6.9.6 Higher-Order Functions

As a functional programming language, Scheme allows the definition of higher-order functions, i.e., functions that take functions as arguments and/or return functions. Utilities to derive procedures from other procedures are provided and described below.

Scheme Procedure: const value

Return a procedure that accepts any number of arguments and returns value.

(procedure? (const 3))        ⇒ #t
((const 'hello))              ⇒ hello
((const 'hello) 'world)       ⇒ hello

Scheme Procedure: negate proc

Return a procedure with the same arity as proc that returns the not of proc’s result.

(procedure? (negate number?)) ⇒ #t
((negate odd?) 2)             ⇒ #t
((negate real?) 'dream)       ⇒ #t
((negate string-prefix?) "GNU" "GNU Guile")
                              ⇒ #f
(filter (negate number?) '(a 2 "b"))
                              ⇒ (a "b")

Scheme Procedure: compose proc rest ...

Compose proc with the procedures in rest, such that the last one in rest is applied first and proc last, and return the resulting procedure. The given procedures must have compatible arity.

(procedure? (compose 1+ 1-)) ⇒ #t
((compose sqrt 1+ 1+) 2)     ⇒ 2.0
((compose 1+ sqrt) 3)        ⇒ 2.73205080756888
(eq? (compose 1+) 1+)        ⇒ #t

((compose zip unzip2) '((1 2) (a b)))
                             ⇒ ((1 2) (a b))

Scheme Procedure: identity x

Return X.

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