[Haskell-cafe] local type denotation

[Brent Yorgey]

Turn on the ScopedTypeVariables extension (e.g. by putting {-#
LANGUAGE ScopedTypeVariables #-} at the top of your file), and add an
explicit 'forall a.' to the type signature of f.

-Brent

On Wed, Nov 14, 2012 at 04:03:57PM +0400, Serge D. Mechveliani wrote:
> Please,
> how to correctly set an explicit type for a local value in the body of 
> a polymorphic function?
> 
> 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
>                                                         
>   main = putStr $ shows (f (D2 (1 :: Int) succ)) "\n"
>                                                                
> 
> This is compiled by    ghc --make Main
> 
> Now I need, for a certain reason, to explicitly set the type for  y  in 
> `let',  with the meaning: 
> "this very `a' which is in the signature for  f"
> (and I think that this type Haskell assignes to  y  in  "y = g x").
> 
> I need to declare this type in a separate line:  y :: <what ever it is>.
> 
> Both  `y :: a'  and  `y :: Eq a => a'  are not compiled.
> 
> Please, copy the answer to  mechvel at botik.ru
> 
> Thanks,
> 
> ------
> Sergei
> 
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