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

Ketil Malde ketil at malde.org
Wed Sep 21 00:04:50 CEST 2011

>> You forgot "confusing"?

> I didn't forget it

Sorry, I should have written "omitted".

> perhaps we ought to ask what the cause of the confusion is.

tr.v. e·nu·mer·at·ed, e·nu·mer·at·ing, e·nu·mer·ates
  1. To count off or name one by one; list: A spokesperson enumerated the strikers' demands.
  2. To determine the number of; count.

Regardless of how easily the children you teach get it, to me
enumerating is a different thing from repeatedly adding one.

>> Expecting Enum to enumerate all inhabitants of
>> a type seems very reasonable to me, and seems to hold for all
>> non-floating point types.

> Floating point (and fixed point, for that matter) types approximate real
> numbers, which of course have no possible enumeration of all values.
> It seems to me particularly pointless to define an Enum
> instance that focuses on, above all else, the inaccuracy of that
> approximation.

Yes.  But we need an Enum instance to get the syntactic sugar of
[1..10], so one is defined anyway.

> Perhaps I was understating the case in saying the
> behavior was established but undocumented; rather, it's explicitly
> documented in the Haskell Report.

Absolutely, it is, as I've said, the reality - like it or not.

>> Or just avoid Enum, and define "range" or something similar instead.

> If Haskell defined list syntax in terms of something that's not called
> Enum, that would be fine.  Renaming is never all that big a deal.  But
> the list sugar is a big deal, and I don't think there's any point at all
> in leaving the list sugar associated with something as minor as building
> a representation of the inaccuracy of your approximations.

I must admit I don't understand this comment.  If the fixpoint library
wants to provide the functionality (producing all values between two
points), and can't/shouldn't use Enum, surely it must provide a
different function, and let go of the list sugar?

If I haven't seen further, it is by standing in the footprints of giants

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