Re: [Haskell-cafe] Bug with [Double]

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

On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov 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