[Hugs-bugs] RE: Enum Int problem

[Simon Peyton-Jones]

| The arithmetic overflow occurs as part of an intermediate result in
the
| implementation of the arithmetic sequence, and not in the elements of
the
| sequence itself. It seems to me that it is worthwhile to fix this, and
| clarify the definition in the Haskell report so that an Int arithmetic
| sequence [start, next .. end] corresponds to the subsequence of the
Integer
| sequence that are actually Ints in the natural inclusion of Int in
Integer.

I do rather agree with this, and I'm fixing GHC to agree with it.  (A
nice displacement activity.)

Malcolm: something for the report errata?

Simon

| In terms of the implementation, it is an inexpensive check and does
not
| involve doing the intermediate computation in the Integer type. For
example,
| for ascending sequences (next >= start) where step = next - start >=
0, one
| just checks if element + step  < element to check for overflow and
thus stop
| the sequence at element.
| 
| Bo
| 
| 
| -----Original Message-----
| From: Alastair Reid [mailto:alastair at reid-consulting-uk.ltd.uk]
| Sent: Wednesday, August 18, 2004 3:59 AM
| To: Bo Ilic
| Cc: hugs-bugs at haskell.org; glasgow-haskell-bugs at haskell.org
| Subject: Re: Enum Int problem
| 
| 
| [copied to hugs-bugs]
| 
| You shouldn't hope to get any useful results from Int after an
arithmetic
| overflow since the Haskell report explicitly says that the results are
| unspecified.  If you want predictable results, you should use Integer
or
| Foreign.Int32.  The latter is part of the foreign interface extension
which
| is implemented by nhc, ghc and Hugs.
| 
| I believe that Hugs is giving a correct answer in this case in that
nothing
| strange happens until after the overflow occurs.  Of course the answer
is
| not
| _useful_ but, as I said, you shouldn't expect useful results after an
| overflow occurs.
| 
| I think that GHC may be giving an incorrect answer because I think the
0
| should appear in the list but it would take a very careful examination
of
| how
| the Haskell report specifies the Enum Int instance to be sure.
| 
| --
| Alastair Reid
| 
| ps When I say 'after an overflow', what I mean is that the value of X
is
| undefined if the value of X depends on the value of Y and Y
overflowed.
| 
| On Friday 13 August 2004 19:39, you wrote:
| > Using GHC 6.2.1 interpreter, the fragment [minBound :: Int, 0 ..]
returns
| > [-2147483648].
| >
| > Using Hugs, the returned value is the infinite list  [-2147483648,
0,
| > -2147483648, 0, ...].
| >
| > However, I would expect to get [-2147483648, 0]. Similar problems
occur
| > with the other arithmetic sequence functions for Int when the
increment is
| > greater than what is representable in the type.
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