Monthly Downloads: 7
Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Control     Freer    
Latest version: v0.3.0.1

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Freer Effects: Extensible Effects with Freer Monads

Haskell Programming Language BSD3 License

Hackage Stackage LTS 8 Stackage Nightly Hackage Dependencies Build


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

Much of the implementation is a repackaging and cleaning up of the reference materials provided here.


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 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)
    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)

Combining with Transformers

You already have some mtl code and are afraid that combining effects with your current tranformer stack would not be possible? Package freer-effects-extra has some mtl-related and other goodies.


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.


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


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:

There will be deviations from the source.

*Note that all licence references and agreements mentioned in the freer-effects README section above are relevant to that project's source code only.