The effin package

[Tags:bsd3, library]

This package implements extensible effects, an alternative to monad transformers. The original paper can be found at The main differences between this library and the one described in the paper are that this library does not use the Typeable type class, does not require that effects implement the Functor type class, and has a simpler API for handling effects.

For example, the following code implements a handler for exceptions:

 newtype Exception e = Throw e

 runException :: Effect (Exception e :+ es) a -> Effect es (Either e a)
 runException = eliminate
     (\x -> return (Right x))
     (\(Throw e) k -> return (Left e))

Compare this to the corresponding code in extensible-effects (

 runExc :: Typeable e => Eff (Exc e :> r) a -> Eff r (Either e a)
 runExc = loop . admin
     loop (Val x) = return (Right x)
     loop (E u)   = handleRelay u loop (\(Exc e) -> return (Left e))

In particular, effect implementors are not required to do any recursion, thereby making effect handlers more composeable.


Dependencies base (>=4.7 && <4.10), mtl (>=2.1 && <3) [details]
License BSD3
Copyright (c) 2014 Anthony Vandikas
Author Anthony Vandikas
Category Control, Effect
Home page
Source repository head: git clone git://
Uploaded Sun Mar 5 15:19:00 UTC 2017 by AnthonyVandikas
Distributions LTSHaskell:, NixOS:, Stackage:, Tumbleweed:
Downloads 1755 total (25 in the last 30 days)
0 []
Status Docs available [build log]
Last success reported on 2017-03-05 [all 1 reports]
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