Re: [Haskell-cafe] instance Enum Double considered not entirely great?

On Tuesday 20 September 2011, 17:39:49, Ketil Malde wrote:

Chris Smith writes:

...

It would be a shame if we lost an occasionally useful and easy to read

You forgot "confusing"? Expecting Enum to enumerate all inhabitants of a type seems very reasonable to me, and seems to hold for all non-floating point types.

And Rational (more generally, Ratio a). (Why does everybody forget that?) Enumerating all inhabitants of a type (within some range) is only possible if there are well-defined successors and predecessors (modulo bounds). For Double and Float, there are (excepting NaNs), so it could be done, but arguably that would be *far less* useful than the current instances. For Rational, no such luck.

A numeric range [a..a+n] might be expected to have a+n+1 elements, but that doesn't hold either for Float and Double. I think Enum for floating point values is broken

Yes, it is. Like Eq and Ord.

- but it is reality, so we need to deal with it.

Like Eq and Ord, it's just too damn convenient to have it. So much nicer to write [0, 0.25 .. 1000] instead of numericEnumFromThenTo 0 0.25 1000 and (x /= y) instead of doublesDifferentOrNaN x y