[ ghc-Bugs-652927 ] Integer -> Int64 sometimes wrong

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Bugs item #652927, was opened at 2002-12-12 21:24
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Category: hslibs/lang
Group: 5.04.2
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Mike Gunter (magunter)
>Assigned to: Simon Marlow (simonmar)
Summary: Integer -> Int64 sometimes wrong

Initial Comment:
Conversion from Integer to Int64 values sometimes
produces the wrong
value.  In particular, it seems to fail on
small-magnitude negative
values whose representation is large.  E.g.

  > (fromIntegral ((2^30 -1 + 2^30) - (2^30 + 2^30 ::
Integer))) :: Data.Int.Int64
  4294967295
  > (fromIntegral ((2^30 - 2 + 2^30) - (2^30 - 1 + 2^30
:: Integer))) :: Data.Int.Int64
  -1

The Haskell-level workaround is to do the conversion as
a non-negative
number then negate if needed:

  > let fis (i::Integer) = (fromIntegral (signum i) *
fromIntegral (abs i)) :: Data.Int.Int64
  > fis ((2^30 -1 + 2^30) - (2^30 + 2^30 :: Integer))
  -1


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>Comment By: Simon Marlow (simonmar)
Date: 2002-12-13 14:26

Message:
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user_id=48280

Fixed; thanks.


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