This package implements extensible effects, an 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, 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
(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.
