Freer: Extensible Effects with Freer Monads

Description

The freer-simple library (a fork of 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
• 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-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:

{-# 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. 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.

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 by Ixperta Solutions, which in turn is a fork of 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
• Data.FTCQueue maps to FTCQueue1
• Control.Monad.Freer* maps to Eff1.hs

There will be deviations from the source.