Re: [Haskell-beginners] Making a generic interpreter

6 May 2011

      Hi Patrick,
thanks for taking your time!

On Fri, May 6, 2011 at 18:45, Patrick LeBoutillier
 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?

Re: [Haskell-beginners] Making a generic interpreter

Erik Helin