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

[Casey McCann]

On Tue, Sep 20, 2011 at 12:47 PM, Paterson, Ross <R.Paterson at city.ac.uk> wrote:
> Daniel Fischer writes:
>>> 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.
>
> .. only more so.  And the brokenness has infected Rational: try
>
> [1,3..20]::[Rational]

I actually think the brokenness of Ord for floating point values is
worse in many ways, as demonstrated by the ability to insert a value
into a Data.Set.Set and have other values "disappear" from the set as
a result. Getting an unexpected element in a list doesn't really seem
as bad as silently corrupting entire data structures.

For numeric ranges of fractional values, I expect that what one
typically wants is either "the two end points, and N intermediate
values evenly spaced" or "the starting value, and successive
increments up to some cut-off", with the default increment being 1 or
-1 as appropriate. The current Enum instance splits the difference and
gets you some of both, except that you might get the wrong values or
something past the cut-off. Similarly, the current Ord instance splits
the difference between a coherent total order (what Ord instances
should be) and the semantically correct partial order (what floats
should have, as defined in the IEEE standard).

- C.