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