Re: [Haskell-cafe] Algebraic Effects?

On Sep 18, 2018, at 2:06 AM, Alexis King wrote:

- The ecosystem of EE libraries is a mess.

Yes, it is rather unclear to me whether exploring any of these is is worth the effort, if so which, or how to use them given sometimes scant documentation.

There are extensible-effects, freer, freer-effects, freer-simple, and others. As far as I can tell, extensible-effects is based on free monads, and freer and freer-effects are both unmaintained.

I took a quick look at: https://hackage.haskell.org/package/extensible-0.4.10. The author claims good performance: https://www.schoolofhaskell.com/user/fumieval/extensible/the-world-s-fastest... I've not tried any benchmarks or yet any non-trivial code using this library. The documentation is rather minimal, but I got the below to compile and run: {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} module Main where import Control.Monad.Reader (ask) import Control.Monad.State (get, put) import Control.Monad.Writer (tell) import Control.Monad.IO.Class (liftIO) import Data.Monoid (Sum(..)) import Data.Extensible import Data.Extensible.Effect.Default type IOeff = "IO" :> IO type KitchenSink a = Eff '[ReaderDef Int, StateDef Int, WriterDef (Sum Int), IOeff] a type JustIO a = Eff '[IOeff] a type Result a = ((a, Int), Sum Int) handler :: KitchenSink a -> JustIO (Result a) handler = runWriterDef . flip runStateDef 3 . flip runReaderDef 5 main :: IO () main = do x <- retractEff $ handler $ do liftIO $ putStrLn "running" s <- get r <- ask tell (Sum s) tell (Sum $ s + 1) put $! s + r return $ "magic" print x it outputs: running (("magic",8),Sum {getSum = 7}) With this library, at least when building effects out of mtl transformers, the order of the effects in the Eff type declaration has to match in reverse order the composition of the "runFooDef" functions. That is, the types: Eff '[ReaderDef r, StateDef s] and Eff '[StateDef s, ReaderDef r] are not the same. Perhaps this is a feature?

My recommendation: if the performance of using EE is acceptable in your application AND you are willing to pay the cost of less ecosystem support (which in practice means needing to write adapters to mtl style libraries and having access to less documentation), I would strongly recommend the freer-simple extensible effect library. MASSIVE DISCLAIMER: I am the author and maintainer of freer-simple! However, I have a few reasons to believe I am not wholly biased:

Thanks. I'll take a look. Any comments on similarities to or differences from the "extensible" package above?

The distinguishing features of freer-simple are better documentation

Barring major downsides, that's a compelling difference.

and a dramatically different (and hopefully easier to understand) API for defining new effects compared to other extensible effects libraries. For details, see the freer-simple module documentation on Hackage here:

https://hackage.haskell.org/package/freer-simple/docs/Control-Monad-Freer.ht...

If you have any further questions, I’m happy to answer them, but this email is long enough already! Hopefully it isn’t too overwhelming.

Much appreciated. Still trying to figure out whether to look into this further. My project runs a concurrent computation for many hours, allocating and freeing terabytes of memory over its lifetime: 5,019,533,368,664 bytes allocated in the heap 162,945,132,824 bytes copied during GC 73,229,680 bytes maximum residency (3421 sample(s)) 4,150,592 bytes maximum slop 356 MB total memory in use (83 MB lost due to fragmentation) One concern for me is whether using Effects is likely to cause more allocations and work for the GC, or is the memory pressure about the same? -- Viktor.