defining (-> Bool) as a set

[Simon Peyton-Jones]

Hal,=20

[I think this sort of question would be better on the haskell-cafe
list.]

I don't think what you want can be done directly.  It's the old
thing about not having lambdas at the type level.  You want:

	instance Eq a =3D> Coll (\x. x -> Bool) a where ...

and you just can't do that.   You *can* abstract the second argument
of (->):

	instance Eq a =3D> Coll ((->) Bool) a where ...

but not the first.  It's a well known shortcoming in Haskell, that you
can
partially apply type constructors, but you can't do argument
permutation.

I know of no good solution.  Adding type lambdas in their full glory
makes type inference pretty much impossible.  What we'd like is
a compromise.  Maybe someone can invent one.  But take care.
The ground is littered with corpses.

Simon


| -----Original Message-----
| From: Hal Daume III [mailto:hdaume@ISI.EDU]=20
| Sent: 23 April 2002 01:30
| To: Jorge Adriano
| Cc: Haskell Mailing List
| Subject: Re: defining (-> Bool) as a set
|=20
|=20
| Yeah, both options suggested are valid, of course.  But I=20
| really don't want to have a constructor and I'm using Edison=20
| where Coll is defined something like:
|=20
| class Coll c e where
|   empty :: c e
|   insert :: c e -> e -> c e
|=20
| etc., which precludes the fun dep solution.
|=20
|  - Hal
|=20
| --
| Hal Daume III
|=20
|  "Computer science is no more about computers    | hdaume@isi.edu
|   than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
|=20
| On Tue, 23 Apr 2002, Jorge Adriano wrote:
|=20
| >=20
| > > class Collection e ce | ce -> e where
| > >     empty :: ce
| > >     insert :: e -> ce -> ce
| > >     member :: e -> ce -> Bool
| > >
| > > instance Eq a =3D> Collection a (a -> Bool) where
| > >     empty =3D (\x -> False)
| > >     insert e f =3D (\x -> if x =3D=3D e then True else f x)
| > >     member e f =3D f e
| >=20
| > This is way better than my solution...
| >=20
| > I had never used multi-parameter classes before, so I forgot the=20
| > functional
| > dependency (right name? the "|ce->e"), and there was=20
| obviously no need for my=20
| > extra constructor.
| >=20
| > J.A.
| >=20
|=20
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|=20