Freer: Extensible Effects with Freer Monads
freer | 0.2.4.1 |
---|---|
Maintainer | Allele Dev (allele.dev@gmail.com) |
Funding | $0 USD |
Copyright | Copyright (C) 2017 Allele Dev |
License | BSD3 |
Freer is an implementation of "Freer Monads, More Extensible Effects". Much of the implementation is a repackaging and cleaning up of the reference materials provided here.
Features
The key features of Freer are:
- An efficient effect system for Haskell as a library
- Implementations for several common Haskell monad instances:
- Reader
- Writer
- State
- StateRW: State in terms of Reader/Writer
- Trace
- Exception
- Core components for defining your own Effects
Example: Teletype DSL
Here's what using Freer looks like:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
module Teletype where
import Eff
import Eff.Internal
import System.Exit hiding (ExitSuccess)
--------------------------------------------------------------------------------
-- Effect Model --
--------------------------------------------------------------------------------
data Teletype s where
PutStrLn :: String -> Teletype ()
GetLine :: Teletype String
ExitSuccess :: Teletype ()
putStrLn' :: Member Teletype r => String -> Eff r ()
putStrLn' = send . PutStrLn
getLine' :: Member Teletype r => Eff r String
getLine' = send GetLine
exitSuccess' :: Member Teletype r => Eff r ()
exitSuccess' = send ExitSuccess
--------------------------------------------------------------------------------
-- Effectful Interpreter --
--------------------------------------------------------------------------------
runTeletype :: Eff '[Teletype] w -> IO w
runTeletype (Val x) = return x
runTeletype (E u q) = case extract u of
(PutStrLn msg) -> putStrLn msg >> runTeletype (qApp q ())
GetLine -> getLine >>= \s -> runTeletype (qApp q s)
ExitSuccess -> exitSuccess
--------------------------------------------------------------------------------
-- Pure Interpreter --
--------------------------------------------------------------------------------
runTeletypePure :: [String] -> Eff '[Teletype] w -> [String]
runTeletypePure inputs req =
reverse . snd $ run (handleRelayS (inputs, []) (\s _ -> pure s) go req)
where
go :: ([String], [String])
-> Teletype 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)
Contributing
Contributions are welcome! Documentation, examples, code, and feedback - they all help.
Be sure to review the included code of conduct. This project adheres to the Contributor's Covenant. By participating in this project you agree to abide by its terms.
This project is not currently actively maintained.
Developer Setup
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 GHC
- stack build
- stack clean
- stack haddock : builds documentation
- stack test
- stack bench
- stack ghci : start a REPL instance
Licensing
This project is distrubted under a BSD3 license. See the included LICENSE file for more details.
Acknowledgements
This package would not be possible without the paper and the reference implementation. In particular:
- Data.Open.Union maps to OpenUnion41.hs
- Data.FTCQueue maps to FTCQueue1
- Eff* maps to Union1.hs
There will be deviations from the source.