Re: [Haskell-cafe] local type denotation

14 Nov 2012


      {-# LANGUAGE ScopedTypeVariables #-}

Отправлено с iPhone

14.11.2012, в 16:03, "Serge D. Mechveliani"  написал(а):
...
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@botik.ru
Thanks,
------
Sergei
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Re: [Haskell-cafe] local type denotation

MigMit