Re: [Haskell-cafe] Yet Another Forkable Class
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
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: 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.
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.
To be honest I'm not so sure about these "effects"... Simply the fact that
the Member class needs -XOverlappingInstances means that we cannot have
duplicate or polymorphic effects. It will arbitrarily pick the first match
in the former and fail to compile in the latter case.
Furthermore I don't really understand the way open sums are implemented.
These unions should be disjoint, but the way they're implemented in the
paper they try to be "true" unions which cannot be done as that would need
type equality (-XOverlappingInstances is a hack around this)
A correct disjoint open sum would behave well with duplicate and
polymorphic types in the type list. For example we should be able to
project the open sum equivalent of Either String String into the second
String but we cannot with the implementation in the paper. This means we
need to ~index~ the type list instead of picking the result type and
"trying for equality" with each entry. Something like this:
http://lpaste.net/92069
Of course this is very inconvenient and simply replaces the monad
transformers' lifts with a static index into the "effect" list.
In general I think there is no convenient way of stacking effects that is
also type safe. At some point we have to disambiguate which effect we are
trying to use one way or the other. The implementation in the paper simply
picks a heuristic and chooses the first effect that seems to match and
discards the others.
On 22 August 2013 12:15, 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.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
-- Alberto.
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I must stress that OpenUnion1.hs described (briefly) in the paper is only one implementation of open unions, out of many possible. For example, I have two more implementations. A year-old version of the code implemented open unions *WITHOUT* overlapping instances or Typeable. http://okmij.org/ftp/Haskell/extensible/TList.hs The implementation in the paper is essentially the one described in the full HList paper, Appendix C. The one difference is that the HList version precluded duplicate summands. Adding the duplication check to OpenUnion1 takes three lines of code. I didn't add them because it didn't seem necessary, or even desired. I should further stress, OverlappingInstances are enabled only within one module, OpenUnion1.hs. The latter is an internal, closed module, not meant to be modified by a user. No user program needs to declare OverlappingInstances in its LANGUAGES pragma. Second, OverlappingInstances are used only within the closed type class Member. This type class is not intended to be user-extensible; the programmer need not and should not define any more instances for it. The type class is meant to be closed. So Member emulates closed type families implemented in the recent version of GHC. With the closed type families, no overlapping instances are needed.
Simply the fact that the Member class needs -XOverlappingInstances means that we cannot have duplicate or polymorphic effects. It will arbitrarily pick the first match in the former and fail to compile in the latter case. Of course we can have duplicate layers. In that case, the dynamically closest handler wins -- which sounds about right (think of reset in delimited control). The file Eff.hs even has a test case for that, tdup. BTW, I'm not sure of the word 'pick' -- the Member class is a purely compile-time constraint. It doesn't do any picking -- it doesn't do anything at all at run-time.
For example we should be able to project the open sum equivalent of Either String String into the second String but we cannot with the implementation in the paper. You inject a String or a String, and you will certainly project a String (the one your have injected). What is the problem then? You can always project what you have injected. Member merely keeps track of what types could possibly be injected/projected. So, String + String indeed should be String.
By polymorphic effects you must mean first-class polymorphism (because the already implemented Reader effect is polymorphic in the environment). First of all, there are workarounds. Second, I'm not sure what would be a good example of polymorphic effect (aside from ST-monad-like).
To be honest I'm not so sure about these "effects"... Haskell Symposium will have a panel on effect libraries in Haskell. It seems plausible that effects, one way or the other, will end ip in Haskell. Come to Haskell Symposium, tell us your doubts and concerns. We want to hear them.
On Fri, 2013-08-23 at 08:06 +0000, oleg@okmij.org wrote:
It will arbitrarily pick the first match in the former and fail to compile in the latter case. Of course we can have duplicate layers. In that case, the dynamically closest handler wins -- which sounds about right (think of reset in delimited control).
Did anyone ever consider using type-level literals (strings) to 'name' effects (or transformer layers when using monad stacks)? A stupid example (OTOH) could be updateStats :: (Member (State "min" Int) r, Member (State "max" Int) r) => Int -> Eff r () updateStats i = do min <- askMin max <- askMax when (i < min) $ putMin i when (i > max) $ putMax i where askMin :: Member (State "min" Int) r => Eff r Int askMin = ask putMax :: Member (State "max" Int) r => Int -> Eff r () putMax = put -- askMax, putMin accordingly Using constraint synonyms/ConstraintKinds (e.g. type StateMax r = Member (State "max" Int) r) might reduce some notation overhead. Just a thought. Nicolas
Hi.
On 23 August 2013 13:29, Nicolas Trangez
Did anyone ever consider using type-level literals (strings) to 'name' effects (or transformer layers when using monad stacks)?
Edwin Brady had this in his effects library in Idris. http://www.idris-lang.org/documentation/effects/ Ozgur.
having recently taken over as maintainer for the extensible-effects library, i'm looking to address some of the current implementation concerns. specifically: 1] the use/need for Typeable in Data.OpenUnion oleg@okmij.org writes:
I must stress that OpenUnion1.hs described (briefly) in the paper is only one implementation of open unions, out of many possible. For example, I have two more implementations. A year-old version of the code implemented open unions *WITHOUT* overlapping instances or Typeable. http://okmij.org/ftp/Haskell/extensible/TList.hs
how does the TList.hs implementation compare with, say, OpenUnion2.hs? neither require OverlappingInstances, and the TList implementation also does away with the Typeable constraint. are there reasons why it might not make sense to use TList.hs as the only/default implementation of Data.OpenUnion? 2] scope for impredicative/first-class polymorphism
By polymorphic effects you must mean first-class polymorphism (because the already implemented Reader effect is polymorphic in the environment). First of all, there are workarounds.
what are the "workarounds" in question?
Second, I'm not sure what would be a good example of polymorphic effect (aside from ST-monad-like).
the paper mentioned "explicitly marking state, and providing an allocation system using monadic regions". is this related to http://okmij.org/ftp/Haskell/regions.html#light-weight and if so, what work needs to be done to apply those ideas to extensible-effects? -- Suhail
Hi Suhail,
On Fri, Nov 28, 2014 at 4:52 AM, Suhail Shergill
having recently taken over as maintainer for the extensible-effects library, i'm looking to address some of the current implementation concerns. specifically:
1] the use/need for Typeable in Data.OpenUnion how does the TList.hs implementation compare with, say, OpenUnion2.hs? neither require OverlappingInstances, and the TList implementation also does away with the Typeable constraint. are there reasons why it might not make sense to use TList.hs as the only/default implementation of Data.OpenUnion?
You need to write an instance of TCode for every different "effect" included in the union for the lookup to work. Check out this example usage of TList.hs: mkV :: Int -> ([] :> Maybe :> Void) Int mkV 1 = H [1,2,3] mkV 2 = T (H (Just 5)) mkV 3 = T (T (undefined :: Void Int)) -- | >>> test1 -- [Just [1,2,3],Nothing,Nothing] test1 :: [Maybe [Int]] test1 = map (prj . mkV) [1 .. 3] -- | >>> test2 -- [Nothing,Just (Just 5),Nothing] test2 :: [Maybe (Maybe Int)] test2 = map (prj . mkV) [1 .. 3] type instance TCode [] = Z type instance TCode Maybe = S Z If you instead had type instance TCode [] = Z type instance TCode Maybe = Z then test2 would not typecheck, and the type error doesn't suggest (to me) that the TCode instances are wrong. I think you're better off depending on a type family Eq :: Bool where Eq x x = True Eq x y = False Or the equivalent with overlapping instances if the code is supposed to work with ghc-7.6. Another objection about TList is that it is a linked list, so operations with types at the "end" of the union are probably relatively slow at runtime, since you end up pattern matching on "n" T constructors in some cases. It might be faster to have more of that traversal done at compile time as in: http://code.haskell.org/HList/Data/HList/Variant.hs Or with unions that use Typeable. I'm not sure about your other questions. Regards, Adam
adam vogt
You need to write an instance of TCode for every different "effect" included in the union for the lookup to work. ...
thanks for the explanation; the Includes instances make more sense now
I think you're better off depending on a
type family Eq :: Bool where Eq x x = True Eq x y = False
Or the equivalent with overlapping instances if the code is supposed to work with ghc-7.6.
yes, i've updated the code on hackage to fall back on the overlapping instances implementation for ghc-7.6
Another objection about TList is that it is a linked list, so operations with types at the "end" of the union are probably relatively slow at runtime, since you end up pattern matching on "n" T constructors in some cases.
that may just be a limitation for the code being written for ghc-7.4 iiuc
It might be faster to have more of that traversal done at compile time as in:
thanks for the additional reference. i've been meaning to read up on HList; perhaps this will be my segue
Or with unions that use Typeable.
actually with Typeable being kind polymorphic in ghc-7.8, the improved deriving code, -XAutoDeriveTypeable and -XStandaloneDeriving i don't see much of a drawback with the above approach.
I'm not sure about your other questions.
yes, they were more directed at oleg (in the cc), but figured others might have made their way through these waters before me, so it couldn't hurt to ask. cheers -- Suhail
First of all, thank you indeed for taking over as the maintainer of the extensible-effects library. Thank you for sending me your questions directly.
1] the use/need for Typeable in Data.OpenUnion
I must stress that OpenUnion1.hs described (briefly) in the paper is only one implementation of open unions, out of many possible. For example, I have two more implementations. A year-old version of the code implemented open unions *WITHOUT* overlapping instances or Typeable. http://okmij.org/ftp/Haskell/extensible/TList.hs
how does the TList.hs implementation compare with, say, OpenUnion2.hs? neither require OverlappingInstances, and the TList implementation also does away with the Typeable constraint. are there reasons why it might not make sense to use TList.hs as the only/default implementation of Data.OpenUnion?
Although TList doesn't require neither overlapping instances nor Typeable, it is not so easy to use since one has to `register' any new type or effects. To show that one can indeed implement the interface of OpenUnion.hs as it is *without* Typeable or overlapping instances, I have just written http://okmij.org/ftp/Haskell/extensible/OpenUnion4.hs It is meant to be a drop-in replacement for OpenUnion2.hs. At least for my Eff code, it does indeed act that way. Nothing in the code has to be changed save for the import declaration. On the down side, in OpenUnion4.hs, projections and injections take linear time in the size of the union. How much difference it really makes in practice is unclear (since the projection and injection operations can be computed statically). Only benchmarks could tell.
2] scope for impredicative/first-class polymorphism
By polymorphic effects you must mean first-class polymorphism (because the already implemented Reader effect is polymorphic in the environment). First of all, there are workarounds.
what are the "workarounds" in question?
Well, OpenUnion4 is one workaround. Since it does not use Typeable, there are no problems with (ST s)--like types.
the paper mentioned "explicitly marking state, and providing an allocation system using monadic regions". is this related to http://okmij.org/ftp/Haskell/regions.html#light-weight and if so, what work needs to be done to apply those ideas to extensible-effects?
I'm afraid that the answer is `just do it'. I thought about the implementation and jotted down some notes somewhere. There didn't seem to be any significant problems. Alas I hadn't have the time since to pursue this further. Thank you!
oleg@okmij.org writes:
First of all, thank you indeed for taking over as the maintainer of the extensible-effects library.
you're welcome! and thank you for the library in question to begin with; i intend to port the ideas to scala as well.
To show that one can indeed implement the interface of OpenUnion.hs as it is *without* Typeable or overlapping instances, I have just written http://okmij.org/ftp/Haskell/extensible/OpenUnion4.hs
thanks for the concrete code sample. also, iiuc, if i were to back-port this code to ghc versions 7.6 and before, i should be able to do away with dependence on closed type families replacing it with overlapping instances. correct? if so, i'll try and expose that point in the design space as well since that would permit us to use effects in the return type of other 'Eff's even in systems not supporting closed type families. currently we have this for ghc-7.8 as can be seen in the contrived test here: https://github.com/bfops/extensible-effects/blob/master/test/Test.hs#L163
It is meant to be a drop-in replacement for OpenUnion2.hs. At least for my Eff code, it does indeed act that way. Nothing in the code has to be changed save for the import declaration. On the down side, in OpenUnion4.hs, projections and injections take linear time in the size of the union. How much difference it really makes in practice is unclear (since the projection and injection operations can be computed statically). Only benchmarks could tell.
agreed. i am also contemplating breaking out open unions into an independent library. -- Suhail
On 02/12/14 17:31, oleg@okmij.org wrote:
To show that one can indeed implement the interface of OpenUnion.hs as it is *without* Typeable or overlapping instances, I have just written http://okmij.org/ftp/Haskell/extensible/OpenUnion4.hs
It is interesting to note that this is exactly the solution (to almost the same problem) that Andres Löh and I came up with last spring: http://bit.ly/1xgo7fQ Roman
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.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
-- Alberto.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
participants (9)
-
adam vogt -
Alberto G. Corona -
John ExFalso -
Nicolas Trangez -
oleg＠okmij.org -
Ozgur Akgun -
Roman Cheplyaka -
Suhail Shergill -
suhorng Y