Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!mintaka!bloom-beacon!eru!hagbard!sunic!mcsun!ukc!warwick!nott-cs!piaggio!anw
From: anw@maths.nott.ac.uk (Dr A. N. Walker)
Newsgroups: comp.lang.misc
Subject: Re: type checking problem
Message-ID: <1991May21.144739.23901@maths.nott.ac.uk>
Date: 21 May 91 14:47:39 GMT
References: <1991May16.182932.26327@kodak.kodak.com>
Reply-To: anw@maths.nott.ac.uk (Dr A. N. Walker)
Organization: Maths Dept., Nott'm Univ., UK.
Lines: 48

In article <1991May16.182932.26327@kodak.kodak.com> cok@islsun.Kodak.COM
(David Cok) writes:

>					  Given, say, myf of four arguments,
>one would write
>		optimum_x = optimize(myf(#1,a,b,c))
>in which myf(#1,a,b,c) is a function of one argument obtained by supplying
>values for three of the four arguments of f.

	Cf Algol Bulletin #37, July 1974, p 24 [the third time I've given
this reference in a few weeks!] for the Algol partial parametrisation
proposals:

		optimum_x := optimise (myf ( , a, b, c))

On the other hand, good luck in finding a commercially available Algol
compiler with p-p implemented.

> [2nd style of solution]				       The problem
>also requires flexibility in number of arguments which is not common in
>dynamically typed languages.

	Yes, it's a nice example.  I don't know of many real situations
in which the programmer may reasonably claim ignorance of both the number
and type of the parameters to a procedure.  I suspect that a good solution
would require a much more visible type system than is common in compiled
languages [statically or dynamically typed], which is why it's easier in
interpreted languages.  It can *nearly* be done in Algol with modals
(same issue of Algol Bulletin, and same caution as above):

	PROC optimise = (MODE Z, PROC (REAL, REF Z) REAL f, REF Z args) REAL:
	  (REAL result, x;
	   WHILE result := f (x := pick_an_x, args);
		 NOT converged (x, result)
	      DO SKIP OD; x);

which allows things like

	MODE THISZ = STRUCT (REAL p, COMPL q, CHAR f);
	optimum_x := optimise (THISZ, myf, THISZ := (a, b, c))

[I think -- no way of trying it here] at the expense of some restrictions
on the nature of "myf".  [Note that "Z" needn't be instantiated as a
STRUCTure, it could be eg an array of UNIONs.]

-- 
Andy Walker, Maths Dept., Nott'm Univ., UK.
anw@maths.nott.ac.uk