Arc: Evaluation

Evaluation

Arc provides several syntax innovations beyond Scheme.

Square-bracket anonymous functions

Anonymous functions on one variable can be created with square brackets; the underscore _ is the variable. For example, [+ 1 _] is equivalent to (fn (_) (+ 1 _)):

arc> (map [+ 1 _] '(10 20 30))
(11 21 31)

Special syntax

The "special syntax" characters are :, ~, ., and !. The ~ character provides Boolean complement of a procedure; ~a is equivalent to (complement a)

arc> (~odd 3)
nil
arc> (~even 3)
t

The : character implements function composition; a:b is equivalent to (compose a b) and applies a to the result of applying b:

arc> (odd:sqrt 4)
nil
arc> (odd:sqrt 9)
t

The syntax a.b is equivalent to (a b). It is intended for structure access:

arc> (= x '(a b c d))
(a b c d)
arc> x.2
c

The syntax a!b is equivalent to (a 'b). It is intended for structure access where quoting is needed:

arc> (= tb (obj a 10 b 20 c 30))
#hash((c . 30) (a . 10) (b . 20))
arc> tb!a
10

eval expression Evaluates the expression.	>(eval '(+ 1 2)) 3
apply fn arglist Applies the function to the arguments.	>(apply + '(1 2)) 3
ssexpand symbol Expands special syntax (: ! . or ~). The : character indicates composition. The ~ indicates complementing. The . applies the first value to the remainder. The ! is like . except the arguments are quoted.	>(ssexpand 'x:~y:z) (compose x (complement y) z) >(ssexpand '+.1.2) (+ 1 2) >(ssexpand '+!1!2) (+ (quote 1) (quote 2)) >(ssexpand 'cons!a!b) (cons (quote a) (quote b))
ssyntax symbol Tests if the symbol contains special syntax (: ! . or ~).	>(ssyntax 'x:y) t
compose [function] ... Composes the functions.	>((compose sqrt:len) "abcd") 2
complement function Creates a procedure that is the complement of function.	>(map (complement odd) '(1 2 3 4 5)) (nil t nil t nil)
idfn x Identity function; returns x.

Copyright 2008 Ken Shirriff.