Hi,
I wonder why the following code doesn't typecheck in GHC 7.4.1: {-# LANGUAGE GADTs,RankNTypes #-} data T a where T1 :: (forall b. b -> b) -> (forall a. Int -> T a) {- Error: Data constructor `T1' returns type `forall a. Int -> T a' instead of an instance of its parent type `T a' In the definition of data constructor `T1' In the data type declaration for `T' Failed, modules loaded: none. -} While:{-# LANGUAGE GADTs,RankNTypes #-} f :: (forall b. b -> b) -> (forall a. Int -> Maybe a) f = undefined {- ghci> :t f f :: (forall b. b -> b) -> Int -> Maybe a -}Thanks, Shayan _______________________________________________ Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
It really seems to me that the error message you've got explains everything quite clear. Отправлено с iPad
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If GHC handles the explicit "forall" in constructor "T1" in the same way as it does for function "f", we have:
data T a where T1 :: (forall b. b -> b) -> Int -> T a Which is totally fine! The main question is then why the "forall"s are handled differently? On Tue, Jul 31, 2012 at 9:07 PM, MigMit <[hidden email]> wrote:
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In reply to this post by Shayan Najd Javadipour-2
On Tue, Jul 31, 2012 at 2:59 PM, Shayan Najd Javadipour <[hidden email]> wrote:
This looks to me like other cases where GHC requires an exact type match even though you used something equivalent. Similarly, for example, it rejects (contrived example)
foo :: Num a => a -> a -> a foo 0 0 = -1 foo = (+) because the explicit arity of the cases must match exactly, even though (+)'s type matches the required arity. I am under the impression that it's difficult to make those kinds of things work nicely in the typechecker.
brandon s allbery [hidden email] wandering unix systems administrator (available) (412) 475-9364 vm/sms _______________________________________________ Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
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