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From: Paul Duquennoy <paul.duquennoy@wanadoo.fr>
Subject: Re: Question for the folks who designed Ad
Date: 1999/04/29
Message-ID: <3727FEB6.6ECEB1F5@wanadoo.fr>#1/1
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References: <7g72rf$hjh3@ftp.kvaerner.com> <7g8roe$igd1@svlss.lmms.lmco.com>
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Reply-To: paul.duquennoy@wanadoo.fr
NNTP-Posting-Date: 29 Apr 1999 06:37:54 GMT
Newsgroups: comp.lang.ada
Date: 1999-04-29T06:37:54+00:00
List-Id: <comp.lang.ada>

David Kristola wrote:
> 
> In article hjh3@ftp.kvaerner.com, "Tarjei Tj�stheim Jensen" <tarjei.jensen@kvaerner.no> () writes:
> >
> >Robert Dewar wrote :
> >>
> >>In other words:
> >>
> >>  a(5)
> >>
> >>is a reference to a mapping, and you want the range value
> >>given the domain value 5. Whether this is implemented by
> >>an array or a function is an implementation detail that
> >>should not affect the syntactic form of reference.
> >
> >
> >What would a(5) := 4; mean then? You cannot assign to a function as far as I
> >know. Which in my opinion voids the function opposition to the common
> >convention of using square brackets for array indices.
> >
> >I don't mind if the function addicts still writes a(5) := 4, if I can write
> >a[5] := 4.
> >
> >Greetings,
> >
> >
> >
> 
> a(5).some_value := 4;
> 
> Maybe the function a returns a pointer to a record structure...
> 

Absolutely, and this is useful. An early version of a package can use an array 
to implement a feature, but later it can be changed to a more sophisticated
approach 
using a cache (e.g. if there are too many unused entries in the array) without
disturbing the existing code.

It is then clear that the 'a(5)' is a way to access some data, regardless of
its 
implementation. Ada allows very minimal impact on the code in such circumstances.

The 'auto-dereferencing' of pointers in Ada (vs '->' in C) also gives more flexibility
to change the implementation from actual data to accesses and vice-versa.