factor/extra/rosetta-code/y-combinator/y-combinator.factor

! Copyright (c) 2012 Anonymous
! See http://factorcode.org/license.txt for BSD license.
USING: fry kernel math ;
IN: rosetta-code.y-combinator

! http://rosettacode.org/wiki/Y_combinator

! In strict functional programming and the lambda calculus,
! functions (lambda expressions) don't have state and are only
! allowed to refer to arguments of enclosing functions. This rules
! out the usual definition of a recursive function wherein a
! function is associated with the state of a variable and this
! variable's state is used in the body of the function.

! The Y combinator is itself a stateless function that, when
! applied to another stateless function, returns a recursive
! version of the function. The Y combinator is the simplest of the
! class of such functions, called fixed-point combinators.

! The task is to define the stateless Y combinator and use it to
! compute factorials and Fibonacci numbers from other stateless
! functions or lambda expressions.

: Y ( quot -- quot )
    '[ [ dup call call ] curry @ ] dup call ; inline

: almost-fac ( quot -- quot )
    '[ dup zero? [ drop 1 ] [ dup 1 - _ call * ] if ] ;

: almost-fib ( quot -- quot )
    '[ dup 2 >= [ 1 2 [ - _ call ] bi-curry@ bi + ] when ] ;