@hackage effin0.2.1.2

This package implements extensible effects, and alternative to monad transformers. The original paper can be found at http://okmij.org/ftp/Haskell/extensible/exteff.pdf. The main differences between this library and the one described in the paper are that this library does not use the Typeable type class, and has a simpler API for handling effects.

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

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

Compare this to the corresponding code in extensible-effects (http://hackage.haskell.org/package/extensible-effects):

runExc :: Typeable e => Eff (Exc e :> r) a -> Eff r (Either e a)
runExc = loop . admin
  where
    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.