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polysemy is a library for writing high-power, low-boilerplate domain specific languages. It allows you to separate your business logic from your implementation details. And in doing so, polysemy lets you turn your implementation code into reusable library code.

It's like mtl but composes better, requires less boilerplate, and avoids the O(n^2) instances problem.

It's like freer-simple but more powerful.

It's like fused-effects but with an order of magnitude less boilerplate.

Additionally, unlike mtl, polysemy has no functional dependencies, so you can use multiple copies of the same effect. This alleviates the need for ugly hacks band-aids like classy lenses, the ReaderT pattern and nicely solves the trouble with typed errors.

Concerned about type inference? polysemy comes with its companion polysemy-plugin, which helps it perform just as well as mtl's! Add polysemy-plugin to your package.yaml or .cabal file's dependencies section to use. Then turn it on with a pragma in your source files:

{-# OPTIONS_GHC -fplugin=Polysemy.Plugin #-}

Or by adding -fplugin=Polysemy.Plugin to your package.yaml/.cabal file ghc-options section.


  • Effects are higher-order, meaning it's trivial to write bracket and local as first-class effects.
  • Effects are low-boilerplate, meaning you can create new effects in a single-digit number of lines. New interpreters are nothing but functions and pattern matching.

Tutorials and Resources


Make sure you read the Necessary Language Extensions before trying these yourself!

Teletype effect:

{-# LANGUAGE TemplateHaskell, LambdaCase, BlockArguments, GADTs
           , FlexibleContexts, TypeOperators, DataKinds, PolyKinds, ScopedTypeVariables #-}

import Polysemy
import Polysemy.Input
import Polysemy.Output

data Teletype m a where
  ReadTTY  :: Teletype m String
  WriteTTY :: String -> Teletype m ()

makeSem ''Teletype

teletypeToIO :: Member (Embed IO) r => Sem (Teletype ': r) a -> Sem r a
teletypeToIO = interpret \case
  ReadTTY      -> embed getLine
  WriteTTY msg -> embed $ putStrLn msg

runTeletypePure :: [String] -> Sem (Teletype ': r) a -> Sem r ([String], a)
runTeletypePure i
  -- For each WriteTTY in our program, consume an output by appending it to the
  -- list in a ([String], a)
  = runOutputMonoid pure
  -- Treat each element of our list of strings as a line of input
  . runInputList i
  -- Reinterpret our effect in terms of Input and Output
  . reinterpret2 \case
      ReadTTY -> maybe "" id <$> input
      WriteTTY msg -> output msg

echo :: Member Teletype r => Sem r ()
echo = do
  i <- readTTY
  case i of
    "" -> pure ()
    _  -> writeTTY i >> echo

-- Let's pretend
echoPure :: [String] -> Sem '[] ([String], ())
echoPure = flip runTeletypePure echo

pureOutput :: [String] -> [String]
pureOutput = fst . run . echoPure

-- echo forever
main :: IO ()
main = runM . teletypeToIO $ echo

Resource effect:

{-# LANGUAGE TemplateHaskell, LambdaCase, BlockArguments, GADTs
           , FlexibleContexts, TypeOperators, DataKinds, PolyKinds
           , TypeApplications #-}

import Polysemy
import Polysemy.Input
import Polysemy.Output
import Polysemy.Error
import Polysemy.Resource

-- Using Teletype effect from above

data CustomException = ThisException | ThatException deriving Show

program :: Members '[Resource, Teletype, Error CustomException] r => Sem r ()
program = catch @CustomException work \e -> writeTTY $ "Caught " ++ show e
  work = bracket (readTTY) (const $ writeTTY "exiting bracket") \input -> do
    writeTTY "entering bracket"
    case input of
      "explode"     -> throw ThisException
      "weird stuff" -> writeTTY input *> throw ThatException
      _             -> writeTTY input *> writeTTY "no exceptions"

main :: IO (Either CustomException ())
  = runFinal
  . embedToFinal @IO
  . resourceToIOFinal
  . errorToIOFinal @CustomException
  . teletypeToIO
  $ program


Friendly Error Messages

Free monad libraries aren't well known for their ease-of-use. But following in the shoes of freer-simple, polysemy takes a serious stance on providing helpful error messages.

For example, the library exposes both the interpret and interpretH combinators. If you use the wrong one, the library's got your back:

    :: forall r a
     . Sem (Resource ': r) a
    -> Sem r a
runResource = interpret $ \case

makes the helpful suggestion:

• 'Resource' is higher-order, but 'interpret' can help only
  with first-order effects.
    use 'interpretH' instead.
• In the expression:
      $ \case

Necessary Language Extensions

You're going to want to stick all of this into your package.yaml file.

  ghc-options: -O2 -flate-specialise -fspecialise-aggressively
    - DataKinds
    - FlexibleContexts
    - GADTs
    - LambdaCase
    - PolyKinds
    - RankNTypes
    - ScopedTypeVariables
    - TypeApplications
    - TypeOperators
    - TypeFamilies

Building with Nix

The project provides a basic nix config for building in development. It is defined as a flake with backwards compatibility stubs in default.nix and shell.nix.

To build the main library or plugin:

nix-build -A polysemy
nix-build -A polysemy-plugin

Flake version:

nix build
nix build '.#polysemy-plugin'

To inspect a dependency:

nix repl

> p = import ./.
> p.unagi-chan

To run a shell command with all dependencies in the environment:

nix-shell --pure
nix-shell --pure --run 'cabal v2-haddock polysemy'
nix-shell --pure --run ghcid

Flake version:

nix develop -i # just enter a shell
nix develop -i -c cabal v2-haddock polysemy
nix develop -i -c haskell-language-server-wrapper # start HLS for your IDE

What about performance?

Previous versions of this README mentioned the library being zero-cost, as in having no visible effect on performance. While this was the original motivation and main factor in implementation of this library, it turned out that optimizations we depend on, while showing amazing results in small benchmarks, don't work in bigger, multi-module programs, what greatly limits their usefulness.

What's more interesting though is that this isn't a polysemy-specific problem - basically all popular effects libraries ended up being bitten by variation of this problem in one way or another, resulting in visible drop in performance compared to equivalent code without use of effect systems.

Why did nobody notice this?

One factor may be that while GHC's optimizer is very, very good in general in optimizing all sorts of abstraction, it's relatively complex and hard to predict - authors of libraries may have not deemed location of code relevant, even though it had big effect at the end. The other is that maybe it doesn't matter as much as we like to tell ourselves. Many of these effects libraries are used in production and they're doing just fine, because maximum performance usually matters in small, controlled areas of code, that often don't use features of effect systems at all.

Acknowledgements, citations, and related work

The following is a non-exhaustive list of people and works that have had a significant impact, directly or indirectly, on polysemy’s design and implementation: