On Tue, Jun 5, 2012 at 5:29 PM, Bas van Dijk
Hello,
I have the following program:
-------------------------------------------------------------------------- {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-}
import Data.Proxy (Proxy) import Data.Typeable (Typeable, TypeRep, typeOf)
data ProxyWrapper constraint = forall a. constraint a => ProxyWrapper (Proxy a)
I must be missing something, but this seems a bit useless to me. You have a phantom type parameter on Proxy, and then you're hiding it. So when you pattern match on ProxyWrapper you recover the fact that there was a type which satisfies the constraint, but you don't know what type it was, and neither do you know about any values which are of the type. What are you trying to do? That said, if you want to be able to recover a Typeable constraint, I don't see any way except for using 'ProxyWrapper (Ext Typeable constraint)' as Andres says or putting 'forall a. (constraint a, Typeable a)' in the definition of ProxyWrapper.
typeOfInnerProxy :: ProxyWrapper constraint -> TypeRep typeOfInnerProxy (ProxyWrapper p) = typeOfArg p
typeOfArg :: forall t a. Typeable a => t a -> TypeRep typeOfArg _ = typeOf (undefined :: a) --------------------------------------------------------------------------
Type checking this gives the following expected type error:
ProxyWrapper.hs:12:37: Could not deduce (Typeable a) arising from a use of `typeOfArg' from the context (constraint a) bound by a pattern with constructor ProxyWrapper :: forall (constraint :: * -> Constraint) a. (constraint a) => Proxy a -> ProxyWrapper constraint, in an equation for `typeOfInnerProxy'
Is there a way to extend the 'constraint' with the 'Typeable' constraint in the type signature of 'typeOfInnerProxy'?
Regards,
Bas