[Haskell-beginners] Convert my own types to the Haskell types using typeclass

[ke dou]

Hi,

I want to define a typeclass that can convert my own types like MyBool,
MyInt, MyOption to according Haskell types -- Bool, Int, Maybe.

Currently I can convert the first two types, but for MyOption, I don't how
to do, since it is a polymophic type.

Here is my toy program:
--------------------------------------------------------------
{-# LANGUAGE TypeFamilies #-}

module Coercible where

import qualified Prelude

data MyBool = MyTrue | MyFalse
data MyInt = Zero | One | Two
data MyOption a =
   Some a
 | None

class Coercible a where
  type Return a :: *
  toHaskell :: a -> (Return a)

instance Coercible MyBool where
  type Return MyBool = Prelude.Bool
  toHaskell MyTrue = Prelude.True
  toHaskell MyFalse = Prelude.False

instance Coercible MyInt where
  type Return MyInt = Prelude.Int
  toHaskell Zero = 0
  toHaskell One = 1
  toHaskell Two = 2

instance Coercible (MyOption a) where
  type Return (MyOption a) = Prelude.Maybe a
  toHaskell (Some a) = Prelude.Just a
  toHaskell None = Prelude.Nothing
--------------------------------------------------------------------------
The problem occurs when I am trying to use "toHaskell (Some MyBool)", the
error message is "Not in scope: data constructor `MyBool'".
Any hints will be appreciated !!

Best,
--Ke
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