! Copyright (c) 2012 Anonymous
! See http://factorcode.org/license.txt for BSD license.
USING: combinators 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

! factorial sequence
: almost-fac ( quot -- quot )
    '[ dup zero? [ drop 1 ] [ dup 1 - @ * ] if ] ;

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

! Ackermann–Péter function
:: almost-ack ( quot -- quot )
    [
        {
          { [ over zero? ] [ nip 1 + ] }
          { [ dup zero? ] [ [ 1 - ] [ drop 1 ] bi* quot call ] }
          [ [ drop 1 - ] [ 1 - quot call ] 2bi quot call ]
        } cond
    ] ;