[Haskell] Type of y f = f . f

[Pedro Vasconcelos]

On Mon, 28 Feb 2005 03:50:14 -0500
Jim Apple <japple at freeshell.org> 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. From this you can see that
(y head) and (y fst) cannot be typed, whereas (y id) can. BTW, this
function is usually named 'twice'.

Best regards,

Pedro

-- 
Pedro Vasconcelos, School of Computer Science, University of St Andrews
-----------------------------------------------------------------------
"The difference between Theory and Practice 
is greater in Practice than in Theory."