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From: acha@CS.CMU.EDU (Anurag Acharya)
Newsgroups: comp.lang.c,comp.lang.functional
Subject: Re: function composition in C
Message-ID: <ACHA.91Feb28002548@DRAVIDO.CS.CMU.EDU>
Date: 28 Feb 91 05:25:48 GMT
References: <6873@munnari.oz.au>
Sender: acha@dravido.soar.cs.cmu.edu
Organization: Carnegie Mellon University
Lines: 44
In-reply-to: aet@felix.ee.mu.OZ.AU's message of 25 Feb 91 05:21:30 GMT

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)));
 }

int sqr(x)
int x;
{
 return x * x;
}

int cube(x)
int x;
{
 return x * x * x;
}

main()
{
  printf("The value is %d\n",compose(sqr,cube,2) );
}

try it out. it works. this version of compose will work for any pair of 
int->int functions. liberal use of casts will allow you to write a general
compose as well.

anurag