[Haskell-cafe] Bug with [Double]

[Roman Leshchinskiy]

On 19/05/2010, at 23:44, Ben Millwood wrote:

> On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov <sergueyz at gmail.com> wrote:
>> 
>> PS
>> Rationals:
>> Prelude> [1,1+2/3..10] :: [Rational]
>> [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
>> 3,25 % 3,9 % 1,29 % 3,31 % 3]
>> 
>> Same result.
> 
> This sounds like a bug to me. The section of the Haskell Report that
> deals with the Enum class mentions Float and Double, not Rational, and
> there's really no sensible reason why Rationals would exhibit this
> behaviour given that they don't have rounding error.

From Section 12.1 of the Library Report:

instance  (Integral a)  => Enum (Ratio a)  where
    succ x           =  x+1
    pred x           =  x-1
    toEnum           =  fromIntegral
    fromEnum         =  fromInteger . truncate	-- May overflow
    enumFrom         =  numericEnumFrom	 -- These numericEnumXXX functions
    enumFromThen     =  numericEnumFromThen	 -- are as defined in Prelude.hs
    enumFromTo       =  numericEnumFromTo	-- but not exported from it!
    enumFromThenTo   =  numericEnumFromThenTo

The numericEnum functions are defined in Section 8 of the Language Report and have semantics required for Float and Double.

Roman