Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!mcvax!hp4nl!htsa!fransvo
From: fransvo@htsa.uucp (Frans van Otten)
Newsgroups: comp.lang.modula2
Subject: Re: Are procedure types breaking modules?
Keywords: procedure types, modules
Message-ID: <849@htsa.uucp>
Date: 24 Apr 89 10:39:38 GMT
References: <688@tuvie>
Reply-To: fransvo@htsa.UUCP (Frans van Otten)
Organization: AHA-TMF (Technical Institute), Amsterdam, The Netherlands
Lines: 44

Ulrich Neumerkel writes:

>When considering 6.8 of the Modula-2 report only procedures local to
>other procedures and standard procedures are forbidden to be assigned
>to a variable of a procedure type. This means that it is possible
>to call a procedure which is not exported from another module, when
>you pass the procedure dynamically. I do not believe that this
>feature is really undesirable, however was it really intended?

It probably was not intended.  I think Wirth thinks of a lot of
nice features for a programming language, but doesn't consider
all consequences before publishing.  The fact that he manages
to write a lot of contradictionary things (even within one book)
supports this.  Also, he is not clear on why some things are or
are not allowed.  Why is it that you are not allowed to assign a
local procedure to a procedure variable ?  Why can't a standard
procedure be assigned to a procedure variable ?

>The problem that might arise here is that there might be
>introduced invisible dependencies. The alternative would be a complicated 
>dataflow analysis or even a runtime check. 
>
>o	Is this a desirable feature, have you used it in "real" programs?
>o	Are there any alternatives to it?

You could call a procedure which is exported from another module.
When calling it you give a parameter indicating which (unexported)
function should be called.  Then, using a case statement or an
array of which the component is of the procedure type, you have
(almost) the same result.

>Just to cite Wirth (3rd ed. p84) [...checking no absoulte safeguard...]
>>After all, it [module concept] concerns formal, syntactic aspects only; it 
>>does not cover the semantics. <Well, we are here on the razors egde...>

Don't use single quotes of Wirth !  That is highly dangerous !
Read all of what he writes, then try to figure out what he might
have meant using your imagination and logics !

-- 
	Frans van Otten                         Don't blame me,  I'm
	Algemene Hogeschool Amsterdam           just another Modula 2
	Technische en Maritieme Faculteit       compiler writer !
	fransvo@htsa.uucp