There is nothing to fix. It is doing exactly as you intended. You are confusing types vs values. Integer is a type, 1, 2, and 3 are values. toHaskell takes a type (MyOption a) and converts it to a type (Maybe a). But in your actual code you will use values ie. toHaskell (Some 'a' :: MyOption Char) and get back a (Just 'a' :: Maybe Char). On Fri, Apr 11, 2014 at 1:13 PM, ke dou wrote:

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.

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Thank you very much! That is exactly what I want. Best, Ke On Fri, Apr 11, 2014 at 2:43 PM, Magnus Therning wrote:

On Fri, Apr 11, 2014 at 12:50:45PM -0400, ke dou wrote:

...

instance Coercible (MyOption a) where type Return (MyOption a) = Prelude.Maybe a toHaskell (Some a) = Prelude.Just a toHaskell None = Prelude.Nothing

The basic issue has been solved in another thread, but wouldn't you want this one to be something like this:

instance Coercible a => Coercible (MyOption a) where type Return (MyOption a) = Prelude.Maybe (Return a) toHaskell (Some a) = Prelude.Just (toHaskell a) toHaskell None = Prelude.Nothing

/M

-- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus@therning.org jabber: magnus@therning.org twitter: magthe http://therning.org/magnus

What gets measured, gets done. -- Tom Peters

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