[Haskell-beginners] Making a generic interpreter

[Erik Helin]

Hi Patrick,
thanks for taking your time!

On Fri, May 6, 2011 at 18:45, Patrick LeBoutillier
<patrick.leboutillier at gmail.com> wrote:
> I don't see how you can make the StackBool something else than Bool.
> The CMP and BRANCH operators seem to me very Boolean by definition.
> Can you provide an example of how it would work?
>
> Patrick
>

I actually can :) I've been hacking today, and the result is:

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}

class (Show a) => AbsInteger a b | a -> b where
    (+) :: a -> a -> a
    (==) :: a -> a -> b
    absInteger :: Integer -> a

class (Show a) => AbsBool a where
    cond :: a -> [Code] -> [Code] -> [Code]
    absBool :: Bool -> a

instance AbsBool Bool where
    cond b c1 c2 = if b then c1 else c2
    absBool b = b

instance AbsInteger Integer Bool where
    a + b = (Prelude.+) a b
    a == b = (Prelude.==) a b
    absInteger a = a

Now, if I would like to use my own integer type or my own bool type, I can do:

data MyInteger = MyInteger Integer deriving (Show)
data MyBool = MyBool Bool deriving (Show)

instance AbsBool MyBool where
    cond (MyBool b) c1 c2 = if b then c1 else c2
    absBool b = MyBool b

instance AbsInteger MyInteger MyBool where
    (MyInteger a) + (MyInteger b) = MyInteger $ (Prelude.+) a b
    (MyInteger a) + (MyInteger b) = MyBool $ (Prelude.==) a b
    absInteger a = MyInteger a

The eval function has to be tweaked a little bit, but it is almost
plug'n'play :)

However, due to the functional dependency of AbsInteger, this does not
allow me to use
MyInteger and Bool, since there can only be one pair of an integer
type and a bool type, and Integer and Bool are already instantiating
AbsInteger .

Do you know some of way of making it possible to have several integer
type and boolean type pairs instantiating the class AbsInteger?