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The word ‘good’ has many meanings. For example, if a man were to shoot his grandmother at a range of five hundred yards, I should call him a good shot, but not necessarily a good man.

Gilbert K. Chesterton


polysemy is a library for writing high-power, low-boilerplate, zero-cost, 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 and 35x faster.

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? Check out polysemy-plugin, which should 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.
  • Effects are zero-cost, meaning that GHC1 can optimize away the entire abstraction at compile time.

1: Unfortunately this is not true in GHC 8.6.3, but will be true in GHC 8.10.1.


Raghu Kaippully wrote a beginner friendly tutorial.


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

Teletype effect:

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

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
  = runOutputMonoid pure  -- For each WriteTTY in our program, consume an output by appending it to the list in a ([String], a)
  . runInputList i         -- Treat each element of our list of strings as a line of input
  . reinterpret2 \case     -- Reinterpret our effect in terms of Input and Output
      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 #-}
{-# LANGUAGE LambdaCase, BlockArguments #-}
{-# LANGUAGE 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)
  where 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 ())
main =
  . 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

Likewise it will give you tips on what to do if you forget a TypeApplication or forget to handle an effect.

Don’t like helpful errors? That’s OK too — just flip the error-messages flag and enjoy the raw, unadulterated fury of the typesystem.

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

Stellar Engineering - Aligning the stars to optimize polysemy away

Several things need to be in place to fully realize our performance goals:

  • GHC Version
    • GHC 8.9+
  • Your code
    • The module you want to be optimized needs to import Polysemy.Internal somewhere in its dependency tree (sufficient to import Polysemy)
  • GHC Flags
    • -O or -O2
    • -flate-specialise (this should be automatically turned on by the plugin, but it’s worth mentioning)
  • Plugin
    • -fplugin=Polysemy.Plugin
  • Additional concerns:
    • additional core passes (turned on by the plugin)


Changelog for polysemy (2019-10-29)

  • Polysemy now works on GHC 8.8.1 (thanks to @googleson78 and @sevanspowell)
  • Exported MemberWithError from Polysemy
  • Added rewrite and transform interpretation combinators (2019-10-22)

  • Fixed a bug in resourceToIO and resourceToIOFinal that prevented the finalizers from being called in BracketOnError when the computation failed due to a Sem failure
  • Added atomicGets (thanks to @googleson78)
  • Added sequenceConcurrently to Polysemy.Async (thanks to @spacekitteh) (2019-09-15)

  • Added InterpreterFor (thanks to @bolt12)
  • Bumped bounds for first-class-families (2019-09-04)

Breaking Changes

  • All lower- interpreters have been deprecated, in favor of corresponding -Final interpreters.
  • runFixpoint and runFixpointM have been deprecated in favor of fixpointToFinal.
  • The semantics for runNonDet when <|> is used inside a higher-order action of another effect has been changed.
  • Type variables for certain internal functions, failToEmbed, and atomicState' have been rearranged.

Other changes

  • Added Final effect, an effect for embedding higher-order actions in the final monad of the effect stack. Any interpreter should use this instead of requiring to be provided an explicit lowering function to the final monad.
  • Added Strategy environment for use together with Final
  • Added asyncToIOFinal, a better alternative of lowerAsync
  • Added errorToIOFinal, a better alternative of lowerError
  • Added fixpointToFinal, a better alternative of runFixpoint and runFixpointM
  • Added resourceToIOFinal, a better alternative of lowerResource
  • Added outputToIOMonoid and outputToIOMonoidAssocR
  • Added stateToIO
  • Added atomicStateToIO
  • Added runWriterTVar, writerToIOFinal, and writerToIOAssocRFinal
  • Added writerToEndoWriter
  • Added subsume operation
  • Exposed raiseUnder/2/3 in Polysemy (2019-08-15)

Breaking Changes

  • MonadFail is now implemented in terms of Fail, instead of NonDet (thanks to @KingoftheHomeless)
  • LastMember has been removed. withLowerToIO and all interpreters that make use of it now only requires Member (Embed IO) r (thanks to @KingoftheHomeless)
  • State and Writer now have better strictness semantics

Other Changes

  • Added AtomicState effect (thanks to @KingoftheHomeless)
  • Added Fail effect (thanks to @KingoftheHomeless)
  • Added runOutputSem (thanks to @cnr)
  • Added modify', a strict variant of modify (thanks to @KingoftheHomeless)
  • Added right-associative variants of runOutputMonoid and runWriter (thanks to @KingoftheHomeless)
  • Added runOutputMonoidIORef and runOutputMonoidTVar (thanks to @KingoftheHomeless)
  • Improved Fixpoint so it won’t always diverge (thanks to @KingoftheHomeless)
  • makeSem will now complain if DataKinds isn’t enabled (thanks to @pepegar) (2019-07-24)

Breaking Changes

  • Renamed Lift to Embed (thanks to @googleson78)
  • Renamed runAsyncInIO to lowerAsync
  • Renamed runAsync to asyncToIO
  • Renamed runBatchOutput to runOutputBatched
  • Renamed runConstInput to runInputConst
  • Renamed runEmbed to runEmbedded (thanks to @googleson78)
  • Renamed runEmbedded to lowerEmbedded
  • Renamed runErrorAsAnother to mapError
  • Renamed runErrorInIO to lowerError
  • Renamed runFoldMapOutput to runOutputMonoid
  • Renamed runIO to embedToMonadIO
  • Renamed runIgnoringOutput to ignoreOutput
  • Renamed runIgnoringTrace to ignoreTrace
  • Renamed runInputAsReader to inputToReader
  • Renamed runListInput to runInputList
  • Renamed runMonadicInput to runInputSem
  • Renamed runOutputAsList to runOutputList
  • Renamed runOutputAsTrace to outputToTrace
  • Renamed runOutputAsWriter to outputToWriter
  • Renamed runResourceBase to resourceToIO
  • Renamed runResourceInIO to lowerResource
  • Renamed runStateInIORef to runStateIORef
  • Renamed runTraceAsList to runTraceList
  • Renamed runTraceAsOutput to traceToOutput
  • Renamed runTraceIO to traceToIO
  • Renamed sendM to embed (thanks to @googleson78)
  • The NonDet effect is now higher-order (thanks to @KingoftheHomeless)

Other Changes

  • Added evalState and evalLazyState
  • Added runNonDetMaybe (thanks to @KingoftheHomeless)
  • Added nonDetToError (thanks to @KingoftheHomeless)
  • Haddock documentation for smart constructors generated via makeSem will no longer have weird variable names (thanks to @TheMatten) (2019-07-08)

Breaking Changes

  • Added a Pass constructor to Writer (thanks to @KingoftheHomeless)
  • Fixed a bug in runWriter where the MTL semantics wouldn’t be respected (thanks to @KingoftheHomeless)
  • Removed the Censor constructor of Writer (thanks to @KingoftheHomeless)
  • Renamed Yo to Weaving
  • Changed the visible type applications for asks, gets, and runErrorAsAnother

Other Changes

  • Fixed haddock generation (2019-07-04)

Breaking Changes

  • Changed the type of runBatchOutput to be more useful (thanks to @Infinisil)

Other Changes

  • THE ERROR MESSAGES ARE SO MUCH BETTER :party: :party: :party:
  • Added runEmbedded to Polysemy.IO
  • Added runOutputAsList to Polysemy.Output (thanks to @googleson78)
  • Asymptotically improved the performance of runTraceAsList (thanks to @googleson78) (2019-06-28)

  • New combinators for Polysemy.Error: fromEither and fromEitherM (2019-06-27)

  • Fixed a bug where intercept and interceptH wouldn’t correctly handle higher-order effects (2019-06-26)

Breaking Changes

  • Removed the internal Effect machinery

New Effects and Interpretations

  • New effect; Async, for describing asynchronous computations
  • New interpretation for Resource: runResourceBase, which can lower Resource effects without giving a lowering natural transformation
  • New interpretation for Trace: runTraceAsList
  • New combinator: withLowerToIO, which is capable of transforming IO-invariant functions as effects.

Other Changes

  • Lots of hard work on the package and CI infrastructure to make it green on GHC 8.4.4 (thanks to @jkachmar)
  • Changed the order of the types for runMonadicInput to be more helpful (thanks to @tempname11)
  • Improved the error machinery to be more selective about when it runs
  • Factored out the TH into a common library for third-party consumers (2019-06-12)

Breaking Changes

  • Renamed runResource to runResourceInIO

Other Changes

  • Added runResource, which runs a Resource purely
  • Added onException, finally and bracketOnError to Resource
  • Added a new function, runResource which performs bracketing for pure code (2019-06-09)

  • Fixed a type error in the benchmark caused by deprecation of Semantic (2019-06-01)

Breaking Changes

  • Removed all deprecated names
  • Moved Random effect to polysemy-zoo

Other Changes

  • makeSem can now be used to create term-level operators (thanks to @TheMatten) (2019-05-30)

  • Added getInspectorT to the Tactical functions, which allows polysemy code to be run in external callbacks
  • A complete rewrite of Polysemy.Internal.TH.Effect (thanks to @TheMatten)
  • Fixed a bug in the TH generation of effects where the splices could contain usages of effects that were ambiguous (2019-05-27)

  • Fixed a bug in the Alternative instance for Sem, where it would choose the last success instead of the first
  • Added MonadPlus and MonadFail instances for Sem (2019-05-23)

Breaking Changes

  • Lower precedence of .@ and .@@ to 8, from 9

Other Changes

  • Fixed a serious bug in interpretH and friends, where higher-order effects would always be run with the current interpreter.
  • Users need no longer require inlineRecursiveCalls — the polysemy-plugin- will do it automatically when compiling with -O
  • Deprecated inlineRecursiveCalls; slated for removal in the next version (2019-05-18)

  • Give explicit package bounds for dependencies
  • Haddock improvements
  • Remove Typeable machinery from Polysemy.Internal.Union (thanks to @googleson78) (2019-04-26)

  • runInputAsReader, runTraceAsOutput and runOutputAsWriter have more generalized types
  • Added runStateInIO
  • Added runOutputAsTrace
  • Added Members (thanks to @TheMatten) (2019-04-14)

  • Added runIO interpretation (thanks to @adamConnerSax)
  • Minor documentation fixes (2019-04-11)

  • Initial release

Unreleased changes

  • Changed the tyvars of fromEitherM, runErrorAsAnother, runEmbedded, asks and gets