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From: volpe@kirkwood.crd.ge.com (Christopher R Volpe)
Newsgroups: comp.lang.c,comp.lang.functional
Subject: Re: function composition in C
Message-ID: <17191@crdgw1.crd.ge.com>
Date: 28 Feb 91 22:34:32 GMT
References: <6873@munnari.oz.au> <ACHA.91Feb28002548@DRAVIDO.CS.CMU.EDU>
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Reply-To: volpe@kirkwood.crd.ge.com (Christopher R Volpe)
Followup-To: comp.lang.c
Lines: 58

In article <ACHA.91Feb28002548@DRAVIDO.CS.CMU.EDU>, acha@CS.CMU.EDU
(Anurag Acharya) writes:
|>In article <6873@munnari.oz.au> aet@felix.ee.mu.OZ.AU (bert) writes:
|>> Does anyone know how to write a compose function in C,
|>> without writing a Scheme interpreter to do it in.
|>> A call should look something like this:
|>> (compose(sqr,cube)) (2) which would evaluate to 64.
|>> "compose" is a one-liner in any functional language,
|>> but I suspect that it is, given the above constraint, impossible in C.
|>
|>Nope. it is possible.
|>
|>Example:
|>
|>#include <stdio.h>
|>
|>int compose (foo,bar,baz)
|>int (*foo) ();
|>int (*bar) ();
|>int baz;
|>{
|>   return ((*foo) ((*bar)(baz)));
|> }

This is not a compose function, because it does not take two functions
are return a FUNCTION. Compose should behave as the original poster
specified: (compose(sqr,cube))(2) == 64.

Try something along the lines of the following:
(Assume a stack of function pointers with push and pop functions)

typedef int (*pfi)(int) /* pointer to funtion taking int and returning int */

void push(pfi);
pfi  pop(void);

int compose_aux(int arg)
{
  pfi f1,f2;
  f2 = pop();
  f1 = pop();
  return f1(f2(arg));
}
 
pfi compose(pfi f1, pfi f2)
{
  push(f1);
  push(f2);
  return compose_aux;
}

This is just the basic concept, I haven't tested it. 

                                            
==================
Chris Volpe
G.E. Corporate R&D
volpecr@crd.ge.com