Copyright	(c) Michael Szvetits 2020
License	BSD3 (see the file LICENSE)
Maintainer	typedbyte@qualified.name
Stability	stable
Portability	portable
Safe Haskell	None
Language	Haskell2010

Control.Effect.Reader

Contents

Tagged Reader Effect
Untagged Reader Effect
Convenience Functions
Interpretations
Tagging and Untagging

Description

The reader effect, similar to the MonadReader type class from the mtl library.

Synopsis

class Monad m => Reader' tag r m | tag m -> r where
- ask' :: m r
- local' :: (r -> r) -> m a -> m a
- reader' :: (r -> a) -> m a
type Reader r = Reader' G r
ask :: Reader r m => m r
local :: Reader r m => (r -> r) -> m a -> m a
reader :: Reader r m => (r -> a) -> m a
asks' :: forall tag r m a. Reader' tag r m => (r -> a) -> m a
asks :: Reader r m => (r -> a) -> m a
runReader' :: forall tag r m a. r -> (Reader' tag r `Via` ReaderT r) m a -> m a
runReader :: r -> (Reader r `Via` ReaderT r) m a -> m a
tagReader' :: forall new r m a. Via (Reader' G r) (Tagger G new) m a -> m a
retagReader' :: forall tag new r m a. Via (Reader' tag r) (Tagger tag new) m a -> m a
untagReader' :: forall tag r m a. Via (Reader' tag r) (Tagger tag G) m a -> m a

Tagged Reader Effect

class Monad m => Reader' tag r m | tag m -> r where Source #

An effect that adds an immutable state (i.e., an "environment") to a given computation. The effect allows to read values from the environment, pass values from function to function, and execute sub-computations in a modified environment.

Minimal complete definition

(ask' | reader'), local'

Methods

ask' :: m r Source #

Gets the environment.

local' Source #

Arguments

:: (r -> r)	The function to modify the environment.
-> m a	The sub-computation to run in the modified environment.
-> m a	The result of the sub-computation.

Executes a sub-computation in a modified environment.

reader' Source #

Arguments

:: (r -> a)	The projection function to apply to the environment.
-> m a	The result of the projection.

Gets a specific component of the environment, using the provided projection function.

Instances

Instances details

Monad m => Reader' (tag :: k) r (ReaderT r m) Source #
Instance details Defined in Control.Effect.Reader Methods ask' :: ReaderT r m r Source # local' :: (r -> r) -> ReaderT r m a -> ReaderT r m a Source # reader' :: (r -> a) -> ReaderT r m a Source #
Control (Reader' tag r) t m => Reader' (tag :: k) r (Via eff t m) Source #
Instance details Defined in Control.Effect.Reader Methods ask' :: Via eff t m r Source # local' :: (r -> r) -> Via eff t m a -> Via eff t m a Source # reader' :: (r -> a) -> Via eff t m a Source #
Handle (Reader' tag r) t m => Reader' (tag :: k) r (Via (Reader' tag r) t m) Source #
Instance details Defined in Control.Effect.Reader Methods ask' :: Via (Reader' tag r) t m r Source # local' :: (r -> r) -> Via (Reader' tag r) t m a -> Via (Reader' tag r) t m a Source # reader' :: (r -> a) -> Via (Reader' tag r) t m a Source #
Reader' new r m => Reader' (tag :: k2) r (Tagger tag new m) Source #
Instance details Defined in Control.Effect.Reader Methods ask' :: Tagger tag new m r Source # local' :: (r -> r) -> Tagger tag new m a -> Tagger tag new m a Source # reader' :: (r -> a) -> Tagger tag new m a Source #

Untagged Reader Effect

If you don't require disambiguation of multiple reader effects (i.e., you only have one reader effect in your monadic context), it is recommended to always use the untagged reader effect.

type Reader r = Reader' G r Source #

ask :: Reader r m => m r Source #

local :: Reader r m => (r -> r) -> m a -> m a Source #

reader :: Reader r m => (r -> a) -> m a Source #

Convenience Functions

If you don't require disambiguation of multiple reader effects (i.e., you only have one reader effect in your monadic context), it is recommended to always use the untagged functions.

asks' Source #

Arguments

:: forall tag r m a. Reader' tag r m
=> (r -> a)	The projection function to apply to the environment.
-> m a	The result of the projection.

Gets a specific component of the environment, using the provided projection function.

asks :: Reader r m => (r -> a) -> m a Source #

The untagged version of asks'.

Interpretations

runReader' Source #

Arguments

:: forall tag r m a. r	The initial environment.
-> (Reader' tag r `Via` ReaderT r) m a	The program whose reader effect should be handled.
-> m a	The program with its reader effect handled.

Runs the reader effect.

runReader :: r -> (Reader r `Via` ReaderT r) m a -> m a Source #

The untagged version of runReader'.

Tagging and Untagging

Conversion functions between the tagged and untagged reader effect, usually used in combination with type applications, like:

    tagReader' @"newTag" program
    retagReader' @"oldTag" @"newTag" program
    untagReader' @"erasedTag" program

tagReader' :: forall new r m a. Via (Reader' G r) (Tagger G new) m a -> m a Source #

retagReader' :: forall tag new r m a. Via (Reader' tag r) (Tagger tag new) m a -> m a Source #

untagReader' :: forall tag r m a. Via (Reader' tag r) (Tagger tag G) m a -> m a Source #