Here's a type that fits:
d :: forall b a t c. (F t c b, F t a c) => t -> a -> b
from the following code:
>-# OPTIONS -fglasgow-exts #-}
>module Main where
>
>main :: IO ()
>main = putStrLn "OK"
>
>data ID = ID
>data HEAD = HEAD
>data FST = FST
>
>class F t a b | t a -> b where
>f :: t ->
Jon Fairbairn writes:
> On 2005-02-28 at 18:03GMT Ben Rudiak-Gould wrote:
> > Pedro Vasconcelos wrote:
> > >Jim Apple <[EMAIL PROTECTED]> wrote:
> > >>Is there a type we can give to
> > >>
> > >>y f = f . f
> > >>
> > >>y id
> > >>y head
> > >>y fst
> > >>
> > >>are all typeable?
> > >
On 2005-02-28 at 18:03GMT Ben Rudiak-Gould wrote:
> Pedro Vasconcelos wrote:
> >Jim Apple <[EMAIL PROTECTED]> wrote:
> >>Is there a type we can give to
> >>
> >>y f = f . f
> >>
> >>y id
> >>y head
> >>y fst
> >>
> >>are all typeable?
> >
> >Using ghci:
> >
> >Prelude> let y f = f.f
>
Pedro Vasconcelos wrote:
>Jim Apple <[EMAIL PROTECTED]> wrote:
>>Is there a type we can give to
>>
>>y f = f . f
>>
>>y id
>>y head
>>y fst
>>
>>are all typeable?
>
>Using ghci:
>
>Prelude> let y f = f.f
>Prelude> :t y
>y :: forall c. (c -> c) -> c -> c
>
>So it admits principal type (a->a) -> a->a
The name y suggests that you want to define the fixpoint combinator.
This works as follows:
Prelude> let y f = f (y f)
Prelude> :type y
y :: forall t. (t -> t) -> t
Prelude> y (\fac n -> if n == 0 then 1 else n*fac(n-1)) 10
3628800
Prelude>
Till
Pedro Vasconcelos wrote:
On Mon, 28 Feb 2005 03:50:14
On Mon, 28 Feb 2005 03:50:14 -0500
Jim Apple <[EMAIL PROTECTED]> wrote:
> Is there a type we can give to
>
> y f = f . f
>
> y id
> y head
> y fst
>
> are all typeable?
>
Using ghci:
Prelude> let y f = f.f
Prelude> :t y
y :: forall c. (c -> c) -> c -> c
So it admits principal type (a->a) ->
Is there a type we can give to
y f = f . f
y id
y head
y fst
are all typeable?
Jim Apple
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