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From: markh@csd4.csd.uwm.edu (Mark William Hopkins)
Newsgroups: comp.lang.c,comp.lang.functional,comp.theory
Subject: Re: function composition in C
Message-ID: <9754@uwm.edu>
Date: 26 Feb 91 02:22:02 GMT
References: <6873@munnari.oz.au>
Sender: news@uwm.edu
Followup-To: comp.lang.c
Organization: University of Wisconsin - Milwaukee
Lines: 53

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.
...

You could do it compile-time like this example for composing x^2 and 2*x:
------------------------------------------------------------

#include <stdio.h>

int Compose(F, G, X)
int (*F)(), (*G)(); int X;
{ return (*F)((*G)(X)); }

static int Square(X)
int X;
{ return X*X; }

static int Double(X)
int X;
{ return 2*X; }

static int Square_Double(X)
int X;
{ return Compose(Square, Double, X); }

static int Double_Square(X)
int X;
{ return Compose(Double, Square, X); }

main() {
   printf("%d, %d", Square_Double(5), Double_Square(5));
}

------------------------------------------------------------

The declarations above for Square_Double, and Double_Square would be
compile-time lambda abstractions (with X being the variable abstracted
out), but C does not support run-time lambda abstraction.

You'd have to be able to write something like:

int (*Compose(int (*F)(), int (*G)()))() {
   int (*H)(int X) { return (*F)((*G)(X)); }
   return H;
}

with embedded run-time function declarations...


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