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Alexis King
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README.md
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README.md
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# Freer: Extensible Effects with Freer Monads [](https://travis-ci.org/lexi-lambda/freer-simple)
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# freer-simple — a friendly effect system for Haskell [](https://travis-ci.org/lexi-lambda/freer-simple)
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# Description
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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.:
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- [Freer Monads, More Extensible Effects](http://okmij.org/ftp/Haskell/extensible/more.pdf)
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- [Reflection without Remorse](http://okmij.org/ftp/Haskell/zseq.pdf)
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- [Extensible Effects](http://okmij.org/ftp/Haskell/extensible/exteff.pdf)
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Much of the implementation is a repackaging and cleaning up of the reference materials provided [here](http://okmij.org/ftp/Haskell/extensible/).
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# Features
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The `freer-simple` library is an implementation of an *extensible effect system* for Haskell, a general-purpose way of tracking effects at the type level and handling them in different ways. The concept of an “effect” is very general: it encompasses the things most people consider side-effects, like generating random values, interacting with the file system, and mutating state, but it also includes things like access to an immutable global environment and exception handling.
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The key features of `freer-simple` are:
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- An efficient effect system for Haskell as a library.
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- Implementations for several common Haskell monads as effects:
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- `Reader`
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- `Writer`
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- `State`
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- `Trace`
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- `Error`
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- Core components for defining your own Effects.
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# Example: Console DSL
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- Implementations for several common Haskell monads as effects, including `Reader`, `Writer`, `State`, `Error`, and others.
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Here's what using `freer-simple` looks like:
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- A combinator language for defining your own effects, designed to make simple, common use cases easy to read and write.
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[**For more details, see the package documentation on Hackage.**](https://hackage.haskell.org/package/freer-simple)
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## Code example
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```haskell
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{-# LANGUAGE GADTs #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE TypeOperators #-}
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE GADTs #-}
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{-# LANGUAGE LambdaCase #-}
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module Console where
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{-# LANGUAGE TemplateHaskell #-}
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{-# LANGUAGE TypeOperators #-}
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import qualified Prelude
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import qualified System.Exit
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import Prelude hiding (putStrLn, getLine)
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import Control.Monad.Freer
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import Control.Monad.Freer.TH
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import Control.Monad.Freer.Error
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import Control.Monad.Freer.State
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import Control.Monad.Freer.Writer
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import System.Exit hiding (ExitCode(ExitSuccess))
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--------------------------------------------------------------------------------
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-- Effect Model --
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@ -48,24 +40,16 @@ data Console r where
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PutStrLn :: String -> Console ()
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GetLine :: Console String
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ExitSuccess :: Console ()
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putStrLn' :: Member Console effs => String -> Eff effs ()
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putStrLn' = send . PutStrLn
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getLine' :: Member Console effs => Eff effs String
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getLine' = send GetLine
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exitSuccess' :: Member Console effs => Eff effs ()
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exitSuccess' = send ExitSuccess
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makeEffect ''Console
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--------------------------------------------------------------------------------
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-- Effectful Interpreter --
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--------------------------------------------------------------------------------
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runConsole :: Eff '[Console, IO] a -> IO a
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runConsole = runM . interpretM (\case
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PutStrLn msg -> putStrLn msg
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GetLine -> getLine
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ExitSuccess -> exitSuccess)
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PutStrLn msg -> Prelude.putStrLn msg
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GetLine -> Prelude.getLine
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ExitSuccess -> System.Exit.exitSuccess)
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--------------------------------------------------------------------------------
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-- Pure Interpreter --
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@ -82,37 +66,6 @@ runConsolePure inputs req = snd . fst $
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go ExitSuccess = throwError ()
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```
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# Contributing
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## Acknowledgements
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Contributions are welcome! Documentation, examples, code, and feedback - they all help.
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## Developer Setup
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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.
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The key commands are:
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- `stack setup` — install required version of GHC compiler
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- `stack build` — builds project, dependencies are automatically resolved
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- `stack test` — builds project, its tests, and executes the tests
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- `stack bench` — builds project, its benchmarks, and executes the benchamks
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- `stack ghci` — start a REPL instance with a project modules loaded
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- `stack clean`
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- `stack haddock` — builds documentation
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More information about `stack` can be found in its [documentation](https://haskellstack.org/).
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# Licensing
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This project is distributed under a BSD3 license. See the included LICENSE file for more details.
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# Acknowledgements
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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:
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- `Data.OpenUnion` maps to [OpenUnion51.hs](http://okmij.org/ftp/Haskell/extensible/OpenUnion51.hs)
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- `Data.FTCQueue` maps to [FTCQueue1](http://okmij.org/ftp/Haskell/extensible/FTCQueue1.hs)
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- `Control.Monad.Freer*` maps to [Eff1.hs](http://okmij.org/ftp/Haskell/extensible/Eff1.hs)
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There will be deviations from the source.
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The `freer-simple` package began 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](http://okmij.org/ftp/Haskell/extensible/more.pdf).
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