RE: inferred type doesn't type-check (using type families)
| I think (untested) that in this particular case you can get around the | problem using scoped type variables: | | g :: forall a. Num a = T a - T a | g x = f x (42 :: a) | | In fact, this seems to be the general pattern for fixing problems like | this with type families: add extra witness arguments which GHC can | use to unify type variables that are hidden inside type family | applications. But in this case all you've done is to make g have an ambiguous type. I've drafted a FAQ about this at http://haskell.org/haskellwiki/GHC/Type_families#Frequently_asked_questions Please, everyone, edit and extend. Simon ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: inferred type doesn't type-check (using type families)
2009/11/3 Daniel Fischer daniel.is.fisc...@web.de: Am Dienstag 03 November 2009 19:28:55 schrieb Roland Zumkeller: Hi, Compiling class WithT a where type T a f :: T a - a - T a f = undefined g x = f x 42 with -XTypeFamilies -fwarn-missing-signatures gives: Inferred type: g :: forall a. (Num a) = T a - T a Adding g :: Num a = T a - T a results in: Couldn't match expected type `T a' against inferred type `T a1' In the first argument of `f', namely `x' Is the inferred type not the right one? Is g typeable? The type function T isn't injective (or, it isn't guaranteed to be), so there's no way to determine which type a to use for 42. I think (untested) that in this particular case you can get around the problem using scoped type variables: g :: forall a. Num a = T a - T a g x = f x (42 :: a) In fact, this seems to be the general pattern for fixing problems like this with type families: add extra witness arguments which GHC can use to unify type variables that are hidden inside type family applications. Cheers, Max ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: inferred type doesn't type-check (using type families)
On Tue, Nov 3, 2009 at 3:20 PM, Max Bolingbroke batterseapo...@hotmail.com wrote: 2009/11/3 Daniel Fischer daniel.is.fisc...@web.de: Am Dienstag 03 November 2009 19:28:55 schrieb Roland Zumkeller: Hi, Compiling class WithT a where type T a f :: T a - a - T a f = undefined g x = f x 42 with -XTypeFamilies -fwarn-missing-signatures gives: Inferred type: g :: forall a. (Num a) = T a - T a Adding g :: Num a = T a - T a results in: Couldn't match expected type `T a' against inferred type `T a1' In the first argument of `f', namely `x' Is the inferred type not the right one? Is g typeable? The type function T isn't injective (or, it isn't guaranteed to be), so there's no way to determine which type a to use for 42. I think (untested) that in this particular case you can get around the problem using scoped type variables: g :: forall a. Num a = T a - T a g x = f x (42 :: a) GHC accepts this, but arguably shouldn't as there's no way to call g. Neither the argument to g nor the context of g has enough information to specify a, so there's no way to know what's intended. In fact, this seems to be the general pattern for fixing problems like this with type families: add extra witness arguments which GHC can use to unify type variables that are hidden inside type family applications. This is true. You can use a dummy argument to force the choice of a, and if the dummy isn't used it gets removed in optimization. data Proxy a -- no values, so should always be removable during optimization g :: forall a. Num a = Proxy a - T a - T a g _ x = f x (42 :: a) -- works fine -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: inferred type doesn't type-check (using type families)
Am Dienstag 03 November 2009 19:28:55 schrieb Roland Zumkeller: Hi, Compiling class WithT a where type T a f :: T a - a - T a f = undefined g x = f x 42 with -XTypeFamilies -fwarn-missing-signatures gives: Inferred type: g :: forall a. (Num a) = T a - T a Adding g :: Num a = T a - T a results in: Couldn't match expected type `T a' against inferred type `T a1' In the first argument of `f', namely `x' Is the inferred type not the right one? Is g typeable? The type function T isn't injective (or, it isn't guaranteed to be), so there's no way to determine which type a to use for 42. Best, Roland ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
