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From: jcm@mstr.hgc.edu (James McKim)
Newsgroups: comp.lang.eiffel
Subject: Re: Functions without side effects (was Old confusion)
Message-ID: <1991Jun10.204452.2717@mstr.hgc.edu>
Date: 10 Jun 91 20:44:52 GMT
References: <1991May30.141218.3446@mstr.hgc.edu>
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In article <1991Jun10.151959.29568@mstr.hgc.edu> jcm@mstr.hgc.edu (James McKim) writes: (This is me, unfortunately!)
>In article <131732@tut.cis.ohio-state.edu> ogden@seal.cis.ohio-state.edu (William F Ogden) writes:
>>OK. Let's look at the side effecting function Pop_of vs. the value
>>returning Pop_Top procedure on stacks of say integers. If we have
>>a stack whose top two entries are say 5 and 3, then, because there's
>>an indeterminate order of evaluation problem even in fairly simple 
>>situations, executing
>>
>>  Dif := Pop_of(S) - Pop_of(S);
>>
>>might cause Dif to be either 2 or -2. In contrast, with the Pop_Top
>>procedure, there can be no uncertainty because a programmer must
>>explicitly sequence the operations.
>>  Pop_Top(S,t1);
>>  Pop_Top(S,t2);
>>  Dif := t1 - t2;   {or t2 - t1}
>>
>
>OK, so when you say 'procedure with a return value' you mean the
>value is returned through arguments of the procedure. Conceptually,
>this is still a function with side effects (the return value is just
>in a different place). The distinction depends on the language
>definition or implementation.
>
Well, it's now Monday afternoon (I should be banned from posting
on Monday morning!) and I now understand (I think) the conceptual
difference. Pop_Top as defined above does _not_ return a value
for immediate use, rather it changes the state of _two_ objects,
the client and the supplier. On the other hand, Pop_of both
changes the state of the stack object and returns  a value for
immediate use, which Bill clearly demonstrates can cause confusion.
Now, did I make the same error in another post this morning...?
>>
>>--
>>
>>/Bill
>
>-- Jim
>
-- Me again




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Internet:  jcm@mstr.hgc.edu _Teach_ people to fish and they eat for a lifetime.