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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: A function that cannot be called?
Date: Tue, 23 Oct 2018 19:49:22 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:54698
Date: 2018-10-23T19:49:22+02:00
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On 23.10.18 16:57, Dmitry A. Kazakov wrote:
> On 2018-10-23 16:35, Jere wrote:
> 
>> The difficulty with compile time comes from your requirements I think.
>> In order for the function to exist, the compiler needs to have it
>> actually be callable.
> 
> In which sense a non-callable function may exist? What exactly is the meaning of "callable"?

The function is supposed to exist just like the empty function
in {} -> {} in mathematics, I think.

"Callable" is probably meaningful in contexts involving types,
such as argument type pattern matching in functionist
languages. Can't say if lazyness is needed in addition.
Maybe "used" is a better word than "called".

functions in Ada that take no argument but that still return
something seem weird in a different way. If one restricts one's
views to the "D -> E" pattern as the only respectalbe way of
describing a function, then what is the proper signature of

   "function of no argument, but returning T" ?

(Which is nice in that it allows naming expressions:

      function Pi return Float is (355.0 / 113.0); )

There might be another good bit in Ada in this regard, namely
pure functions, to be got from a pragma, thus

package Nope is

     pragma Pure (Nope);

     function Source return Integer;

end Nope;

I can't say whether this particular one approximates Haskell's

    absurd :: Void -> a

but providing an Ada body for function Source from the pure
package above and one that is actually impure is a little tricky.