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From: Harald Korneliussen <vintermann@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: not null
Date: Fri, 6 Mar 2009 05:05:40 -0800 (PST)
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Date: 2009-03-06T05:05:40-08:00
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On Mar 6, 1:43=A0pm, Jacob Sparre Andersen <spa...@nbi.dk> wrote:
> Harald Korneliussen wrote:
> > You can use algebraic data types to do that in a type safe manner.
> > Haskell, Ocaml and many smaller functional languages do: If a
> > function returns a "Maybe Integer" type, the compiler will warn you
> > if you pretend it's an Integer without dealing with the "Nothing"
> > case.
>
> Can this be implemented as a static check? =A0(Or is there a good reason
> that it is only a warning?)
>
> It would be nice(TM) if Ada compilers could do something like that for
> with null access types.
>

It can be enforced, of course. It could be implemented in Ada, but to
do it without serious program analysis, I think there would need to be
a special function that separates the cases (Null and not null) and
sends the program down one path or the other - naturally, with only
the not-null path having access to a variable of the base type. The
compiler could then insure that (almost) the only operation that could
be performed on the possibly-null type was this function.

In Haskell and Ocaml this is done with pattern matching on types.