A library for writing extensible algebraic effects and handlers. Similar to extensible-effects but with deep handlers.


LTS Haskell 10.10:
Stackage Nightly 2018-03-10:
Latest on Hackage:

See all snapshots effect-handlers appears in

MIT licensed by Andraz Bajt, Blaz Repas
Maintained by Andraz Bajt
This version can be pinned in stack with:effect-handlers-,3992


This is an extensible effects library for Haskell taking inspiration from the Eff language.

See these papers for the ideas and theory behind the library:

Implementation wise it’s most close to extensible-effects (also see the Extensible Effects paper) but it implements deep handlers instead of shallow.

What does this library provide?

There is the Eff monad type and modules for pre-implemented effects.

  • Exception
  • IO
  • Reader
  • Search
  • State
  • Writer

It is easy to define your own effects and combine them.


Most of the types are inferred, you only need to provide enough to tell the compiler how to specialize some effect handlers (e.g. readerHandler).

import Control.Effects.Cont.Eff
import Control.Effects.Cont.Reader
import Control.Effects.Cont.Exception

program = do
  v <- ask
  if v < 15 
  then throw $ show v
  else return (v+1)

run n = runPure 
                 . handle exceptionHandler 
                 . handle (readerHandler n)

res :: Integer -> Either String Integer
res n = run n program


Haddock docs are available online