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

- data Eff r a
- data VE w r
- class Member t r
- class Member t r => SetMember set t r | r set -> t
- data Union r v
- data a :> b
- inj :: (Functor t, Typeable1 t, Member t r) => t v -> Union r v
- prj :: (Typeable1 t, Member t r) => Union r v -> Maybe (t v)
- prjForce :: (Typeable1 t, Member t r) => Union r v -> (t v -> a) -> a
- decomp :: Typeable1 t => Union (t :> r) v -> Either (Union r v) (t v)
- send :: (forall w. (a -> VE w r) -> Union r (VE w r)) -> Eff r a
- admin :: Eff r w -> VE w r
- run :: Eff () w -> w
- interpose :: (Typeable1 t, Functor t, Member t r) => Union r v -> (v -> Eff r a) -> (t v -> Eff r a) -> Eff r a
- handleRelay :: Typeable1 t => Union (t :> r) v -> (v -> Eff r a) -> (t v -> Eff r a) -> Eff r a
- unsafeReUnion :: Union r w -> Union t w

# Documentation

Basic datatype returned by all computations with extensible effects.
The type `r`

is the type of effects that can be handled,
and `a`

is the type of value that is returned.

The

determines whether `Member`

t r`t`

is anywhere in the sum type `r`

.

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

`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

A sum data type, for composing effects

prj :: (Typeable1 t, Member t r) => Union r v -> Maybe (t v)Source

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

prjForce :: (Typeable1 t, Member t r) => Union r v -> (t v -> a) -> aSource

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

decomp :: Typeable1 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 :: (forall w. (a -> VE w r) -> Union r (VE w r)) -> Eff r aSource

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

admin :: Eff r w -> VE w rSource

Tell an effectful computation that you're ready to start running effects and return a value.

interpose :: (Typeable1 t, Functor t, Member t r) => Union r v -> (v -> Eff r a) -> (t v -> Eff r a) -> Eff r aSource

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.

:: Typeable1 t | |

=> Union (t :> r) v | Request |

-> (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 wSource

Juggle types for a Union. Use cautiously.