[Haskell-cafe] local type denotation
Richard O'Keefe
ok at cs.otago.ac.nz
Wed Nov 14 23:13:56 CET 2012
On 15/11/2012, at 1:03 AM, Serge D. Mechveliani wrote:
> Please,
> how to correctly set an explicit type for a local value in the body of
> a polymorphic function?
Other people have told you how to do it.
I'd like to tell you why you don't need to.
>
> Example (tested under ghc-7.6.1):
>
> data D a = D1 a | D2 a (a -> a)
>
> f :: Eq a => D a -> a
> f (D1 x) = x
> f (D2 x g) = let -- y :: Eq a => a
> y = g x
> in if x == y then x else g y
You say that you want y to have exactly the type a.
Look around. Is there some data in scope with that type?
Yes: (D2 x g) :: a => x :: a.
So you just want to say "y has the same type as x".
There's a Prelude function
asTypeOf :: a -> a -> a
asTypeOf x y = x
So e1 `asTypeOf` e2 gives you the value of e1,
having first ensured that e1 and e2 have the same type.
So
f :: Eq a => D a -> a
f (D1 x) = x
f (D2 x g) = if x == y then x else g y
where y = g x `asTypeOf` x
You apparently already know that you don't need any of this
(thanks to x == y), but want to be explicit. The question
is how explicit you want to be. Using asTypeOf is sort of
half way between implicit typing and showing the type you
want _as_ a type.
>
The other question, I suppose, is _why_ you want to be explicit?
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