extensible-effects- An Alternative to Monad Transformers

Safe HaskellTrustworthy



Original work available at http://okmij.org/ftp/Haskell/extensible/tutorial.html. This module implements extensible effects as an alternative to monad transformers, as described in http://okmij.org/ftp/Haskell/extensible/exteff.pdf and http://okmij.org/ftp/Haskell/extensible/more.pdf.

Extensible Effects are implemented as typeclass constraints on an Eff[ect] datatype. A contrived example can be found under Control.Eff.Example. To run the effects, consult the tests.



type Arr r a b = a -> Eff r b Source #

A monadic library for communication between a handler and its client, the administered computation

Effectful arrow type: a function from a to b that also does effects denoted by r

arr :: (a -> b) -> Arrs r a b Source #

ident :: Arrs r a a Source #

single :: Arr r a b -> Arrs r a b Source #

first :: Arr r a b -> Arr r (a, c) (b, c) Source #

comp :: Arrs r a b -> Arrs r b c -> Arrs r a c Source #

type Arrs r a b = FTCQueue (Eff r) a b Source #

An effectful function from a to b that is a composition of several effectful functions. The paremeter r describes the overall effect. The composition members are accumulated in a type-aligned queue

data Eff r a Source #

The Eff monad (not a transformer!). It is a fairly standard coroutine monad where the type r is the type of effects that can be handled, and the missing type a (from the type application) is the type of value that is returned. It is NOT a Free monad! There are no Functor constraints.

The two constructors denote the status of a coroutine (client): done with the value of type a, or sending a request of type Union r with the continuation Arrs r b a. Expressed another way: an Eff can either be a value (i.e., Val case), or an effect of type Union r producing another Eff (i.e., E case). The result is that an Eff can produce an arbitrarily long chain of Union r effects, terminated with a pure value.

Potentially, inline Union into E


Val a 
E (Union r b) (Arrs r b a) 


Monad (Eff r) Source # 


(>>=) :: Eff r a -> (a -> Eff r b) -> Eff r b #

(>>) :: Eff r a -> Eff r b -> Eff r b #

return :: a -> Eff r a #

fail :: String -> Eff r a #

Functor (Eff r) Source #

Eff is still a monad and a functor (and Applicative) (despite the lack of the Functor constraint)


fmap :: (a -> b) -> Eff r a -> Eff r b #

(<$) :: a -> Eff r b -> Eff r a #

Applicative (Eff r) Source # 


pure :: a -> Eff r a #

(<*>) :: Eff r (a -> b) -> Eff r a -> Eff r b #

(*>) :: Eff r a -> Eff r b -> Eff r b #

(<*) :: Eff r a -> Eff r b -> Eff r a #

qApp :: Arrs r b w -> b -> Eff r w Source #

Application to the `generalized effectful function' Arrs r b w

qComp :: Arrs r a b -> (Eff r b -> Eff r' c) -> Arr r' a c Source #

Compose effectful arrows (and possibly change the effect!)

send :: Member t r => t v -> Eff r v Source #

Send a request and wait for a reply (resulting in an effectful computation).

run :: Eff '[] w -> w Source #

The initial case, no effects. Get the result from a pure computation.

The type of run ensures that all effects must be handled: only pure computations may be run.

handle_relay :: (a -> Eff r w) -> (forall v. t v -> Arr r v w -> Eff r w) -> Eff (t ': r) a -> Eff r w Source #

A convenient pattern: given a request (open union), either handle it or relay it.

handle_relay_s :: s -> (s -> a -> Eff r w) -> (forall v. s -> t v -> (s -> Arr r v w) -> Eff r w) -> Eff (t ': r) a -> Eff r w Source #

Parameterized handle_relay

interpose :: Member t r => (a -> Eff r w) -> (forall v. t v -> Arr r v w -> Eff r w) -> Eff r a -> Eff r w Source #

Add something like Control.Exception.catches? It could be useful for control with cut.

Intercept the request and possibly reply to it, but leave it unhandled (that's why we use the same r all throuout)