[Haskell-cafe] Haskell Report Infelicity?

[Levent Erkok]

There's an existing ticket about this, filed 7 years ago:
https://ghc.haskell.org/trac/ghc/ticket/3070

On Mon, Oct 26, 2015 at 1:49 PM, Dominic Steinitz <dominic at steinitz.org>
wrote:

> I have probably misunderstood but is the following expected? floor, round,
> truncate and ceiling give similar results (all methods of the RealFrac
> class).
>
> properFraction :: (Integral b, RealFrac a) => a -> (b, a)
>
> *Girsanov> properFraction (-1 / 0)
> (-17976931348623159077293051907890247336179769789423065727343008115773267
> 5805500963132708477322407536021120113879871393357658789768814416622492847
> 4306394741243777678934248654852763022196012460941194530829520850057688381
> 5068234246288147391311054082723716335051068458629823994724593847971630483
> 5356329624224137216,0.0)
> *Girsanov> properFraction (1 / 0)
> (179769313486231590772930519078902473361797697894230657273430081157732675
> 8055009631327084773224075360211201138798713933576587897688144166224928474
> 3063947412437776789342486548527630221960124609411945308295208500576883815
> 0682342462881473913110540827237163350510684586298239947245938479716304835
> 356329624224137216,0.0)
> *Girsanov> properFraction (0 / 0)
> (-26965397022934738615939577861835371004269654684134598591014512173659901
> 3708251444699062715983611304031680170819807090036488184653221624933739271
> 1459592111865666518401372982279144533294018691411791796244281275086532572
> 2602351369432221086966581124085574502576602687944735992086890771957445725
> 3034494436336205824,0.0)
>
> The Haskell Report "6.4.6 Coercions and Component Extraction" is
> silent on what to expect with NaN, Infinity and -Infinity so I think it
> should
> be amended. Alternatively, one could expect implementations to throw a
> runtime error?
>
> In ghc, the problem is here https://phabricator.haskell.org/D160a
>
> /* This is expected to replace uses of __decodeDouble_2Int() in the long
> run */
> StgInt
> __decodeDouble_Int64 (StgInt64 *const mantissa, const StgDouble dbl)
> {
>     if (dbl) {
>         int exp = 0;
>         *mantissa = (StgInt64)scalbn(frexp(dbl, &exp), DBL_MANT_DIG);
>         return exp-DBL_MANT_DIG;
>     } else {
>         *mantissa = 0;
>         return 0;
>     }
> }
>
> I believe frexp [1] and scalbn [2] both pass through NaN, Infinity and
> -Infinity and I
> think type coercion in C is undefined for these values.
>
> Furthermore exp is undefined for these values [1].
>
> 1. http://en.cppreference.com/w/cpp/numeric/math/frexp
> 2. http://en.cppreference.com/w/cpp/numeric/math/scalbn
>
> Dominic Steinitz
> dominic at steinitz.org
> http://idontgetoutmuch.wordpress.com
>
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