# Freer: Extensible Effects with Freer Monads [](https://travis-ci.org/lexi-lambda/freer-simple)
# Description
The `freer-simple` library (a fork of [`freer-effects`](http://hackage.haskell.org/package/freer-effects)) is an implementation of an effect system for Haskell, which is based on the work of Oleg Kiselyov et al.:
- [Freer Monads, More Extensible Effects](http://okmij.org/ftp/Haskell/extensible/more.pdf)
- [Reflection without Remorse](http://okmij.org/ftp/Haskell/zseq.pdf)
- [Extensible Effects](http://okmij.org/ftp/Haskell/extensible/exteff.pdf)
Much of the implementation is a repackaging and cleaning up of the reference materials provided [here](http://okmij.org/ftp/Haskell/extensible/).
# Features
The key features of `freer-simple` are:
- An efficient effect system for Haskell as a library.
- Implementations for several common Haskell monads as effects:
- `Reader`
- `Writer`
- `State`
- `Trace`
- `Error`
- Core components for defining your own Effects.
# Example: Console DSL
Here's what using `freer-simple` looks like:
```haskell
{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE LambdaCase #-}
module Console where
import Control.Monad.Freer
import Control.Monad.Freer.Error
import Control.Monad.Freer.State
import Control.Monad.Freer.Writer
import System.Exit hiding (ExitCode(ExitSuccess))
--------------------------------------------------------------------------------
-- Effect Model --
--------------------------------------------------------------------------------
data Console r where
PutStrLn :: String -> Console ()
GetLine :: Console String
ExitSuccess :: Console ()
putStrLn' :: Member Console effs => String -> Eff effs ()
putStrLn' = send . PutStrLn
getLine' :: Member Console effs => Eff effs String
getLine' = send GetLine
exitSuccess' :: Member Console effs => Eff effs ()
exitSuccess' = send ExitSuccess
--------------------------------------------------------------------------------
-- Effectful Interpreter --
--------------------------------------------------------------------------------
runConsole :: Eff '[Console, IO] a -> IO a
runConsole = runM . interpretM (\case
PutStrLn msg -> putStrLn msg
GetLine -> getLine
ExitSuccess -> exitSuccess)
--------------------------------------------------------------------------------
-- Pure Interpreter --
--------------------------------------------------------------------------------
runConsolePure :: [String] -> Eff '[Console] w -> [String]
runConsolePure inputs req = snd . fst $
run (runWriter (runState inputs (runError (reinterpret3 go req))))
where
go :: Console v -> Eff '[Error (), State [String], Writer [String]] v
go (PutStrLn msg) = tell [msg]
go GetLine = get >>= \case
[] -> error "not enough lines"
(x:xs) -> put xs >> pure x
go ExitSuccess = throwError ()
```
# Contributing
Contributions are welcome! Documentation, examples, code, and feedback - they all help.
## Developer Setup
The easiest way to start contributing is to install [stack](https://haskellstack.org/). Stack can install GHC/Haskell for you, and automates common developer tasks.
The key commands are:
- `stack setup` — install required version of GHC compiler
- `stack build` — builds project, dependencies are automatically resolved
- `stack test` — builds project, its tests, and executes the tests
- `stack bench` — builds project, its benchmarks, and executes the benchamks
- `stack ghci` — start a REPL instance with a project modules loaded
- `stack clean`
- `stack haddock` — builds documentation
More information about `stack` can be found in its [documentation](https://haskellstack.org/).
# Licensing
This project is distributed under a BSD3 license. See the included LICENSE file for more details.
# Acknowledgements
The `freer-simple` package started as a fork of [freer-effects](http://hackage.haskell.org/package/freer-effects) by Ixperta Solutions, which in turn is a fork of [freer](http://hackage.haskell.org/package/freer) by Allele Dev. All implementations are based on the paper and reference implementation by Oleg Kiselyov. In particular:
- `Data.OpenUnion` maps to [OpenUnion51.hs](http://okmij.org/ftp/Haskell/extensible/OpenUnion51.hs)
- `Data.FTCQueue` maps to [FTCQueue1](http://okmij.org/ftp/Haskell/extensible/FTCQueue1.hs)
- `Control.Monad.Freer*` maps to [Eff1.hs](http://okmij.org/ftp/Haskell/extensible/Eff1.hs)
There will be deviations from the source.