[Haskell-cafe] Interpreter with Cont

David Menendez dave at zednenem.com
Mon Nov 21 06:08:56 CET 2011

On Sat, Nov 19, 2011 at 3:29 PM, Felipe Almeida Lessa
<felipe.lessa at gmail.com> wrote:
> On Sat, Nov 19, 2011 at 6:08 PM, Tim Baumgartner
> <baumgartner.tim at googlemail.com> 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 <dave at zednenem.com>

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