An implementation of "Freer Monads, More Extensible Effects".

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Freer: Extensible Effects with Freer Monads

Maintainer Allele Dev (allele.dev@gmail.com)
Funding $0 USD
Copyright Copyright (C) 2017 Allele Dev
License BSD3

Freer is an implementation of "Freer Monads, More Extensible Effects". 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 monad instances:
    • Reader
    • Writer
    • State
    • StateRW: State in terms of Reader/Writer
    • Trace
    • Exception
  • Core components for defining your own Effects

Example: Teletype DSL

Here's what using Freer looks like:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
module Teletype where

import Control.Monad.Freer
import Control.Monad.Freer.Internal
import System.Exit hiding (ExitSuccess)

                          -- Effect Model --
data Teletype s where
  PutStrLn    :: String -> Teletype ()
  GetLine     :: Teletype String
  ExitSuccess :: Teletype ()

putStrLn' :: Member Teletype r => String -> Eff r ()
putStrLn' = send . PutStrLn

getLine'  :: Member Teletype r => Eff r String
getLine' = send GetLine

exitSuccess' :: Member Teletype r => Eff r ()
exitSuccess' = send ExitSuccess

                     -- Effectful Interpreter --
runTeletype :: Eff '[Teletype] w -> IO w
runTeletype (Val x) = return x
runTeletype (E u q) = case extract u of
              (PutStrLn msg) -> putStrLn msg  >> runTeletype (qApp q ())
              GetLine        -> getLine      >>= \s -> runTeletype (qApp q s)
              ExitSuccess    -> exitSuccess

                        -- Pure Interpreter --
runTeletypePure :: [String] -> Eff '[Teletype] w -> [String]
runTeletypePure inputs req =
  reverse . snd $ run (handleRelayS (inputs, []) (\s _ -> pure s) go req)
    go :: ([String], [String])
       -> Teletype 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)


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

Be sure to review the included code of conduct. This project adheres to the Contributor's Covenant. By participating in this project you agree to abide by its terms.

This project is not currently actively maintained.

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 GHC
  • stack build
  • stack clean
  • stack haddock : builds documentation
  • stack test
  • stack bench
  • stack ghci : start a REPL instance


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


This package would not be possible without the paper and the reference implementation. In particular:

There will be deviations from the source.