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121 lines
4.0 KiB
Markdown
121 lines
4.0 KiB
Markdown
# Freer: Extensible Effects with Freer Monads
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Freer is an implementation of
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["Freer Monads, More Extensible Effects"](http://okmij.org/ftp/Haskell/extensible/more.pdf). Much
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of the implementation is a repackaging and cleaning up of the
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reference materials provided
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[here](http://okmij.org/ftp/Haskell/extensible/).
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# Features
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The key features of Freer are:
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* An efficient effect system for Haskell as a library
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* Implementations for several common Haskell monad instances:
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* Reader
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* Writer
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* State
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* StateRW: State in terms of Reader/Writer
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* Trace
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* Exception
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* Core components for defining your own Effects
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# Example: Teletype DSL
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Here's what using Freer looks like:
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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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module Teletype where
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import Control.Monad.Freer
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import Control.Monad.Freer.Internal
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import System.Exit hiding (ExitSuccess)
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--------------------------------------------------------------------------------
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-- Effect Model --
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--------------------------------------------------------------------------------
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data Teletype s where
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PutStrLn :: String -> Teletype ()
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GetLine :: Teletype String
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ExitSuccess :: Teletype ()
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putStrLn' :: Member Teletype r => String -> Eff r ()
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putStrLn' = send . PutStrLn
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getLine' :: Member Teletype r => Eff r String
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getLine' = send GetLine
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exitSuccess' :: Member Teletype r => Eff r ()
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exitSuccess' = send ExitSuccess
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--------------------------------------------------------------------------------
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-- Effectful Interpreter --
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--------------------------------------------------------------------------------
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runTeletype :: Eff '[Teletype] w -> IO w
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runTeletype (Val x) = return x
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runTeletype (E u q) = case decomp u of
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Right (PutStrLn msg) -> putStrLn msg >> runTeletype (qApp q ())
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Right GetLine -> getLine >>= \s -> runTeletype (qApp q s)
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Right ExitSuccess -> exitSuccess
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Left _ -> error "This cannot happen"
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--------------------------------------------------------------------------------
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-- Pure Interpreter --
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--------------------------------------------------------------------------------
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runTeletypePure :: [String] -> Eff '[Teletype] w -> [String]
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runTeletypePure inputs req = reverse (go inputs req [])
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where go :: [String] -> Eff '[Teletype] w -> [String] -> [String]
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go _ (Val _) acc = acc
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go [] _ acc = acc
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go (x:xs) (E u q) acc = case decomp u of
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Right (PutStrLn msg) -> go (x:xs) (qApp q ()) (msg:acc)
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Right GetLine -> go xs (qApp q x) acc
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Right ExitSuccess -> go xs (Val ()) acc
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Left _ -> go xs (Val ()) acc
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```
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# Contributing
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Contributions are welcome! Documentation, examples, code, and
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feedback - they all help.
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Be sure to review the included code of conduct. This project adheres
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to the [Contributor's Covenant](http://contributor-covenant.org/). By
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participating in this project you agree to abide by its terms.
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## Developer Setup
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The easiest way to start contributing is to install
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[stack](https://github.com/commercialhaskell/stack). stack can install
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GHC/Haskell for you, and automates common developer tasks.
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The key commands are:
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* stack setup : install GHC
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* stack build
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* stack clean
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* stack haddock : builds documentation
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* stack test
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* stack bench
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* stack ghci : start a REPL instance
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# Licensing
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This project is distrubted under a BSD3 license. See the included
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LICENSE file for more details.
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# Acknowledgements
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This package would not be possible without the paper and the reference
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implementation. In particular:
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* Data.Open.Union maps to [OpenUnion41.hs](http://okmij.org/ftp/Haskell/extensible/OpenUnion41.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 [Union1.hs](http://okmij.org/ftp/Haskell/extensible/Eff1.hs)
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There will be deviations from the source.
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