extensible-effects- An Alternative to Monad Transformers

Safe HaskellTrustworthy



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

Extensible Effects are implemented as typeclass constraints on an Eff[ect] datatype. A contrived example is:

{-# LANGUAGE FlexibleContexts #-}
import Control.Eff
import Control.Eff.Lift
import Control.Eff.State
import Control.Monad (void)
import Data.Typeable

-- Write the elements of a list of numbers, in order.
writeAll :: (Typeable a, Member (Writer a) e)
         => [a]
         -> Eff e ()
writeAll = mapM_ putWriter

-- Add a list of numbers to the current state.
sumAll :: (Typeable a, Num a, Member (State a) e)
       => [a]
       -> Eff e ()
sumAll = mapM_ (onState . (+))

-- Write a list of numbers and add them to the current state.
writeAndAdd :: (Member (Writer Integer) e, Member (State Integer) e)
            => [Integer]
            -> Eff e ()
writeAndAdd l = do
    writeAll l
    sumAll l

-- Sum a list of numbers.
sumEff :: (Num a, Typeable a) => [a] -> a
sumEff l = let (s, ()) = run $ runState 0 $ sumAll l
           in s

-- Safely get the last element of a list.
-- Nothing for empty lists; Just the last element otherwise.
lastEff :: Typeable a => [a] -> Maybe a
lastEff l = let (a, ()) = run $ runWriter $ writeAll l
            in a

-- Get the last element and sum of a list
lastAndSum :: (Typeable a, Num a) => [a] -> (Maybe a, a)
lastAndSum l = let (lst, (total, ())) = run $ runWriter $ runState 0 $ writeAndAdd l
               in (lst, total)



type Eff r = Free (Union r) Source

Basic type returned by all computations with extensible effects. The Eff r type is a type synonym 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.

Expressed another way: an Eff can either be a value (i.e., Pure case), or an effect of type Union r producing another Eff (i.e., Impure case). The result is that an Eff can produce an arbitrarily long chain of Union r effects, terminated with a pure value.

As is made explicit below, the Eff type is simply the Free monad resulting from the Union r functor.

type Eff r a = Free (Union r) a

class (Member' t r ~ True) => Member t r Source

Class Member is defined only for the sake of the interface compatibility with OpenUnion1. Generally, the closed type family Member' below could be used instead.

The Member t r specifies whether t is present anywhere in the sum type r, where t is some effectful type, e.g. Lift IO, State Int`.


(~) Bool (Member' t r) True => Member t r 

class Member t r => SetMember set t r | r set -> t Source

SetMember is similar to Member, but it allows types to belong to a "set". For every set, only one member can be in r at any given time. This allows us to specify exclusivity and uniqueness among arbitrary effects:

-- Terminal effects (effects which must be run last)
data Terminal

-- Make Lifts part of the Terminal effects set.
-- The fundep assures that there can only be one Terminal effect for any r.
instance Member (Lift m) r => SetMember Terminal (Lift m) r

-- Only allow a single unique Lift effect, by making a "Lift" set.
instance Member (Lift m) r => SetMember Lift (Lift m) r


MemberU k set t r => SetMember (k -> * -> *) set t r 

data Union r v Source

Parameter r is phantom: it just tells what could be in the union. Where r is t1 :> t2 ... :> tn, Union r v can be constructed with a value of type ti v. Ideally, we should be able to add the constraint Member t r.

NOTE: exposing the constructor below allows users to bypass the type system. See unsafeReUnion for example.


(MonadBase b m, Typeable (* -> *) m, SetMember ((* -> *) -> * -> *) Lift (Lift m) r) => MonadBase b (Eff r) 
Functor (Union r) 
(Typeable (* -> *) m, MonadIO m, SetMember ((* -> *) -> * -> *) Lift (Lift m) r) => MonadIO (Eff r) 
Typeable (* -> * -> *) Union 

data a :> b infixr 1 Source

A sum data type, for composing effects


Typeable ((* -> *) -> * -> *) (:>) 

inj :: (Functor t, Typeable t, Member t r) => t v -> Union r v Source

Construct a Union.

prj :: (Typeable t, Member t r) => Union r v -> Maybe (t v) Source

Try extracting the contents of a Union as a given type.

prjForce :: (Typeable t, Member t r) => Union r v -> (t v -> a) -> a Source

Extract the contents of a Union as a given type. If the Union isn't of that type, a runtime error occurs.

decomp :: Typeable t => Union (t :> r) v -> Either (Union r v) (t v) Source

Try extracting the contents of a Union as a given type. If we can't, return a reduced Union that excludes the type we just checked.

send :: Union r a -> Eff r a Source

Given a method of turning requests into results, we produce an effectful computation.

run :: Eff Void w -> w Source

Get the result from a pure computation.

interpose :: (Typeable t, Functor t, Member t r) => Union r v -> (v -> Eff r a) -> (t v -> Eff r a) -> Eff r a Source

Given a request, either handle it or relay it. Both the handler and the relay can produce the same type of request that was handled.

handleRelay Source


:: Typeable t 
=> Union (t :> r) v


-> (v -> Eff r a)

Relay the request

-> (t v -> Eff r a)

Handle the request of type t

-> Eff r a 

Given a request, either handle it or relay it.

unsafeReUnion :: Union r w -> Union t w Source

Juggle types for a Union. Use cautiously.