Freer Effects: Extensible Effects with Freer Monads

Description

Library freer-effects is an implementation of effect system for Haskell, which is based on the work of Oleg Kiselyov et al.:

• Freer Monads, More Extensible Effects
• Reflection without Remorse
• 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 monads as effects:
  • Reader
  • Writer
  • State
  • StateRW: State in terms of Reader/Writer.
  • Trace
  • Exception
• Core components for defining your own Effects.

Example: Console DSL

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)

Contributing

Contributions are welcome! Documentation, examples, code, and feedback - they all help.

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 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

For more information about stack tool can be found in its documentation.

Licensing

This project is distrubted under a BSD3 license. See the included LICENSE file for more details.

Acknowledgements

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.OpenUnion maps to OpenUnion51.hs
• Data.FTCQueue maps to FTCQueue1
• Control.Monad.Freer* maps to Eff1.hs

There will be deviations from the source.