For the open union used in extensible effects, apart from using the
Typeable mechanism, is there a more protected way to implement
the open sum type?
I managed to modified the Member class given in the paper, but
ended up having to use the vague OverlappingInstance. That's not
quite what I hope. I'm not even sure whether the instance `Member t (t :>
r)`
is more specific than `Member t (t' :> r)`.
--
suhorng
{-# LANGUAGE KindSignatures, TypeOperators, GADTs, FlexibleInstances,
FlexibleContexts, MultiParamTypeClasses, OverlappingInstances
#-}
-- FlexibleContexts is for Show instance of Union
import Data.Functor
import Control.Applicative -- for several functor instances
-- open union
infixr 2 :>
data (a :: * -> *) :> b
data Union r v where
Elsewhere :: Functor t' => Union r v -> Union (t' :> r) v
Here :: Functor t => t v -> Union (t :> r) v
class Member t r where
inj :: Functor t => t v -> Union r v
prj :: Functor t => Union r v -> Maybe (t v)
instance Member t (t :> r) where
inj tv = Here tv
prj (Here tv) = Just tv
prj (Elsewhere _) = Nothing
-- Note: overlapped by letting t' = t
instance (Functor t', Member t r) => Member t (t' :> r) where
inj tv = Elsewhere (inj tv)
prj (Here _) = Nothing
prj (Elsewhere u) = prj u
decomp :: Functor t => Union (t :> r) v -> Either (Union r v) (t v)
decomp (Here tv) = Right tv
decomp (Elsewhere u) = Left u
-- Auxiliary definitions for tests
data Void
newtype Func a = Func a
instance Show (Union Void a) where
show _ = undefined
instance (Show (t v), Show (Union r v)) => Show (Union (t :> r) v) where
show (Here tv) = "Here " ++ show tv
show (Elsewhere u) = "Elsewhere " ++ show u
instance Functor Func where
fmap f (Func x) = Func (f x)
instance Show a => Show (Func a) where
show (Func a) = show a
type Stk = Maybe :> Either Char :> Func :> Void
type Stk' = Either Char :> Func :> Void -- used in `deTrue`, `deFalse`
unTrue :: Union Stk Bool
unTrue = inj (Func True)
unFalse :: Union Stk Bool
unFalse = inj (Just False)
-- `Func` is repeated
un5 :: Union (Maybe :> Func :> Either Char :> Func :> Void) Int
un5 = inj (Func 5)
maybe2 :: Maybe (Func Int)
maybe2 = prj un5
maybeTrue :: Maybe (Func Bool)
maybeTrue = prj unTrue
maybeFalse1 :: Maybe (Func Bool)
maybeFalse1 = prj unFalse
maybeFalse2 :: Maybe (Maybe Bool)
maybeFalse2 = prj unFalse
deTrue :: Either (Union Stk' Bool) (Maybe Bool)
deTrue = decomp unTrue
deFalse :: Either (Union Stk' Bool) (Maybe Bool)
deFalse = decomp unFalse
2013/8/22 Alberto G. Corona
The paper is very interesting:
http://www.cs.indiana.edu/~sabry/papers/exteff.pdf
It seems that the approach is mature enough and it is better in every way than monad transformers, while at the same time the syntax may become almost identical to MTL for many uses.
I only expect to see the library in Hackage with all the blessings, and with all the instances of the MTL classes in order to make the transition form monad transformers to ExtEff as transparent as possible
2013/8/22
Perhaps effect libraries (there are several to choose from) could be a better answer to Fork effects than monad transformers. One lesson from the recent research in effects is that we should start thinking what effect we want to achieve rather than which monad transformer to use. Using ReaderT or StateT or something else is an implementation detail. Once we know what effect to achieve we can write a handler, or interpreter, to implement the desired operation on the World, obeying the desired equations. And we are done.
For example, with ExtEff library with which I'm more familiar, the Fork effect would take as an argument a computation that cannot throw any requests. That means that the parent has to provide interpreters for all child effects. It becomes trivially to implement:
Another example would be a child that should not be able to throw errors as opposed to the parent thread. It is possible to specify which errors will be allowed for the child thread (the ones that the parent will be willing to reflect and interpret). The rest of errors will be statically prohibited then.
instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) where fork (ReaderT f) = liftIO . forkIO . f =<< getSink
This is a good illustration of too much implementation detail. Why do we need to know of (Sink p) as a Reader layer? Would it be clearer to define an Effect of sending to the socket? Computation's type will make it patent the computation is sending to the socket. The parent thread, before forking, has to provide a handler for that effect (and the handler will probably need a socket).
Defining a new class for each effect is possible but not needed at all. With monad transformers, a class per effect is meant to hide the ordering of transformer layers in a monad transformer stack. Effect libraries abstract over the implementation details out of the box. Crutches -- extra classes -- are unnecessary. We can start by writing handlers on a case-by-case basis. Generalization, if any, we'll be easier to see. From my experience, generalizing from concrete cases is easier than trying to write a (too) general code at the outset. Way too often, as I read and saw, code that is meant to be reusable ends up hardly usable.
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-- Alberto.
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