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From: aglew@ccvaxa.UUCP
Newsgroups: net.lang
Subject: Proc types
Message-ID: <800016@ccvaxa>
Date: Sun, 13-Jul-86 11:48:00 EDT
Article-I.D.: ccvaxa.800016
Posted: Sun Jul 13 11:48:00 1986
Date-Received: Mon, 14-Jul-86 22:28:18 EDT
Lines: 44
Nf-ID: #N:ccvaxa:800016:000:1678
Nf-From: ccvaxa.UUCP!aglew    Jul 13 10:48:00 1986


The discussion about how to implement pointers to functions with enumerated
classes in Ada got me to thinking...

I really like pointers to functions. Don't take them away. Of course, I want
full specification of argument types, even varargs...

However, sometimes a pointer to a whole class of functions is too loose.
It would be nice to be able to constrain it further. Eg. instead of all
int(int)s, only to particular int(int) functions (that you know maintain
the necessary invariant?...)

You can do this with a type declaration for those particular int(int)s you
will permit, but (1) types proliferate, and it can be damned hard to invent
a meaningful name for a type that you're using on an ad-hoc basis, 
and (2) this may constrain sharing functions between modules that might
require a different type-name. Also (3) it violates a rule that I am fond of
in language design: if you can bind something to a symbol, then it should be
possible to replace uses of that symbol by some literal.

So, what I'm looking for in this situation is something like
(veuillez excusez my mixture of langages)

    VAR p: POINTER TO int(int) FROM { func1, func2, func3 ... }

or 

    TYPE p = POINTER TO int(int) SATISFYING constraint;

where you might declare that you have satisfied the constraint at the
procedure body:

    PROCEDURE func1(int a) RETURNS int SATISFIES constraint;


In many ways this is like the problem of specifying non-contiguous 
subsets of the integers as types.


Has anyone seen languages that do something like this?


Andy "Krazy" Glew. Gould CSD-Urbana.    USEnet:  ihnp4!uiucdcs!ccvaxa!aglew
1101 E. University, Urbana, IL 61801    ARPAnet: aglew@gswd-vms