Testing equality between two heterogeneous values

existential-type, ghc, haskell

Solution

It's possible if you add a `Typeable` constraint:

import Data.Typeable

data Object = forall a. (Shape a, Typeable a) => Object a

instance Eq Object where
  Object x == Object y =
    case cast y of
      Just y' -> x == y'
      Nothing -> False

Here, `cast y` will return `Just y'` if the desired type of `y'` (inferred from the use of `==` with `x`) matches the actual type of `y`, and `Nothing` otherwise.

Problem

I'm using the `-XExistentialQuantification` GHC extension to create a heterogeneous container for values of a specific type class (`Shape`): ``` -- Container type data Object = forall a. Shape a => Object a -- 'Shape' class. Methods not important class Eq s => Shape s where doStuff :: s -> s ``` Given that all instances of `Shape` are also instances of `Eq`, is there a way to make `Object` an instance of `Eq` as well?

Original source