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

Thank you, Bob. I also noticed that, but don't know how to fix it. Any suggestions? --Ke On Fri, Apr 11, 2014 at 1:05 PM, Bob Ippolito wrote:

On Fri, Apr 11, 2014 at 9:50 AM, ke dou 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.

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners