing stops. The default is not to stop until the end of the file (reading)
or the end of the string (writing).
(4) Telling the position inside the file
fil$(<file name>,TEL)
returns the current value of the file position indicator.
(5) Go to file position
fil$(<file name>,<whence>,<offset>)
sets the current value of the file position indicator to an <offset> based
on the value of <whence>.
Option <whence> is one of the following :
SET start of file
CUR current file position
END end of file. (In some implementations, binary files may not handle END).
For a text file, <offset> must be 0, or the value returned from a call to
fil$(<file name>,TEL), in which case <whence> must be SET.
(6) Reading
fil$([<file name>][,,<number>])
reads (if <mode> permitting) <number> chunks of <size> bytes.
When reading variable sized records (STR), fil$ returns a dot-separated list
of two elements: the found stop character and the read string (which does not
contain the stop character).
{?} fil$("mytext.txt","rb") { Open for reading in binary mode. }
{?} fil$(,STR) { Reading until end-of-line "\n".}
{?} fil$:(?stop.?line) { Read the first line. }
{?} fil$(,SET,-1) { Close the file. }
(7) Writing fil$([<file name>],,<number>,<value>)
writes (if <mode> permitting) <number> chunks of <size> bytes from <value>.
If <type> is "DEC", <number>*<size> must be 1,2,3 or 4. <value> must be an
integer value and is cast to a binary number with at most <number>*<size>*8
bits. This number is stored in <number>*<size> bytes, which in turn are output.
If <number> is greater than 1, the byte(s) with the least significant digits
are output first. In machines with little-endian byte-order, only the
product <number>*<size> matters.
If <type> is "CHR" and the length of <value> is shorter than <number>*<size>,
<value> is padded with spaces (to the right).
(8) Closing the file fil$([<file name>],SET,-1)
An open file is closed by specifying an impossible file position.
flg returns a flagless copy of the expression and a new leaf with the flags of the original expression. These two results are coupled with a dot-operator, the flags to the left and the flagless expression to the right. The result is protected against evaluation by a '=' operator.
{?} flg$(=~#<>?%@a)
{!} (=~#<>%@?).a Use macro evaluation if the expression to be split is the value of a variable:
{?} X=~(%+%)
{!} X
{?} flg$('$X):(=?flags.?expr)
{!} =~.%+%
{?} glf$('($flags.%*%))
{!} =~(%*%)
glf returns a copy of <expression> with <flags> added to its flags. If <expression> has one or more flags also present in <flags>, then glf fails. Therefore, glf can be used to test for the presence of one or more flags. The result is protected against evaluation by a '=' operator. The function glf has an effect that is the opposite of flg.
{?} glf$flg$(=?a)
{!} =?a Use macro evaluation if the expression to be split is the value of a variable.
{?} X=?!x
{!} X
{?} flg$('$X):(=?flags.?expr)
{!} =?!.x
{?} glf$('($flags.z))
{!} =?!z
fre$ returns a chunk of memory to the memory pool (heap). The only valid parameter is a return value of alc$. Applying fre$ to a chunk of memory that was never allocated or that has been returned already results in undefined behaviour of the program.
{?} alc$1000:?p {allocate chunk of 1000 bytes from the memory pool}
{?} fre$!p {return this chunk to the memory pool}
get$ reads and interprets characters in a string (internal memory) or file (external memory or keyboard). options :
MEM present :The name of the first parameter is the source of the characters.
(MEMory)
otherwise:A file with the name of the first parameter is the source.
ECH present :The characters are echoed to the screen as they are read.
otherwise:No echo.
VAP present :The characters are read "as is". Between the characters extra
spaces are inserted.
(VAPorised)
otherwise:No extra spaces are added.
STR present :The characters are read into one string.
otherwise:The characters are interpreted as parts of a Bracmat expression
and evaluated after the whole expression has been read.
The VAP option is evaluated before the STR option.
Applications :
get'(matrix,ECH)
Read file "matrix" and evaluate the expressions (delimited by semicolons) therein. If the system finds a syntax error in a multiple expression file, the ECH option makes it easier to locate the error.
get'(matrix,STR):?intern
Read file "matrix" into a string called "intern". If this file contains Bracmat instructions, they hereafter exist in a "sleeping" state in memory.
get$(!intern,MEM)
!intern is, in this example, expanded to an atom with a very long name, namely all of the text of file "matrix". The sleeping expressions are evaluated one after the other, just as if "get'matrix" was evaluated.
get'(,VAP):?space-list
Read characters from standard input (normally keyboard) until next line feed character. Put each character into an atom. Put all atoms into a linear list with space operators. Bind this list to the name "space-list".
get'(")y",MEM)
Read the sleeping "expression" ")y" from memory. The lexical scanner will find an unbalanced right parenthesis, which could mean that this Bracmat session should stop. The "y" confirms this assumption and the program will come to an end immediately. If the "y" hadn't been present, Bracmat would ask "end Bracmat session ? (y/n)" after which the user has to choose. This trick is useful in batch processing.
If the first parameter is <nil> or "stdin" and the MEM option is not used, input is coming from standard input. Take care for putting filenames in double quotes if they contain any characters that can be misunderstood, such as dots, (back) slashes or dollars.
low$ converts a string to all-lowercase.
The characters 'A'-'Z' are converted to 'a'-'z'. Other characters are converted
depending on the presence of the DOS option:
DOS present : Characters 128-255 are assumed to be in CodePage 850
otherwise: Characters 128-255 are assumed to be in ISO 8859 (Latin 1)
⇑ Built-in functions.
⇒ Codepage 850 support$.
As of July 2009 Codepage 850 is not supported, unless bracmat is compiled with #define CODEPAGE850 1.
⇑ Built-in functions.
⇑ Hash tables.
⇐ low$(<atom or nil>[,DOS]).
⇐ upp$(<atom or nil>[,DOS]).
⇐ (myhash..DOS)$.
Outputs all present bindings of one or more variables to standard output. If the first parameter is the zero-length string, then all variables with names starting with a character below ASCII 128 are shown. If a variable has more than one binding (arrays/stacks) then the current value is preceded by a ">"-sign. The second parameter is optional.
{?} lst$help { shows this programme on screen,
unless stdout has been redirected }
{?} lst$(tay,LIN) { listing without indentations of function tay$ }
⇑ Built-in functions.
⇒ option LIN.
If LIN is not present, output is very much indented, sometimes making it more readable for humans. If LIN is present, output is as compact as possible.
⇑ Built-in functions.
⇐ lst$(<variable>[,LIN]).
⇐ put$(<expression>[,LIN]).
⇒ lst$(<variable>,MEM [,LIN]).
⇒ put$(<expression>,MEM [,LIN]).
The difference with the preceding form is, that "output" takes place to memory. What is normally visible on screen is put in one atom, which is the return value of the call to lst. It has the opposite effect of get$(<atom>,MEM). Use : compression of an expression to save space. If the compressed expression is needed, it is decompressed with get$. Compression is typically by a factor of about 5, but may be as large as 16. Expressions with very large atoms (such as this help function) do less well. Example:
{?} lst$(fct,MEM):?sleeping-fct
{?} tbl$(fct,0) { remove function fct$ from memory }
{?} { .. celebrate space-saving, until fct$ is needed .. }
{?} get$(!sleeping-fct,MEM)
⇑ Built-in functions.
⇐ option LIN.
⇒ lst$(<variable>,<file name>,NEW | APP [,LIN]).
This time, output is sent to the named file instead of standard output. The third argument is explained below. Code that has been saved with lst$ can be reloaded with get$.
{?} lst$(,"all",NEW,LIN)
{ write all current code without indentations to file "all" }
{?} lst$(tay,taylor,NEW)
{ save function tay$ to file "taylor" in indented format }
⇑ Built-in functions.
⇐ lst$(<variable>,MEM [,LIN]).
⇒ options NEW and APP.
one of the options NEW and APP must be present :
NEW tells the computer to open a new file or overwrite an old one.
APP directs output to an existing file. If the file does not exist, it is
created first.
⇑ Built-in functions.
⇐ lst$(<variable>,<file name>,NEW | APP [,LIN]).
⇐ put$(<expression>,<file name>,NEW | APP [,LIN]).
mem$ produces a list of all currently existing variables, except those beginning with a character above ASCII 126. The EXT option adds information about the number of occurrences (array or stack size - 1) of those variables which have more than one occurrence and shows which of them is currently in focus (index into array: 0 .. size-1). The predefined function cat$ makes use of mem$.
{?} mem$
{?} mem$EXT
mod$ divides <number> by <divisor>. The rest is returned.
{?} mod$(22,7)
new$ creates a shallow copy of an object and calls the method "new" of the new object, if there is one. With the second form, <args> is passed to the method "new". Example:
{?} (patient=
(name=(first=John),(last=Bull)) {name is a copy, first and last are not}
, (age=20)
, ( new
=
. out$"hello world"
& new$(its.name):(=?(its.name)) {create fresh copies of first and last}
& !arg
: (?(its.name.first).?(its.name.last).?(its.age))
))
{?} new$(patient,(Albert.Keinstein.42)):?x
{?} new$(patient.name):(?Name)
{?} Alice:?(Name..first)
{?} new$(('$patient),(Albert.Keinstein.42)):?y {x and y are identical !}
{?} new$(=(a=1),(b=2)):?ab {"die" is called when ab is reassigned}
{?} 3:?(ab..a)
{?} new$(=(a=1),(b=2),(new=.),(die=.)):(=?cd) {"die" is called at once!}
{?} 3:?(cd.a)
When an object was created with the new$ function, an internal flag is set in the object telling the system that the "die" method must be called just before deletion of the object. The "die" method, like the "new" method, is optional and should be used to do clean-up.
Depending on <size>, a 1, 2 or 4 byte sized integer allocated at <address> is returned.
2 and 4 byte integers may only start at addresses that are multiples of 2 and 4, respectively. <address> is lowered to the nearest allowable value, if needed. Notice that multi byte integers are stored differently in Little Endian (iAPx86, VAX, ARM) and Big Endian (MC680x0) machines. Many operating systems abort programs that try to access non-existent or protected memory areas.
{?} chr$pee$34567 {return value at address 34567 (1 byte) as a character}
{?} pee$(34567,2) {return value at address 34566 (2 bytes)}
{?} pee$(34567,4) {return value at address 34564 (4 bytes)}
Depending on <size>, a 1, 2 or 4 byte sized integer is stored at <address>.
2 and 4 byte integers may only start at addresses that are multiples of 2 and 4, respectively. <address> is lowered to the nearest allowable value, if needed. Notice that multi byte integers are stored differently in Little Endian (iAPx86, VAX, ARM) and Big Endian (MC680x0) machines. Many operating systems abort programs that try to access non-existent or protected memory areas.
{?} pok$(34567,asc$K) {store the internal value of the character K at memory
location 34567 (as 1 byte)}
{?} pok$(34567,-1,4) {store 2^32-1 at memory location 34564
(assuming 1-complement arithmetic)}
Sends <expression> to standard output. The cursor is positioned after the last output character. The predefined function out$ does the same as put$, with the exception that it positions the cursor on the beginning of the next line.
{?} put$("b+a" is after evaluation b+a)
⇑ Built-in functions.
⇒ option LIN.
<expression> is stringized and placed into an atom, which is the return
value of the call. This use of put$ is similar to the str$ function, but
whereas str$ suppresses the space-operator, put$ transfers every character,
including space-operators, to the atom.
{?} put$(this is not Lotus 1 1+1 1+1+1,MEM):?proposition &
{?} put$!proposition
⇑ Built-in functions.
⇐ option LIN.
⇒ put$(<expression>,<file name>,NEW | APP [,LIN]).
Sent <expression> to the named file.
{?} put$(tay$(e^x,x,10),"e.out",APP)
⇑ Built-in functions.
⇐ put$(<expression>,MEM [,LIN]).
⇒ options NEW and APP.
Renames a file or directory or moves a file.
The ren$ function succeeds, unless a syntactic error was made. If there is an
error at the operating system level, one of the following codes is returned:
EACCES
File or directory specified by newname already exists or could not be
created (invalid path); or oldname is a directory and newname specifies
a different path.
ENOENT
File or path specified by oldname not found.
EINVAL
Name contains invalid characters.
If the command succeeds at the operating system level, the value 0 is returned.
Reverses the order of bytes in an atom. This function can be useful in case of a string match that asks for the last occurrence of a pattern. The rev$ function succeeds on all atoms and fails on all other expressions.
sim$ uses the Ratcliff/Obershelp pattern matching algorithm in establishing a measure of the similarity between its two (atomic) arguments. The returned value is an unsimplified fraction. The denominator is the sum of the numbers of characters in both arguments. The numerator is the total number of characters that have been matched successfully. Matching is case-insensitive.
{?} sim$(colour,Color)
{?} den$sim$(,"this is an easy way to find this string's length")
{?} div$(sim$("similarity rounded","and in procents")+1/200,1/100)
str$ "writes" <expression> into one single atom. All atoms, flags and operators, with the exception of the space-operator, are copied to the output string. Main use : pasting of two or more atoms.
{?} n=3
{?} str$(var !n) = seventeen
{?} out$(var3 is !var3)
{?} editfile = mytxts/story
{?} sys$str$("vi " !editfile) { execute UNIX command "vi mytxts/story" }
{?} a=x+2*y;b=2*x+y
{?} put$str$(!a "+" !b " = " !a+!b)
This function is the most operating-system-dependent function in Bracmat. Currently, it is implemented for RISC-OS (Archimedes) and 16-bit MS-DOS versions of Bracmat. All arguments must be integer values. The list of input values (registers r0 and upwards) need not be complete. Missing values are assumed to be zero. Blocks of memory should be passed by allocating memory with alc$ and passing the returned value. The returned value has the form (<error code>.<output value>,<output value>,...) An error code of 0 means that no error is reported. In the MS-DOS version, the input registers are AX,BX,CX,DX,BP,SI,DI,DS,ES and FLAGS, respectively.
{?} {RISC-OS only}
{?} putstr=(loop,c,buf,ret. { goal:copy argument to memoryblock }
alc$(den$sim$(!arg,)+1):?buf:?ret { allocate block for string to fit }
& get$(!arg,MEM,VAP):?arg { argument -> single characters }
& (loop = !arg:%?c ?arg & pok$(!buf.asc$!c.1) & !buf+1:?buf & !loop)
& ~!loop { poke each character into memory block }
& pok$(!buf.0.1) { poke string-delimiting zero }
& !ret) { return pointer to block }
{?} putstr$"OS_EvaluateExpression":?inbuf { create pointer to string }
{?} 57:?"OS_SWINumberFromString" { From manual }
{?} swi$(!"OS_SWINumberFromString".0,!inbuf):(?error.?nummer,?) { find
interrupt number corresponding with the string "OS_EvaluateExpression"}
{?} fre$!inbuf { deallocate block containing copy of input string }
{?} {MS-DOS only}
{?} gotoxy = (VIDEO,setCursorPosition,videoPage0.
{?} 16:?VIDEO { interrupt number 10H }
{?} & 2*256:?setCursorPosition { AH }
{?} & 0*256:?videoPage0 { BH }
{?} & !arg:(?x,?y) { DL and DH }
{?} & swi$(!VIDEO.!setCursorPosition,!videoPage0,0,!x+256*!y)
{?} )
{?} gotoxy$(0,0) & put$(top left corner)
In most environments, sys$ passes its argument to the command line interpreter. Therefore, sys$ has a functionality that very much depends on the operating system in which Bracmat runs. One has to take care for memory limitations and the possibility that sys$ may never return. Possible uses are for example :
The functions put$(<expression>,MEM) or str$<expression> may be used for constructing an argument for sys$ :
{?} file = bracmat.c
{?} !file:((?stem.?)|?stem) {remove file extension and put result in "stem"}
{?} sys$str$("copy " !file " " !stem.bak)
The sys$ function succeeds, unless a syntactic error was made. If there is an error at the operating system level, one of the following codes is returned:
E2BIG
Argument list (which is system-dependent) is too big.
ENOENT
Command interpreter cannot be found.
ENOEXEC
Command-interpreter file has invalid format and is not executable.
ENOMEM
Not enough memory is available to execute command; or available memory
has been corrupted; or invalid block exists, indicating that process
making call was not allocated properly.
If the command succeeds at the operating system level, the value 0 is returned.
The named variable is (re-)sized to an array with <array size> elements.
Resizing always affects the elements with the highest indexes first :
shrinking means that the last elements are lost, expanding creates new
zero-valued elements at the end.
If <array-size> equals zero, the named variable ceases to exist in memory.
This is the only way in Bracmat to get rid of global variables.
Stacks and arrays are exactly the same thing in Bracmat. Therefore, it is not
possible to declare arrays locally.
Bracmat never accesses arrays as a whole; there is always just one element that
is in focus. By issuing an instruction of the form <index>$<array name> or
<index>'<array name> you can explicitly tell Bracmat to put focus on some
element. Bracmat does this automatically in the case of pushing and popping
local variables onto and from a stack.
{?} tbl$(bigarray,16000)
{?} lst$bigarray { this may take a long time to execute }
{?} tbl$(bigarray,0) { remove bigarray }
{?} lst$bigarray
upp$ converts a string to all-uppercase.
The characters 'a'-'z' are converted to 'A'-'Z'. Other characters are converted
depending on the presence of the DOS option:
DOS present : Characters 128-255 are assumed to be in CodePage 850
otherwise: Characters 128-255 are assumed to be in ISO 8859 (Latin 1)
⇑ Built-in functions.
⇒ Codepage 850 support$.
utf$ returns the Unicode code point of the UTF-8 character. The function fails if the string is too short or too long, or if the sequence is an invalid UTF-8 string. It is safe to use utf$ in a pattern: @(!txt:(?%c & utf$!c) ?) If txt starts with an valid UTF-8 sequence, bracmat backtracks until c matches the UTF-8 sequence. If txt starts with a sequence that is not UTF-8, bracmat stops backtracking when that fact has been established.
whl' implements a while ... loop. The expression is repeatedly evaluated until it fails. whl' always fails. Notice that whl' is slightly faster than til' and that both are faster than loops using tail recursion.
Besides hard-coded built-in functions, Bracmat offers a number of soft-coded
functions which behave as user defined functions in all respects. They are
redefinable and removable, for example. Some functions are called by the
interpreter itself and should never be changed by the user. Such functions have
names that start with an 8-bit character with the high-bit set. Bracmat has
been drilled to leave these names out when the user asks for a list of variable
names (lst$ or mem$), so you will not notice their existence. The following
visible functions are predefined:
abs (and sgn) absolute value and sign
cat list existing variable names selectively
cos (and sin) cosine and sine
fct factorising
flt floating point notation of numbers
out output to screen
sub substitution
tay Taylor series
⇑ functions.
sgn$ determines the sign of the numerical factor of <expression>. If the sign is "-", then sgn returns -1. In all other cases the returned value is 1. abs$<expression> is defined as sgn$<expression>*<expression>.
{?} sgn$(-1*i)
{?} abs$(-7*a)
cat$ is like the built-in function mem$, but offers the possibility to exclude names that are not in the first parameter and/or to exclude names that are in the second parameter. The optional third parameter adds information about array size and current index value, e.g. (arg,5,5). If the first parameter is missing, this is taken to mean that NO names are excluded (unless by virtue of the second parameter). You can use cat$ to save the state of the variable space for later use, e.g. removing all variables that have been created since.
{?} cat$(,,EXT):?save-state { mem$EXT is also OK. }
{?} newvar1=12345 { create new variables }
{?} tbl$(newvar2,100)
{?} cat$(,!save-state,EXT) { show the newcomers }
These functions produce cos(expression) and sin(expression), expressed in powers of e. In this way, expressions with goniometric functions can be differentiated and, sometimes, simplified.
fct$ uses some heuristics in trying to factorise <expression>.
{?} 1+(2*a^3+6/7*t)*(3*x+4*y+z^-1)+-1:?sum
{?} fct$!sum
flt$ converts a rational number to a floating point presentation. The result is stored in an atom. This function is meant for output, Bracmat does not use floating point numbers itself.
{?} flt$(123/456,78)
{?} flt$(sub$(tay$(sin$x,x,40).x.11/7),12)
Uses the built-in function put$ to output <expression> to the output stream (usually the screen). Output is ended with a new line. Normally, out$ returns its argument. out$ is a good debugging tool, but put$ is slightly safer, as it handles failing arguments in the correct way, contrary to out$.
{?} put$a & put$b
{?} out$a & out$b
⇑ Predefined functions.
⇒ using out$ as debugging aid.
substitution function
argument 1 : subject
argument 2 : pattern
argument 3 : replacement for subexpressions matched by pattern.
A Taylor expansion is applied to <expression>.
The second argument is the independent variable.
The third argument denotes the number of terms, including vanishing terms.
{?} tay$((cos$x)^-1,x,20)
If you need to manage a large data set it may be a good idea to use a hash- table instead of a list. Storing, retrieving and deleting are costly processes in lists, but cheap in hash tables. Handling hash tables in Bracmat is very simple. You create a hash table as follows
{?} new$hash:?myhash
Hereafter, 'myhash' refers to a hash table and is treated in the same way as
a user defined object. The following methods are defined for hash tables:
find
insert
remove
New
Die
DOS
ISO
casesensitive
forall
⇑ BRACMAT .
Returns a blank-separated list of key-value pairs, all with the same key.
⇑ Hash tables.
Inserts the key Key with the value Value. Multiple values for the same key are possible and the same value can be inserted more than once for the same key.
⇑ Hash tables.
Removes the key with all its values and returns a blank-separated list of key-value pairs.
⇑ Hash tables.
This is a method of the hash class that is called by the system when it evaluates 'new$hash'. It cannot be called from user code.
⇑ Hash tables.
This method is called when a hash object is deleted. It is not directly called from user code. (Compare with a C++ destructor). You can add your own clean up code by writing a 'die' method and adding it to a hash object once it is created. Example:
{?} new$hash:?myhash; { create a hash table myhash }
{?} ((
= ( Insert { Add a method 'Insert' to myhash that
only allows one value per key. }
= K,V
. !arg:(?K.?V)
& (Its..find)$!K:(?.?v)
& out
$ ( str
$ ( "Key "
!K
" already present"
( !V:!v&" with same"
| ", but with different"
)
" value "
!v
)
)
| (Its..insert)$!arg
)
(die=.out$"Oh my dear") { This method is called just before
the object is deleted. }
)
: (=?(myhash.)))
{?} (myhash..Insert)$(X.12); { Insert the value 12 for key X. }
{?} (myhash..Insert)$(X.12); { Try to do it one more time. }
{?} (myhash..Insert)$(X.10); { Try to insert another value
for the same key.}
{?} (myhash..insert)$(Z.1); { Use the built-in insert method.}
{?} (myhash..insert)$(Z.1); { Insert same value again. }
{?} (myhash..insert)$(Z.2); { Also insert a different value. }
{?} (myhash..find)$X; { Show all key-value pairs of X.
(only 1) }
{?} (myhash..find)$Z; { Show all key-value pairs of Z.(3)}
{?} (myhash..remove)$Z:?values;
{?} :?myhash; { Get rid of the hash table. }
⇑ Hash tables.
Make all key access case-insensitive and use DOS codepage 850.
⇑ Hash tables.
⇒ Codepage 850 support$.
Make all key access case-insensitive and use the ISO table ISO8859.
⇑ Hash tables.
Make all key access case sensitive. This the default.
⇑ Hash tables.
Apply the function to all key-value pairs. The function can be specified by its name or by its function body. The forall method finishes when all elements are traversed or before that if the function fails. The behaviour of forall is undefined if the hash table is changed or deleted during the traversal, although this can be done safely. For example may some members be missed and others be processed more than once. Example:
{?} new$hash:?myhash; { create a hash table myhash }
{?} (myhash..insert)$(X.12); { Insert the value 12 for key X. }
{?} (myhash..insert)$(Z.1); { Use the built-in insert method.}
{?} ( (myhash..forall) { Output all key-value pairs. }
$ (
= Key,Value,loop
. ( loop
= !arg:(?Key.?Value) ?arg
& out$(str$("Key=" !Key " Value=" !Value))
& !loop
)
& ~!loop
)
)
⇑ Hash tables.
Bracmat originated from a Basic program that was meant (and able) to do some algebraic calculations in General Relativity. This program could do the mathematical operations that Bracmat can: add, multiply, take powers and logarithms and differentiate. This calculator was not programmable, all program flow had to be done in Basic. It became clear that the program could only solve the simplest algebraic problems. The reason for this was its inability to recognise complex patterns in the subject expressions. All pattern recognition had to be done in Basic and this was a very fault prone business. It would be nice to have an interpreter at hand that could interpret human readable production rules. That is exactly what Bracmat embodies. This program is written in ANSI-C and developed on the fastest home computer that existed at that time (Acorn Archimedes). Although Bracmat is much faster than its predecessor, its main virtue lies in its programmability. The speed at which it processes formulae is not impressive, Bracmat needs a fast machine. Unlike many other algebra systems, its grammar has no relationship to the Algol grammar. Instead, the language component extends the syntax of simple algebraic formulae.
Compared with other algebra systems, Bracmat has few built-in functions and even its set of mathematical operators is small. There are no operators for subtraction and division, for example. Nevertheless, Bracmat is general and flexible enough to be able to solve even problems outside the field of computer algebra in an elegant way. This flexibility on the programming level is traded off against the inability to change the behaviour of the interpreter itself. There are no switches (toggles) that could influence, for example, the order of terms within a polynomial, or whether or not complex products are expanded, or the way backtracking is done. This choice was, of course, easier to implement, but it also has benefits for the user: the working of Bracmat programs is not obscured by deep side effects of switch settings. The only side effects that Bracmat allows are expression binding and change of focus in a multiple valued variable (array indexing, stacking). A later addition is support of objects, i.e. data structures that allow for partial updates. This introduced another kind of side effect.
One peculiar thing about the original, object-less Bracmat is the way in which it manages data:
Leaving one of these features out would have severe consequences for the other features. (2) explains why (1) is true. (3) ensures that (2) is workable: if two pieces of data are created equal they will remain so. This, in turn, explains why (4) is true. (5) almost follows from (3) : in any full fledged programming language, named fields allow all types of actions on the named parts of a data structure that are allowed on whole data structures, that is: creation and destruction. But the possibility to destroy only part of a data structure means that the data structure as a whole is changeable, which violates (3).
In the current version of Bracmat, with objects, the last restriction (5) no longer exists. As a consequence, restrictions (3) and (4) are not true for objects. (2) needs the additional remark that the reference counter for objects is made so big (counts to more than 4000000000), that overflow is practically impossible. Restriction (1) is still true, which means that the Bracmat programmer must take care of the deletion of some pathological structures (to be precise: circular structures, which were non-existent in the object-less Bracmat).
How can a programming language that is created for handling data structures do without named fields ? If I only can create and destroy data, how can I let data evolve gradually, piecemeal ? The answer lies in the well-developed pattern matching mechanism. Change of data is a two step process. In the first step you retrieve, by means of pattern matching, all those parts of the data that you want to keep. The second step is building the new data from the retrieved parts, together with new pieces. Creation of complex data structures from parts is straightforward in Bracmat. For example, the variable Row is a list of three words. We want to change the second word into the word "cat":
{?} the dog runs:?Row {initial creation}
{?} !Row:%?one % %?three {step 1: retrieval of 1st and 3rd word}
{?} !one cat !three:?Row {step 2: reconstruction} This may seem complicated and cumbersome, but look at this:
{?} 123:?a {create three variables a,b and c}
{?} a sentence:?b
{?} (a.silly,data*structure):?c
{?} (!a.!b.!c):(?b.?c.?a) {permutation of 3 values in just 1 "statement"}
Bracmat has no clear genealogy, but it has borrowed features from a number of
programming languages. It is not declarative, like Prolog, nor deeply object
oriented, like Smalltalk, but it is more or less procedural, like the majority
of languages. Below, I have tried to make the origin of some details more
explicit:
C:
Pascal:
locally defined functions and routines
Lisp:
Every similarity to other computer algebra systems is a matter of evolutionary convergence.
⇑ BRACMAT .
You can obtain a copy of Bracmat by sending an e-mail to me (Bart Jongejan) at bart[AT]cst.dk. Please state your hardware/OS.
⇑ BRACMAT .