Control.Monad.MultiReader

The multi-valued version of mtl's Reader / ReaderT / MonadReader

MultiReaderT

newtype MultiReaderT x m a

A Reader transformer monad patameterized by:

• x - The list of types constituting the environment / input (to be read),
• m - The inner monad.

MultiReaderT corresponds to mtl's ReaderT, but can contain a heterogenous list of types.

This heterogenous list is represented using Types.Data.List, i.e:

• Null - The empty list,
• Cons a b - A list where a is an arbitrary type and b is the rest list.

For example,

MultiReaderT (Cons Int (Cons Bool Null)) :: (* -> *) -> (* -> *)

is a Reader wrapper containing the types [Int,Bool].

Constructors

MultiReaderT

Fields

runMultiReaderTRaw :: StateT (HList x) m a

Instances

MonadState s m => MonadState s (MultiReaderT c m)

MonadWriter w m => MonadWriter w (MultiReaderT c m)

(Monad m, ContainsType a c) => MonadMultiReader a (MultiReaderT c m)

MonadTrans (MultiReaderT x)

Monad m => Monad (MultiReaderT x m)

Functor f => Functor (MultiReaderT x f)

(Applicative m, Monad m) => Applicative (MultiReaderT x m)

type MultiReaderTNull = MultiReaderT []

A MultiReader transformer carrying an empty state.

type MultiReader x = MultiReaderT x Identity

A reader monad parameterized by the list of types x of the environment / input to carry.

Similar to Reader r = ReaderT r Identity

MonadMultiReader class

class Monad m => MonadMultiReader a m where

All methods must be defined.

The idea is: Any monad stack is instance of MonadMultiReader a, iff the stack contains a MultiReaderT x with a element of x.

Methods

mAsk

Arguments

:: m a

Access to a specific type in the environment.

Instances

(MonadTrans t, Monad (t m), MonadMultiReader a m) => MonadMultiReader a (t m)

(Monad m, ContainsType a c) => MonadMultiReader a (MultiReaderT c m)

functions

mAskRaw :: Monad m => MultiReaderT a m (HList a)

A raw extractor of the contained HList (i.e. the complete environment).

For a possible usecase, see withMultiReaders.

withMultiReader :: Monad m => x -> MultiReaderT (x : xs) m a -> MultiReaderT xs m a

Adds an element to the environment, thereby transforming a MultiReaderT carrying an environment with types (x:xs) to a a MultiReaderT with xs.

Think "Execute this computation with this additional value as environment".

withMultiReaders :: Monad m => HList xs -> MultiReaderT (Append xs ys) m a -> MultiReaderT ys m a

Adds a heterogenous list of elements to the environment, thereby transforming a MultiReaderT carrying an environment with values over types xs++ys to a MultiReaderT over ys.

Similar to recursively adding single values with withMultiReader.

Note that ys can be Null; in that case the return value can be evaluated further using evalMultiReaderT.

evalMultiReaderT :: Monad m => MultiReaderT [] m a -> m a

Evaluate a computation over an empty environment.

Because the environment is empty, it does not need to be provided.

If you want to evaluate a computation over any non-Null environment, either use

• evalMultiReaderTWithInitial
• simplify the computation using withMultiReader / withMultiReaders, then use evalMultiReaderT on the result.

evalMultiReaderTWithInitial

Arguments

:: Monad m

=> HList a

The initial state

-> MultiReaderT a m b

The computation to evaluate

-> m b

Evaluate a reader computation with the given environment.

mapMultiReaderT :: (m (a, HList w) -> m' (a', HList w)) -> MultiReaderT w m a -> MultiReaderT w m' a'

Map both the return value and the environment of a computation using the given function.

Note that there is a difference to mtl's ReaderT, where it is not possible to modify the environment.