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

[ke dou]

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 <bob at redivi.com> wrote:

> On Fri, Apr 11, 2014 at 9:50 AM, ke dou <kd6ck at 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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