[[email protected]: Re: Enum class]

Dylan Thurston [email protected]
Thu, 25 Oct 2001 18:16:09 +0900


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(I want to trim the headers, but don't know the history of this
thread.  Also cc:ed back to the Haskell list.)

On Thu, Oct 25, 2001 at 11:11:42AM +0200, Ketil Malde wrote:
> Dylan Thurston <[email protected]> writes:
> > I agree that Enum instances for Float/Double are not likely to be
> > useful.
>=20
> From a gut feeling, I could see a use for expressions like
>=20
>         [1.5, 1.6..] and [1.5, 1.6..2.0]
>=20
> (i.e. enumFromThen and enumFromThenTo) but enumFrom and enumFromTo
> making list of rounded integers seems strange to me.
>=20
> It seems to me that enumFromThen and -To could be implemented
> something like:
>=20
>         enumFromThen beg next =3D beg : enumFromThen next (next+delta)
>                 where delta =3D next-beg
>=20
> similarly for enumFromThenTo, of course.
>=20
> i.e. depending only on functionality found in Num.  Why not put these
> functions there, and remove Float and Double as Enum instances?
>=20
> What am I missing?

Currently you can write

data Day =3D Sunday | Monday | Tuesday | Wednesday | Thursday | Friday |
           Saturday

weekdays =3D [Monday..Friday]

which has nothing to do with Num.

Best,
	Dylan Thurston

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