Jon Fairbairn wrote:
If you allow quantification over higher
kinds, you can do something like this:
d f = f . f
d:: âa::*, b::*â*.(b a â a) â b (b a)â a
What's the problem with
d :: (forall c . b c -> c) -> b (b a) -> a
d f = f . f
to which ghci gives the type
d :: forall a b. (forall c. b c ->
On 2005-02-28 at 23:10EST Jim Apple wrote:
> Jon Fairbairn wrote:
> > If you allow quantification over higher
> > kinds, you can do something like this:
> >
> >d f = f . f
> >
> >d:: âa::*, b::*â*.(b a â a) â b (b a)â a
> >
>
> What's the problem with
>
> d :: (forall c . b c -> c) ->
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 ->
Actually none of these seem to work:
>{-# OPTIONS -fglasgow-exts #-}
>
>module Main where
>
>main :: IO ()
>main = putStrLn "OK"
>
>d :: (forall c . b c -> c) -> b (b a) -> a
>d f = f . f
>
>t0 = d id
>t1 = d head
>t2 = d fst
Load this into GHCI and you get:
Test.hs:11:7:
Couldn't match the rigi
Gtk2Hs - A Haskell GUI library based on the Gtk+ GUI Toolkit.
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It is really too bad the 'middle' version does not work, ie
John Fairbarn's version
> d1 :: (forall c . b c -> c) -> b (b a) -> a
> d1 f = f . f
John Meacham's version (dual (?))
> d2 :: (forall c . c -> b c) -> a -> b (b a)
> d2 f = f . f
Or something in the middle
> d3 :: forall e a b . (fo
The first issue of The Monad.Reader is available:
http://www.haskell.org/hawiki/TheMonadReader_2fIssueOne
We'd like to hear what you think:
http://www.haskell.org/hawiki/TheMonadReader_2fIssueOne_2fFeedBack
--
Shae Matijs Erisson - http://www.ScannedInAvian.com/ - Sockmonster once said:
You could
On Mon, Feb 28, 2005 at 11:10:40PM -0500, Jim Apple wrote:
> Jon Fairbairn wrote:
> >If you allow quantification over higher
> >kinds, you can do something like this:
> >
> > d f = f . f
> >
> > d:: âa::*, b::*â*.(b a â a) â b (b a)â a
> >
>
> What's the problem with
>
> d :: (forall c . b c
This message presents polymorphic functions that derive a term for a
given type -- for a class of fully polymorphic functions: proper and
improper combinators. This is better understood on an example:
rtest4 f g = rr (undefined::(b -> c) -> (a -> b) -> a -> c) HNil f g
*HC> rtest4 (:[]) Just
