[Haskell-cafe] Desired behaviour of rounding etc.
daniel.is.fischer at web.de
Fri Oct 8 08:08:01 EDT 2010
The methods of the RealFrac class produce garbage when the value lies
outside the range of the target type, e.g.
Prelude GHC.Float> truncate 1.234e11 :: Int -- 32-bits
and, in the case of truncate, different garbage when the rewrite rule
Prelude GHC.Float> double2Int 1.234e11
I'm currently working on faster implementations of properFraction,
truncate, round, ceiling and floor for Float and Double, so I'd like to
- does it matter at all what garbage is returned in the above case?
- if it does, what is the desired behaviour (at least for Int, I can't
cater for all possibilities)?
On a related note, in my benchmarks,
truncFloatGen :: Integral a => Float -> a
truncFloatGen = fromInteger . truncFloatInteger
truncFloatInteger :: Float -> Integer
truncFloatInteger x =
case decodeFloat x of
(m,e) | e == 0 -> m
| e < 0 ->
let s = -e
in if m < 0
then - ((-m) `shiftR` s)
else m `shiftR` s
| otherwise -> m `shiftL` e
is more than twice as fast as GHC.Float.float2Int, the corresponding for
Double almost twice as fast as double2Int.
Can anybody confirm that the above is faster than float2Int on other
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