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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Not intended for use in medical,
Date: 1997/05/16
Message-ID: <dewar.863787244@merv>#1/1
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Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-05-16T00:00:00+00:00
List-Id: <comp.lang.ada>


Nick asks/says

<<Would you care to give one or two examples?>>

  (of functions with useful side effects)
  No, I don't think so, this horse has been beaten to death too many times
  to be worth discussing any more. Well I will give just one phrase,
  memo functions, that should be enough! Note that this issue was
  discussed in the very early stages of the language design, and everyone
  agrees that there is a necessity in a language like Ada to permit
  functions with side effects. The issue that was discussed was whether
  to distinguish between functions without side effects, and value
  returning procedures with side effects. But even that attempt fails,
  because you can have functions that at a conceptual interface level
  are pure, but their implementations require side effects (memo functions
  are the canonical example). Note that it will work just fine to give
  the pragma Pure_Function in GNAT for a memo function, even though it
  has side effects.

<<Ada.Numerics.Float_Random and Ada.Numerics.Discrete_Random are
grandchildren of the package Ada, which is declared as pure, they must be
pure also, and pure packages are not allowed to have any state (such as
internal generators). Silly me. (Is this it, or is there some other reason
that I have missed?)>>

  The Pure is completely irrelevant here, the problem has no solution in
  pure Ada without using the heap even if you did not have pragma Pure.
  Furthermore the idea that children of a pure package must be pure is
  a pretty serious misconception about the whole structure of child
  packages. It is of course the other way round, you cannot have pure
  children of an impure parent, but OF COURSE you can have impure
  children of pure parents -- what would make you think this was not
  allowed.

<<Quite right. I was forgetting that as the packages
Ada.Numerics.Float_Random and Ada.Numerics.Discrete_Random are
grandchildren of the package Ada, which is declared as pure, they must be
pure also, and pure packages are not allowed to have any state (such as
internal generators). Silly me. (Is this it, or is there some other reason
that I have missed?)>>

  As above, you are confused.

<<However, I did spot that the RM95 has "pragma Pure(Ada.Numerics);" in the
Ada.Numerics package, which seems odd to me, since Ada.Numerics must be
pure anyway (because it's a child of Ada). Why the extra pragma Pure?>>

  If you see something that seems odd to you in the RM, it is quite likely
  that, as in this case, it corresponds to a misunderstanding on your part.
  As per above, there is no extra Pure, and the Pure here is important.