[Haskell-cafe] Haskell Report Infelicity?

26 Oct 2015

      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@steinitz.org
http://idontgetoutmuch.wordpress.com