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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: function composition in C
Message-ID: <10666@dog.ee.lbl.gov>
Date: 6 Mar 91 14:15:15 GMT
References: <6873@munnari.oz.au> <ACHA.91Feb28002548@DRAVIDO.CS.CMU.EDU> <1991Feb28.072757.1904@bingvaxu.cc.binghamton.edu> <15357@smoke.brl.mil> <91060.132635REIDMP@MAINE.BITNET> <347@tslwat.UUCP>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 32
X-Local-Date: Wed, 6 Mar 91 06:15:15 PST

In article <347@tslwat.UUCP> louk@tslwat.UUCP (Lou Kates) writes:
>Perhaps we need to consider what you need to add to C to get
>this with the minimal change to C.

Probably such consideration belongs in alt.lang.cfutures, but since
alt.* groups have weaker propagations...:

>You should be able to do function composition in OO extensions to C.
In particular, consider the class C, each of whose objects has the
>publicly settable function pointers fp and gp and the method c(x)
>which returns f(g(x)) where f and g are the functions pointed at
>by fp and gp.

This only works for a single return type.  Since the type system in
any C-like language is infinitely extensible (each new `struct' is a new
type, for instance), either the composition function must be polymorphic
(choose your favourite other word here for `accepts arbitrary types')
or else you must resort to trickery, e.g., `the machine internally has
only this many types'.

The problem continues (only more so) for binding of arguments, e.g.,
there is no good way to define a function:

	; compose takes a function f of $k$ arguments, a function g of
	; one argument, and arguments $x_1 .. x_k$, and produces a new
	; function of one argument which is equivalent to
	;	(f (g arg) x1 x2 ... xk).

You might as well give in and use Scheme. :-)
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab EE div (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov