On Fri, Apr 11, 2014 at 9:50 AM, ke dou <kd6ck@virginia.edu> wrote:

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 !!

`Some MyBool` isn't valid Haskell. MyBool is the name of a type, there's no binding or constructor with that name. `toHaskell (Some MyTrue)` evaluates to `Just MyTrue` as expected.

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