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From: nick@lfcs.ed.ac.uk (Nick Rothwell)
Newsgroups: comp.lang.misc
Subject: Re: Polymorphism
Message-ID: <1834@etive.ed.ac.uk>
Date: 24 Apr 89 13:08:56 GMT
References: <5957@pdn.paradyne.com< <1810@etive.ed.ac.uk> <5973@pdn.paradyne.com>
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Reply-To: nick@lfcs.ed.ac.uk (Nick Rothwell)
Organization: LFCS Enya Admiration Society
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In article <5973@pdn.paradyne.com> alan@rnms1.paradyne.com (Alan Lovejoy) writes:
><ML is
><certainly polymorphic, although you can't "inspect" all values (they may
><be abstract, or function objects). 
>
>By "inspect" I simply mean "read."  If I can code "IF a = b," then I can
>"inspect" or "read" "a" and "b".

I stand by my words. You can't evaluate "a = b" in ML if a and b are functions,
since equality doesn't make sense. If they're abstract types, then any
default interpretation of "=" may be incorrect (e.g. sets represented as
lists).

><"modification" is only allowed on
><objects created specially for the purpose: how do you modify a function?
>
>By "modification" I simply mean "change."  If I can code "INC(i)", so that
>the value of "i" is incremented to the next "higher" member of the ordered set 
>of values of which "i" is a member,  then I have the ability to "modify" "i".

I stand by my words here as well. If I bind X to 3 (in a functional language),
I don't want X to be alterable.

>Also, I really want to require that modification be possible in principle,
>if the programmer wants to define any operations that modify values of some
>type.

But the point of a high-level language is that it provides a firm semantic
framework for the objects that you're dealing with and the operations that
they support...? I don't think that making "any modification possible in
principle" is desirable. Or feasible, for that matter.

>As for "modifying a function," how about recompiling it?  Or changing the
>input parameter "type" and/or "range" checking?

I distinguish "modification" and "re-binding". If I rebind a function "F",
that declares a new "F", which I can then go on and use. Existing code
doesn't see the change (unless it is also recompiled).

><"Storage in memory" - doesn't make sense for higher-order garbage collected
><languages. 
>Would you accept Smalltalk as a higher-order garbage-collected language?
>What did you think I meant?

ML has objects which accept assignment, but no notion of "memory" or "address"
in the classical sense. The behaviour of ML reference objects is defined
semantically.

>Alan Lovejoy; alan@pdn; 813-530-2211; AT&T Paradyne: 8550 Ulmerton, Largo, FL.

		Nick.

Nick Rothwell,	Laboratory for Foundations of Computer Science, Edinburgh.
		nick@lfcs.ed.ac.uk    <Atlantic Ocean>!mcvax!ukc!lfcs!nick
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