Library freer-effects (actively maintained fork of
freer) is an implementation of
effect system for Haskell, which is based on the work of Oleg Kiselyov et al.:
Much of the implementation is a repackaging and cleaning up of the reference materials provided here.
The key features of Freer are:
- An efficient effect system for Haskell as a library.
- Implementations for several common Haskell monads as effects:
ReaderWriterStateStateRW: State in terms of Reader/Writer.TraceException
- Core components for defining your own Effects.
Here's what using Freer looks like:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
module Console where
import Control.Monad.Freer
import Control.Monad.Freer.Internal
import System.Exit hiding (ExitSuccess)
--------------------------------------------------------------------------------
-- Effect Model --
--------------------------------------------------------------------------------
data Console s where
PutStrLn :: String -> Console ()
GetLine :: Console String
ExitSuccess :: Console ()
putStrLn' :: Member Console r => String -> Eff r ()
putStrLn' = send . PutStrLn
getLine' :: Member Console r => Eff r String
getLine' = send GetLine
exitSuccess' :: Member Console r => Eff r ()
exitSuccess' = send ExitSuccess
--------------------------------------------------------------------------------
-- Effectful Interpreter --
--------------------------------------------------------------------------------
runConsole :: Eff '[Console] w -> IO w
runConsole (Val x) = return x
runConsole (E u q) =
case extract u of
PutStrLn msg -> putStrLn msg >> runConsole (qApp q ())
GetLine -> getLine >>= \s -> runConsole (qApp q s)
ExitSuccess -> exitSuccess
--------------------------------------------------------------------------------
-- Pure Interpreter --
--------------------------------------------------------------------------------
runConsolePure :: [String] -> Eff '[Console] w -> [String]
runConsolePure inputs req =
reverse . snd $ run (handleRelayS (inputs, []) (\s _ -> pure s) go req)
where
go :: ([String], [String])
-> Console v
-> (([String], [String]) -> Arr '[] v ([String], [String]))
-> Eff '[] ([String], [String])
go (is, os) (PutStrLn msg) q = q (is, msg : os) ()
go (i:is, os) GetLine q = q (is, os) i
go ([], _ ) GetLine _ = error "Not enough lines"
go (_, os) ExitSuccess _ = pure ([], os)You already have some mtl code and
are afraid that combining effects with your current tranformer stack would not
be possible? Package
freer-effects-extra has some
mtl-related and other goodies.
Contributions are welcome! Documentation, examples, code, and feedback - they all help.
The easiest way to start contributing is to install stack. Stack can install GHC/Haskell for you, and automates common developer tasks.
The key commands are:
stack setup– install required version of GHC compilerstack build– builds project, dependencies are automatically resolvedstack test– builds project, its tests, and executes the testsstack bench– builds project, its benchmarks, and executes the benchamksstack ghci– start a REPL instance with a project modules loadedstack cleanstack haddock– builds documentation
For more information about stack tool can be found in its
documentation.
This project is distrubted under a BSD3 license. See the included LICENSE file for more details.
Package freer-effects started as a fork of
freer authored by Allele Dev.
This package would not be possible without the paper and the reference implementation. In particular:
Data.OpenUnionmaps to OpenUnion51.hsData.FTCQueuemaps to FTCQueue1Control.Monad.Freer*maps to Eff1.hs
There will be deviations from the source.
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