Malcolm Wallace wrote:
Phil proposes that, although retaining the instances of Enum for Float and Double, we simplify the definitions of the numericEnumFrom family:
numericEnumFromThenTo :: (Fractional a, Ord a) => a -> a -> a -> [a] numericEnumFrom = iterate (+1) numericEnumFromThen n m = iterate (+(m-n)) n numericEnumFromTo n m = takeWhile (<= m) (numericEnumFrom n) numericEnumFromThenTo n m p = takeWhile (<= p) (numericEnumFromThen n m)
I'll leave it to the floating-point experts to decide exactly what to do here for Haskell' (but I note that David Roundy's version looks better than the iterate version above, because the errors won't accumulate).
But as maintainer and bug-fixer of the Haskell'98 Report, I have also been asked whether we should make this change retrospectively to the Haskell'98 language (as a "typo"). Since it involves not merely an ordinary library function, but a Prelude function, and moreover a function that is used in the desugaring of syntax, it is less clear to me whether to alter Haskell'98.
We definitely can't make breaking changes to Haskell 98; this would be much more than a "typo". However, this does give us a problem if we decide to make a change here for H', as Neil points out, because there would be two mutually-incompatible instances for Enum Float. We'd need to have a clear distinction between programs that are Haskell 98 and those that are not, with only the former allowed to use the haskell98 package. Cheers, Simon