Safe Haskell  None 

Language  Haskell2010 
The multivalued version of mtl's Reader / ReaderT / MonadReader
 newtype MultiReaderT x m a = MultiReaderT {
 runMultiReaderTRaw :: StateT (HList x) m a
 type MultiReaderTNull = MultiReaderT []
 type MultiReader x = MultiReaderT x Identity
 class Monad m => MonadMultiReader a m where
 mAsk :: m a
 mAskRaw :: Monad m => MultiReaderT a m (HList a)
 withMultiReader :: Monad m => x > MultiReaderT (x : xs) m a > MultiReaderT xs m a
 withMultiReaders :: Monad m => HList xs > MultiReaderT (Append xs ys) m a > MultiReaderT ys m a
 evalMultiReaderT :: Monad m => MultiReaderT [] m a > m a
 evalMultiReaderTWithInitial :: Monad m => HList a > MultiReaderT a m b > m b
 mapMultiReaderT :: (m (a, HList w) > m' (a', HList w)) > MultiReaderT w m a > MultiReaderT w m' a'
MultiReaderT
newtype MultiReaderT x m a Source
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:
 The empty list,Null
 A list whereCons
a ba
is an arbitrary type andb
is the rest list.
For example,
MultiReaderT (Cons Int (Cons Bool Null)) :: (* > *) > (* > *)
is a Reader wrapper containing the types [Int,Bool].
MultiReaderT  

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 [] Source
A MultiReader transformer carrying an empty state.
type MultiReader x = MultiReaderT x Identity Source
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 Source
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.
(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) Source
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 Source
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 Source
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 Source
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 nonNull environment, either use
evalMultiReaderTWithInitial
 simplify the computation using
withMultiReader
/withMultiReaders
, then useevalMultiReaderT
on the result.
evalMultiReaderTWithInitial Source
:: 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' Source
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.