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
{-# LANGUAGE ScopedTypeVariables #-}
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14.11.2012, в 16:03, "Serge D. Mechveliani"
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
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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@botik.ru
Thanks,
------ Sergei
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
You need to enable ScopedTypeVariables, and add a forall to introduce the
type variable at the top level. The local variable will then be the *same*
'a' instead of a fresh one:
{-# LANGUAGE ScopedTypeVariables #-}
data D a = D1 a | D2 a (a -> a)
f :: forall a. 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"
On Wed, Nov 14, 2012 at 1:03 PM, Serge D. Mechveliani
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
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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?
participants (5)
-
Brent Yorgey -
Erik Hesselink -
MigMit -
Richard O'Keefe -
Serge D. Mechveliani