# Type dispatch based on runtime value

Suppose we're implementing an interpreter for the bytecode of some stack machine. Let every element of the stack be a Word8, and let the stack be represented by a list:

type Stack = [Word8]


We'll need opcodes for addition, subtraction, and unary negation. However, apart from acting on Word8s, we might want to treat the top two elements together as a Word16, and then the next two as another Word16, and then do something to those. To keep things simple:

data DataSize = OneByte | TwoBytes | FourBytes deriving (Eq, Ord, Read, Show)

data OpCode = ... | Add DataSize | Subtract DataSize | Negate DataSize | ...


Add OneByte performs addition of Word8s, Add TwoBytes performs addition of Word16s, etc.

We'll use some monad to store the state of our computation. It doesn't really matter what we use, so we'll just represent it with M here. Our function to execute one instruction might be

exec :: OpCode -> M ()


and ideally we'd like to write

exec (Add size) = perform get2 (+) size
exec (Subtract size) = perform get2 (-) size
exec (Negate size) = perform get1 negate size


where perform get2 f size pops two objects of the given size, applies f to them, then pushes the result on the stack.

The question, now, is how close we can come to implementing perform. Intuitively, the type is something like

-- Stackable is some class of things we can push and pop.
-- c is any constraint.  In the examples above, it would be num.
-- fn{a} is a type with one free variable a.  Haskell does not allow
-- generalising this way, which is why I'm using the non-existent fn{a}
-- notation.
perform :: (forall a. (Stackable a, c a) => fn{a} -> Proxy a -> M ())
-> (forall a. c a => fn{a})
-> Size
-> M ()


For any choice of fn, this function is easily implemented:

perform act f = go
where go OneByte = act f (Proxy :: Proxy Word8)
where go TwoBytes = act f (Proxy :: Proxy Word16)
where go FourBytes = act f (Proxy :: Proxy Word32)


Unfortunately, we can't just say forall fn (nor, for that matter, forall c). Here's a solution I came up with to allow a single definition of perform in the presence of this. I've simplified a little to reduce the amount of code, so M is IO and we assume all values involved support plenty of operations.

{-# LANGUAGE RankNTypes, ConstraintKinds, TypeFamilies, EmptyDataDecls #-}
{-# LANGUAGE FlexibleInstances, ViewPatterns #-}

import Data.Proxy

-- This is the runtime value we'll use to decide what type we're operating on.
data DataSize = DSInt | DSDouble
-- I haven't found a way to support arbitrary constraints, so the solution
-- for now is to throw in a bunch that I know the types involved will support.
type SimpleNum a = (Num a, Eq a, Ord a, Read a, Show a)

-- We use this as a placeholder for a in the types
data Hole

-- This is meant to be something like a lambda on the type level.
-- fn should be a type with some "holes" in it, and Lambda fn a should
-- be isomorphic to the type with the holes filled with a.
data family Lambda fn a

data instance Lambda (Hole -> r) a = LambdaTakingHole { getLambdaTakingHole :: a -> Lambda r a }
data instance Lambda Hole a = LambdaHole { getLambdaHole :: a }
data instance Lambda (IO ()) a = LambdaIO { getLambdaIO :: IO () }

-- This is a helper typeclass to allow uniform conversion to and from
-- values of the form Lambda fn a.  With the recursion going on, this makes
-- wrapping far easier.

class LambdaLike fn where
-- UnderlyingFun fn a is the type that Lambda fn a should be isomorphic to.
-- wrapf and unwrapf are the isomorphism.
type family UnderlyingFun fn a
wrapf :: UnderlyingFun fn a -> Lambda fn a
unwrapf :: Lambda fn a -> UnderlyingFun fn a

instance LambdaLike r => LambdaLike (Hole -> r) where
type UnderlyingFun (Hole -> r) a = a -> UnderlyingFun r a
wrapf f = LambdaTakingHole (wrapf . f)
unwrapf (LambdaTakingHole f) = unwrapf . f

instance LambdaLike Hole where
type UnderlyingFun Hole a = a
wrapf = LambdaHole
unwrapf = getLambdaHole

instance LambdaLike (IO ()) where
type UnderlyingFun (IO ()) a = IO ()
wrapf = LambdaIO
unwrapf = getLambdaIO

-- Here are some example functions we could write
-- These are given a specific instantiation of the function they need
-- to work with, and then a proxy to indicate the type involved.

runWith2 :: SimpleNum a => (Lambda (Hole -> IO ()) a) -> Proxy a -> IO ()
runWith2 (unwrapf -> f) _ = f 2

runWithUserInput :: SimpleNum a => (Lambda (Hole -> IO ()) a) -> Proxy a -> IO ()
runWithUserInput (unwrapf -> f) _ = getLine >>= f . read

runWith2andPrint :: SimpleNum a => (Lambda (Hole -> Hole) a) -> Proxy a -> IO ()
runWith2andPrint (unwrapf -> f) _ = print $f 2 runWith2and5 :: SimpleNum a => (Lambda (Hole -> Hole -> IO ()) a) -> Proxy a -> IO () runWith2and5 (unwrapf -> f) _ = f 2 5 perform :: (forall a. SimpleNum a => Lambda fn a -> Proxy a -> IO ()) -> (forall a. SimpleNum a => Lambda fn a) -> DataSize -> IO () perform fn f = go where go DSInt = fn f (Proxy :: Proxy Int) go DSDouble = fn f (Proxy :: Proxy Double) main = do perform runWith2 (wrapf print) DSInt perform runWith2 (wrapf print) DSDouble perform runWith2andPrint (wrapf (+1)) DSInt perform runWith2and5 (wrapf$ \x y -> print $x + y) DSInt perform runWith2and5 (wrapf$ \x y -> print $x * y) DSDouble  Is this a good solution to the problem? Am I reinventing some wheel here? ## 1 Answer I'm not familiar with using Data.Word so I switched to using ints. I think your solution looks to be overkill. How does my solution match your specification? import Control.Monad.State import Data.Maybe import Control.Applicative import Data.List type Stack = [Int] data DataSize = OneByte | TwoBytes | FourBytes deriving (Eq, Ord, Read, Show) data OpCode = Add DataSize | Subtract DataSize | Negate DataSize deriving (Eq, Ord, Read, Show) type M = State Stack exec :: OpCode -> M () exec (Add size) = get2 size (+) exec (Subtract size) = get2 size (-) exec (Negate size) = get1 size negate -- 5 + 6 with one byte test1 :: M () test1 = do push 5 push 6 add OneByte -- 256 + 1 with two bytes test2 :: M () test2 = do push 0 push 1 push 1 push 1 add TwoBytes push x = modify (x:) add = exec . Add sub = exec . Subtract neg = exec . Negate get2 :: DataSize -> (Int -> Int -> Int) -> M () get2 ds f = do (a, b) <- (,) <$> getArg ds <*> getArg ds
modify (to8 (f a b) ++)

get1 :: DataSize -> (Int -> Int) -> M ()
get1 ds f = do
a <- getArg ds
modify (to8 (f a) ++)

getArg :: DataSize -> M Int
getArg OneByte = takeByte
getArg TwoBytes = to16 <$> takeByte <*> takeByte to8 :: Integral a => a -> [a] to8 = unfoldr f f :: Integral a => (a -> Maybe (a, a)) f bs = case bs == 0 of True -> Nothing False -> Just$ (rem bs 255, div bs 255)

to16 a b = (255 * a) + b

takeByte :: M Int
takeByte = gets head <* modify tail

• Operating on the most general type is an option, and works for integers; could you show how you'd generalise this to floats and doubles, though? Aug 1 '14 at 19:10
• I was under the impression you were using discrete values? If you mean how would you do it with Word8 etc then I assume you can do similar sorts of things with shifting 8 bits rather than doing div 255. Aug 1 '14 at 20:11
• Well, the Word8 vs Int thing isn't going to matter much (seeing as you only use the bottom 8 bits, anyway). However, I don't see how to easily extend your solution to allow adding two doubles, which mine does. Aug 1 '14 at 22:28
• I never suggested it did but I didn't realise from your specification you wanted it to. Aug 2 '14 at 11:38