eveff-0.1.0.0: Efficient effect handlers based on evidence translation.

Copyright (c) 2020 Microsoft Research; Daan Leijen; Ningning Xie MIT xnning@hku.hk; daan@microsoft.com Experimental None Haskell2010

Control.Ev.Eff

Description

Efficient effect handlers based on Evidence translation. The implementation is based on "Effect Handlers, Evidently", Ningning Xie et al., ICFP 2020 (pdf), and the interface and design is described in detail in "Effect Handlers in Haskell, Evidently", Ningning Xie and Daan Leijen, Haskell 2020 (pdf).

An example of defining and using a Reader effect:

{-# LANGUAGE  TypeOperators, FlexibleContexts, Rank2Types  #-}
import Control.Ev.Eff

-- A Reader effect definition with one operation ask of type () to a.
data Reader a e ans = Reader{ ask :: Op () a e ans }

greet :: (Reader String :? e) => Eff e String
greet = do s <- perform ask ()
return ("hello " ++ s)

test :: String
test = runEff $handler (Reader{ ask = value "world" })$  -- :: Reader String () Int
do s <- greet                              -- executes in context :: Eff (Reader String :* ()) Int
return (length s)


Enjoy,

Daan Leijen and Ningning Xie, May 2020.

Synopsis

# Effect monad

data Eff e a Source #

The effect monad in an effect context e with result a

Instances
handler (Reader{ ask = value "world" }) $greet  handler :: h e ans -> Eff (h :* e) ans -> Eff e ans Source # Use handler hnd action to handle effect h with handler hnd in action (which has h in its effect context now as h :* e). For example: data Reader a e ans = Reader { ask :: Op () a e ans } reader :: a -> Eff (Reader a :* e) ans -> Eff e ans reader x = handler (Reader{ ask = value x })  handlerRet :: (ans -> a) -> h e a -> Eff (h :* e) ans -> Eff e a Source # Handle an effect h over action and tranform the final result with the return function ret. For example: data Except a e ans = Except { throwError :: forall b. Op a b e ans } exceptMaybe :: Eff (Except a :* e) ans -> Eff e (Maybe ans) exceptMaybe = handlerRet Just (Except{ throwError = except (\_ -> return Nothing) })  handlerHide :: h (h' :* e) ans -> Eff (h :* e) ans -> Eff (h' :* e) ans Source # Define a handler h that hides the top handler h' from its action, while keeping h' is still visible in the operation definitions of h. This is used to implement locally isolated state for handlerLocal using the regular local state. In particular, handlerLocal is implemented as: handlerLocal :: a -> (h (Local a :* e) ans) -> Eff (h :* e) ans -> Eff e ans handlerLocal init h action = local init (handlerHide h action)  mask :: Eff e ans -> Eff (h :* e) ans Source # Mask the top effect handler in the give action (i.e. if a operation is performed on an h effect inside e the top handler is ignored). # Defining operations data Op a b e ans Source # The abstract type of operations of type a to b, for a handler defined in an effect context e and answer type ans. value :: a -> Op () a e ans Source # Create an operation that always resumes with a constant value (of type a). (see also the perform example). function :: (a -> Eff e b) -> Op a b e ans Source # Create an operation that takes an argument of type a and always resumes with a result of type b. These are called tail-resumptive operations and are implemented more efficient than general operations as they can execute in-place (instead of yielding to the handler). Most operations are tail-resumptive. (See also the handlerLocal example). except :: (a -> Eff e ans) -> Op a b e ans Source # Create an operation that never resumes (an exception). (See handlerRet for an example). operation :: (a -> (b -> Eff e ans) -> Eff e ans) -> Op a b e ans Source # Create an fully general operation from type a to b. the function f takes the argument, and a resumption function of type b -> Eff e ans that can be called to resume from the original call site. For example: data Amb e ans = Amb { flip :: forall b. Op () Bool e ans } xor :: (Amb :? e) => Eff e Bool xor = do x <- perform flip () y <- perform flip () return ((x && not y) || (not x && y)) solutions :: Eff (Amb :* e) a -> Eff e [a] solutions = handlerRet (\x -> [x])$
Amb{ flip = operation (\() k -> do{ xs <- k True; ys <- k False; return (xs ++ ys)) }) }


# Local state

data Local a e ans Source #

The type of the built-in state effect. (This state is generally more efficient than rolling your own and usually used in combination with handlerLocal to provide local isolated state)

local :: a -> Eff (Local a :* e) ans -> Eff e ans Source #

Create a local state handler with an initial state of type a.

localRet :: a -> (ans -> a -> b) -> Eff (Local a :* e) ans -> Eff e b Source #

Create a local state handler with an initial state of type a, with a return function to combine the final result with the final state to a value of type b.

handlerLocal :: a -> h (Local a :* e) ans -> Eff (h :* e) ans -> Eff e ans Source #

Create a new handler for h which can access the locally isolated state Local a. This is fully local to the handler h only and not visible in the action as apparent from its effect context (which does not contain Local a).

data State a e ans = State { get :: Op () a e ans, put :: Op a () e ans  }

state :: a -> Eff (State a :* e) ans -> Eff e ans
state init = handlerLocal init (State{ get = function (\_ -> perform lget ()),
put = function (\x -> perform lput x) })

test = runEff $state (41::Int)$
inc                -- see :?


handlerLocalRet :: a -> (ans -> a -> b) -> h (Local a :* e) b -> Eff (h :* e) ans -> Eff e b Source #

Create a new handler for h which can access the locally isolated state Local a. This is fully local to the handler h only and not visible in the action as apparent from its effect context (which does not contain Local a). The ret argument can be used to transform the final result combined with the final state.

lget :: Local a e ans -> Op () a e ans Source #

Get the value of the local state.

lput :: Local a e ans -> Op a () e ans Source #

Set the value of the local state.

lmodify :: Local a e ans -> Op (a -> a) () e ans Source #

Update the value of the local state.

localGet :: Eff (Local a :* e) a Source #

Get the value of the local state if it is the top handler.

localPut :: a -> Eff (Local a :* e) () Source #

Set the value of the local state if it is the top handler.

localModify :: (a -> a) -> Eff (Local a :* e) () Source #

Update the value of the local state if it is the top handler.