Thanks for a fine report, Tim!
> I set USE_DOUBLE_PRECISION in options.h.in (manually, since I can't see
> an appropriate configure switch).
We used to have a switch - but decided to hide it because
GreenCard couldn't cope with the change very well.
> I don't expect
> $ make check
> to pass now, but expect diffs to be confined to less significant digits
> in results. Instead I see many other errors.
>
> Small example:
> Prelude> 1.0 :: Ratio Int
> elicits
> Program error: PreludeRatio.% zero denominator
> instead of
> 1 % 1
Yoiks! I was mightily surprised by this report but, after a bit of
chasing around, I decided that it's more or less the right answer.
The short explanation is that with Ints being 32 bits wide and
Float/Double being 64 bits wide, overflow and underflow are
almost bound to happen when we convert between them.
In this case, what's happening is something like this:
1.0
= "desugaring"
fromDouble 1.0
=
let (m,n) = decodeFloat 1.0 in fromInteger m % (fromInteger 2 ^ (-n))
=
let (m,n) = (4503599627370496,-52) in fromInteger m % (fromInteger 2 ^ (-n))
=
fromInteger 4503599627370496 % (2 ^ 52)
=
<4503599627370496 modulo 2^32> % <2^52 modulo 2^32>
=
<4503599627370496 modulo 2^32> % 0
=
<division by zero>
Possible fixes:
1) Hugs really shouldn't be desugaring 1.0 as "fromDouble 1.0".
It should be desugaring it as "fromRational (1 % 1)".
With this fix, you'd have to work a lot harder before you got an
example which failed as obviously as the above - but you'd still
have problems.
2) We could rewrite doubleToRatio to avoid depending on the size of Int.
The most obvious version is this:
doubleToRatio :: Integral a => Double -> Ratio a
doubleToRatio = fromRational . toRational
This is probably a bit slower - but at least it gives the
right answer.
3) Discourage the use of "Ratio Int"
[This seems like an argument in favour of Jon Fairbairn's proposal that
Standard Haskell should use Integer whenever possible. Hmmm.]
> What's the recommended way to search the archive when accessed via
> web?
I don't think we ever thought of that.
We turned the hugs-bugs mail into a web page because it seemed easy...
but, as always, it's harder than we thought.
Alastair
