问题
I want to write a function with this type signature:
getTypeRep :: Typeable a => t a -> TypeRep
where the TypeRep will be the type representation for a, not for t a. That is, the compiler should automatically return the correct type representation at any call sites [to getTypeRep], which will have concrete types for a.
To add some context, I want to create a "Dynamic type" data type, with the twist that it will remember the top-level type, but not its parameter. For example, I want to turn MyClass a into Dynamic MyClass, and the above function will be used to create instances of Dynamic MyClass that store a representation of the type parameter a.
回答1:
Well, how about using scoped type variables to select the inner component:
{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE ScopedTypeVariables #-}
import Data.Dynamic
import Data.Typeable
getTypeRep :: forall t a . Typeable a => t a -> TypeRep
getTypeRep _ = typeOf (undefined :: a)
Works for me:
*Main> getTypeRep (Just ())
()
*Main> getTypeRep (Just 7)
Integer
*Main> getTypeRep ([True])
Bool
Interesting design.
回答2:
On a tangential note to Don's solution, notice that code rarely require ScopedTypeVariables. It just makes the solution cleaner (but less portable). The solution without scoped types is:
{-# LANGUAGE ExplicitForAll #-}
import Data.Typeable
helper :: t a -> a
helper _ = undefined
getTypeRep :: forall t a. Typeable a => t a -> TypeRep
getTypeRep = typeOf . helper
回答3:
This function (now) exists in Data.Typeable typeRep
来源：https://stackoverflow.com/questions/5924539/haskell-get-typerep-from-concrete-type-instance