Hi
Bringing up an old thread, but this issue just bit me in real life:
[0, 2.5 .. 3.75 ] = [0, 2.5, 5.0]
I was comparative testing some C++ and some Haskell, and its a shame
when its the Haskell that is clearly wrong! I don't think any end
decision was reached in this thread, but I think this is sufficiently
incorrect to call the Haskell 98 behaviour a bug.
Thanks
Neil
On Wed, Oct 22, 2008 at 10:11 AM, Simon Marlow
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
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