Re: [Haskell-cafe] Interpreter with Cont

On Sat, Nov 19, 2011 at 3:29 PM, Felipe Almeida Lessa wrote:

On Sat, Nov 19, 2011 at 6:08 PM, Tim Baumgartner wrote:

...

I have not yet gained a good understanding of the continuation monad, but I wonder if it could be used here. What would a clean solution look like? Perhaps there are other things that need to be changed as well?

Your 'Interaction' data type is actually an instance of the more general "operational monad" (as named by Heinrich Apfelmus) or "prompt monad" (as named by Ryan Ingram).

Both of which are just disguised free monads. For reference: data Free f a = Val a | Wrap (f (Free f a)) foldFree :: Functor f => (a -> b) -> (f b -> b) -> Free f a -> b foldFree v w (Val a) = v a foldFree v w (Wrap t) = w $ fmap (foldFree v w) t instance Functor f => Monad (Free f) where return = Val m >>= f = foldFree f Wrap m To use Free, just find the signature functor for Interaction by replacing the recursive instances with a new type variable, data InteractionF a b x = ExitF b | OutputF b x | InputF (a -> x) instance Functor (InteractionF a b) where fmap f (ExitF b) = ExitF b fmap f (OutputF b x) = OutputF b (f x) fmap f (InputF g) = InputF (f . g) roll :: InteractionF a b (Interaction a b) -> Interaction a b roll (ExitF b) = Exit b roll (OutputF b x) = Output b x roll (InputF g) = Input g type InteractionM a b = Free (InteractionF a b) runM :: InteractionM a b b -> Interaction a b runM = foldFree Exit roll exit :: b -> InteractionM a b c exit b = Wrap (ExitF b) output :: b -> InteractionM a b () output b = Wrap (OutputF b (Val ())) input :: InteractionM a b a input = Wrap (InputF Val) -- Dave Menendez http://www.eyrie.org/~zednenem/