Re: [Haskell] Type of y f = f . f

28 Feb 2005

      On Mon, 28 Feb 2005 03:50:14 -0500
Jim Apple <japple@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
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