Note that using ConstraintKinds in this way has two related downsides:
This is great, thank you!
@tbidne, based on your report of your experience I made some improvements.
withEffToIO_, which is the simplest IO unlifting operation.EffReader instance for MonadUnliftIO that withEffToIO_ should be preferred.makeOp and useImplUnder and recommend them as the way of making dynamic effects. This is more uniform than before, because it extends to dynamic effects that take handles as arguments.Whoa, unused constraint warnings? That’s amazing! One of my biggest struggles with effectful is that I don’t realize when there are unused effects in the constraints. I’m going to have a wonderful time enabling that warning and laughing maniacly while sweeping all the redundant effects from the code base where I use effects. 
On a more serious note, I’d argue against grouping together as well. Listing all effects in a function, be it constraint or arguments, will tell you a lot about what a function, and its transitive calls will do. The absence of an effect tells you what kind of stuff a function won’t do. Questions like “will this touch the database?” would otherwise be answered by a lot of go-to-source.
Having a lot of effects in a type signature could still say something about the cohesion of the function. Maybe it just does too much, and some of the effect uses can be extracted out to another function. To me, effects push (but do not force!) me to think about that more. That does not globally apply though. The functions closest to your runEff will likely still be sitting on a big pile of effects.
I am interested in how the so-called interpose operation can be used in bluefin.
The interpose operation plays a central role in dynamic effects.
It allows for locally modifying the behavior of an effect handler within the scope enclosed by interpose, and can be considered a kind of generalization of the Reader’s local operation.
When users hear that “dynamic effects are possible,” they will expect the functionality provided by interpose.
interpose is usually used as follows:
modifyPlus1 :: Reader Int :> es => Eff es a -> Eff es a
modifyPlus1 = interpose \case
Ask -> (+1) <$> ask
Local f m -> local f m
main = runReader 0 $ modifyPlus1 $ print =<< ask
-- > main
-- 1
In practical terms, for example, it can be used to add logging locally after the fact to an effect:
logWriteDB = interpose \case
WriteDB ... -> do
writeDB ...
log "Wrote to the DB"
Now, in effectful, there is an interpose function at Effectful.Dispatch.Dynamic. On the other hand, it seems that bluefin currently does not have this functionality for general effects, but if you have any ideas, please let me know.
My guess is that (considering that bluefin explicitly propagates evidence via arguments* instead of implicitly holding the environment in a ReaderT IO), in bluefin it would theoretically take the following form:
modifyPlus1
:: Reader Int :> es
=> Reader Int e
-> (Reader Int e -> Eff (e :& es) a)
-> Eff (e :& es) a
modifyPlus1 r f = f $ r{ask = (+1) <$> ask r}
newtype Reader r e = MkReader { ask :: ? r } -- Not quite sure what to do here...
* It seems I had a slight misunderstanding… What bluefin carries around isn’t the handler functions, but just an IORef, right?
I’m not sure that example is particularly interesting, because it only modifies ask. If you want to modify ask you can just use local! It seems that it is the examples that modify local that are interesting. such as your example at What is a higher-order effect? - #18 by ymdfield.
I’m not sure I understand fully what you mean, but what the handle actually is depends on the effect in question. You can see them all in the source. For example, State s is an IORef s, Reader is a State, Exception e is a forall a. e -> IO a that throws an exception, Coroutine a b is a function a -> Eff e b, etc., etc.
Thank you for your explanation.
Yes, my explanation wasn’t very good here. modifyPlus1 is just a toy example to show how to use interpose itself, and you’re right that in this case, simply using local is sufficient. Rather, what I mainly should have asked about was logWriteDB. This is close to the code I showed in another thread, and it’s something used to take logs in a local, monkey-patch-like manner. By using interpose in this way, you can set up hooks for general effects in a unified manner, which I consider one of the convenient features of the effect system. So I’m hoping we can do this in bluefin as well. I’ll read the responses in that thread and think about it a bit more. Thank you.
It seems I was mistaken about it being just an IORef. Good to hear that it depends on the effect (especially interesting in the case of coroutines). Thanks!
If you provide a simple, but fully-working, example then I’ll try to replicate it in Bluefin.
I tried writing it in effectful-2.5:
{-# LANGUAGE GHC2021 #-}
{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
module Main where
import Effectful (Eff, Effect, IOE, MonadIO (liftIO), runEff, type (:>))
import Effectful.Dispatch.Dynamic (interpose_, interpret_)
import Effectful.TH (makeEffect)
import Prelude hiding (log)
data Log :: Effect where
Log :: String -> Log m ()
makeEffect ''Log
runLog :: (IOE :> es) => Eff (Log ': es) a -> Eff es a
runLog = interpret_ \(Log msg) -> liftIO $ putStrLn $ "[INFO] " <> msg
type DBValue = String
data DB :: Effect where
InsertDB :: DBValue -> DB m ()
makeEffect ''DB
runDummyDB :: (IOE :> es) => Eff (DB ': es) a -> Eff es a
runDummyDB = interpret_ \case
InsertDB value -> liftIO $ putStrLn $ "[DummyDB.InsertDB] " <> value
hookLoggingInsertDBLocally :: (DB :> es, Log :> es) => Eff es a -> Eff es a
hookLoggingInsertDBLocally = interpose_ \(InsertDB value) -> do
insertDB value
log $ "Wrote to the DB: " <> value
suppressLog :: (Log :> es) => Eff es a -> Eff es a
suppressLog = interpose_ \(Log _) -> pure ()
-- > main
-- [DummyDB.InsertDB] value1
-- [DummyDB.InsertDB] value2
-- [DummyDB.InsertDB] value3
-- [INFO] Wrote to the DB: value3
-- [DummyDB.InsertDB] value4
-- [INFO] Wrote to the DB: value4
-- [DummyDB.InsertDB] value5
-- [DummyDB.InsertDB] value6
-- [DummyDB.InsertDB] value9
-- [DummyDB.InsertDB] value10
main :: IO ()
main = runEff . runLog . runDummyDB $ do
insertDB "value1"
insertDB "value2"
hookLoggingInsertDBLocally do
insertDB "value3"
insertDB "value4"
insertDB "value5"
insertDB "value6"
suppressLog do
hookLoggingInsertDBLocally do
insertDB "value9"
hookLoggingInsertDBLocally do
suppressLog do
insertDB "value10"
Note that not only are we locally hooking logging with hookLoggingInsertDBLocally, but we’re also using interpose with suppressLog to locally suppress logging. Not only can we add processing, but we can also eliminate the effect itself. From the user’s perspective, we can write code with the intuition of “editing” a program where effects are written sequentially, without resorting to ad-hoc methods like managing state with Reader or State and writing branches with if statements. Imagine that this intuition can be used universally and directly, without being limited to specific effects or use cases. Personally, I think this is somewhat akin to metaprogramming.
If we achieve this in bluefin, I guess it would take the form of an idiom or design pattern where we locally swap or rewrite the value-level effects passed as arguments in some way. However, there might be aspects of bluefin that I’m not aware of that could come into play.
Also, you might not find such “hooks” to be particularly practical. Personally, I find these hooks to be quite convenient, especially for implementing “cross-cutting” concerns like logging, tracing, and profiling, although I acknowledge that I might have some personal bias.
Regarding the part where I said “universally”, ultimately this means being able to write a function that is polymorphic over effects, allowing you to add logging to any arbitrary effect, like the following:
hookLoggingLocallyForAnyEffect ::
forall e es a.
(e :> es, Log :> es, DispatchOf e ~ 'Dynamic, ShowEffect e) =>
Eff es a -> Eff es a
hookLoggingLocallyForAnyEffect = interpose_ @e \e -> do
r <- send e
log $ "Effect executed: " <> showEffect e
pure r
…Strictly speaking, this doesn’t typecheck at the send part (I’m a beginner with effectful…), but it should be possible in other libraries, so I think we can do something similar in effectful. Note that ShowEffect is a type class that would need to be defined separately.
…Strictly speaking, this doesn’t typecheck at the
sendpart (I’m a beginner witheffectful…),
It will work with Add passthrough by arybczak · Pull Request #274 · haskell-effectful/effectful · GitHub if you do this:
hookLoggingLocallyForAnyEffect ::
forall e es a.
(e :> es, Log :> es, DispatchOf e ~ 'Dynamic, ShowEffect e) =>
Eff es a -> Eff es a
hookLoggingLocallyForAnyEffect = interpose @e \env e -> do
r <- passthrough env e
log $ "Effect executed: " <> showEffect e
pure r
For the record, effectful’s interpose also allows one to write purely dynamic handler for Reader:
runPureReader :: r -> Eff (Reader r : es) a -> Eff es a
runPureReader r0 = interpret (handler r0)
where
handler :: r -> EffectHandler (Reader r) handlerEs
handler r env = \case
Ask -> pure r
Local f m -> localSeqUnlift env $ \unlift -> do
unlift $ interpose (handler $ f r) m
so it’s very powerful.
Thank you! I found that very helpful for writing this type of code in Bluefin. The structure of the implementation carries over directly, and I can confirm it has the exact same behaviour. One notable difference is that I explicitly use the H “mutable handle” (I’ll come up with a better name) rather than defining an interpose primitive.
basicLog :: (e :> es) => BasicLog e -> String -> Eff es ()
basicLog b s = makeOp (basicLogImpl (mapHandle b) s)
runBasicLog ::
(e1 :> es) =>
IOE e1 ->
(forall e. BasicLog e -> Eff (e :& es) a) ->
Eff es a
runBasicLog io k =
useImplIn
k
MkBasicLog
{ basicLogImpl = \msg ->
effIO io $ putStrLn $ "[INFO] " <> msg
}
type DBValue = String
newtype BasicDB es = MkBasicDB
{ insertBasicDBImpl ::
forall e.
DBValue ->
Eff (e :& es) ()
}
instance Handle BasicDB where
mapHandle b =
MkBasicDB
{ insertBasicDBImpl = \s -> useImplUnder (insertBasicDBImpl b s)
}
insertBasicDB :: (e :> es) => BasicDB e -> DBValue -> Eff es ()
insertBasicDB d v = makeOp (insertBasicDBImpl (mapHandle d) v)
runDummyBasicDB ::
(e1 :> es) =>
IOE e1 ->
(forall e. BasicDB e -> Eff (e :& es) a) ->
Eff es a
runDummyBasicDB io k =
useImplIn
k
MkBasicDB
{ insertBasicDBImpl = \value ->
effIO io $ putStrLn $ "[DummyDB.InsertDB] " <> value
}
type Log = H BasicLog
log :: (e :> es) => Log e -> String -> Eff es ()
log l msg = do
b <- askH l
basicLog b msg
runLog ::
(e1 :> es) =>
IOE e1 ->
(forall e. Log e -> Eff (e :& es) a) ->
Eff es a
runLog io k = do
runBasicLog io $ \l -> do
runH l $ \h -> do
useImplIn k (mapHandle h)
type DB = H BasicDB
insertDB :: e :> es => DB e -> DBValue -> Eff es ()
insertDB d v = do
b <- askH d
insertBasicDB b v
runDummyDB ::
(e1 :> es) =>
IOE e1 ->
(forall e. DB e -> Eff (e :& es) a) ->
Eff es a
runDummyDB io k = do
runDummyBasicDB io $ \d -> do
runH d $ \h -> do
useImplIn k (mapHandle h)
hookLoggingInsertDBLocally ::
(e1 :> es, e2 :> es) =>
DB e1 ->
Log e2 ->
Eff es a ->
Eff es a
hookLoggingInsertDBLocally d l k = do
orig <- askH d
localH
d
( MkBasicDB
{ insertBasicDBImpl = \value -> do
insertBasicDB orig value
log l $ "Wrote to the DB: " <> value
}
)
k
suppressLog ::
(e1 :> es) =>
Log e1 ->
Eff es a ->
Eff es a
suppressLog l k = do
localH
l
(MkBasicLog {basicLogImpl = \_ -> pure ()})
k
-- > main
-- [DummyDB.InsertDB] value1
-- [DummyDB.InsertDB] value2
-- [DummyDB.InsertDB] value3
-- [INFO] Wrote to the DB: value3
-- [DummyDB.InsertDB] value4
-- [INFO] Wrote to the DB: value4
-- [DummyDB.InsertDB] value5
-- [DummyDB.InsertDB] value7
-- [DummyDB.InsertDB] value9
-- [DummyDB.InsertDB] value10
main :: IO ()
main =
runEff $ \io -> runLog io $ \l -> runDummyDB io $ \d -> do
insertDB d "value1"
insertDB d "value2"
hookLoggingInsertDBLocally d l $ do
insertDB d "value3"
insertDB d "value4"
insertDB d "value5"
insertDB d "value7"
suppressLog l $ do
hookLoggingInsertDBLocally d l $ do
insertDB d "value9"
hookLoggingInsertDBLocally d l $ do
suppressLog l $ do
insertDB d "value10"
Full code. You’ll need to use this branch of Bluefin. H does not exist in a published version.
If you stick handlers in State which is an IORef in disguise, then everything breaks horribly once you introduce multiple threads into the equation.
This is why internals of effectful are more complicated. Because it handles this automatically behind the scenes.
Yes, that’s right, although to be precise I’d say “introduce concurrency” rather than “introduce multiple threads” (not that it really weakens your argument) because Bluefin already uses threads for some functionality (such as connectCoroutines) but the usage is synchronized, not concurrent, thus there is no problem.
We’ll see how big a problem this is in practice. If it becomes a problem then I may have to move to an effectful-style implementation where the mutable state is stored in Eff, and can thus be cloned when spawning a new thread. However, I am hopeful that I will discover new concurrency patterns that don’t require cloning all state, because that may be wasteful, especially when there are a lot of States in scope when dealing with graph traversals and such.
Thank you for converting the example into bluefin! Indeed, I think this corresponds well with the original code. It doesn’t seem like any ad-hoc patterns specialized for the case are being used; it looks generic. Regarding hookLoggingLocallyForAnyEffect, I’ve come to think that perhaps it might be possible to handle it well by using something like a type class for the data types of compound effects.
For the record, here’s how to use it (I haven’t tested it because passthrough wasn’t available in effectful-2.5, but typechecks except for the passthrough part, so it should work):
-- > main
-- [DummyDB.InsertDB] value1
-- [DummyDB.InsertDB] value2
-- [INFO] Effect executed: InsertDB value2
-- [DummyDB.InsertDB] value3
-- [INFO] Effect executed: InsertDB value3
-- [DummyDB.InsertDB] value4
main :: IO ()
main = runEff . runLog . runDummyDB $ do
insertDB "value1"
hookLoggingLocallyForAnyEffect @DB do
insertDB "value2"
insertDB "value3"
insertDB "value4"
class ShowEffect e where
showEffect :: e m a -> String
instance ShowEffect DB where
showEffect (InsertDB x) = "InsertDB " <> x
hookLoggingLocallyForAnyEffect ::
forall e es a.
(e :> es, Log :> es, DispatchOf e ~ 'Dynamic, ShowEffect e) =>
Eff es a ->
Eff es a
hookLoggingLocallyForAnyEffect = interpose @e \env e -> do
r <- passthrough env e
log $ "Effect executed: " <> showEffect e
pure r
That’s really helpful. Thanks!
I wonder what other effects are like that, so far examples are logging and tracing. For me, those could come built-in with Eff itself. I really don’t see much value in having this type Log :> es => Eff es () compared to Eff es (). Looking at the 23-effect function, I’d throw structured concurrency to the pile as well. Why would I care if a function spawns threads if they can’t escape its scope? Those are innocent effects that don’t help me understand what a function is doing and what important things it is messing with. Then Eff could provide concurrency primitives that are safe with regard to this built-in functionality. Making other effects safe would be up to the users, they created them, so let them take care of it.
This is a problem for a general purpose effects library, as folks want different loggers and tracers, but as a basis for building applications, I think it would be just fine.
I’ve added this litmus test to the test suite.
I really don’t see much value in having this type
Log :> es => Eff es ()compared toEff es ()
I think it depends exactly what Log does. Given that forall es. Eff es r can be converted to r (via runPureEff) if Log is present but not tracked in the types then it had really better be pretty benign. If it’s as benign as Debug.Trace then that’s fine, of course, but then there’s not a lot of point to it at all: you may as well be using Debug.Trace. If it’s logging to files or network connections then that’s much more dubious. In such a case I’d rather have it tracked in the type.
47 posts were split to a new topic: Why use effects?
There would be no runPureEff for this. There would be only runMyAppEff.
The point is, it doesn’t. Or, rather, if I decide, for my application, that logging is benign (whatever it does), then I might just not want to track it.
This is not a very fleshed out idea, but I feel like something being an effect in theory/being provided as such by a library, doesn’t make it an (important) effect for me. How to generalize this to a useful library? I have no idea.