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mirror of https://github.com/idris-lang/Idris2.git synced 2024-12-19 09:12:34 +03:00
Idris2/libs/base/Control/Monad/Reader.idr

82 lines
2.4 KiB
Idris

module Control.Monad.Reader
import Control.Monad.Identity
import Control.Monad.Trans
||| A computation which runs in a static context and produces an output
public export
interface Monad m => MonadReader stateType m | m where
||| Get the context
ask : m stateType
||| `local f c` runs the computation `c` in an environment modified by `f`.
local : MonadReader stateType m => (stateType -> stateType) -> m a -> m a
||| The transformer on which the Reader monad is based
public export
record ReaderT (stateType : Type) (m : Type -> Type) (a : Type) where
constructor MkReaderT
runReaderT' : stateType -> m a
public export
implementation Functor f => Functor (ReaderT stateType f) where
map f (MkReaderT g) = MkReaderT (\st => map f (g st))
public export
implementation Applicative f => Applicative (ReaderT stateType f) where
pure x = MkReaderT (\st => pure x)
(MkReaderT f) <*> (MkReaderT a) =
MkReaderT (\st =>
let f' = f st in
let a' = a st in
f' <*> a')
public export
implementation Monad m => Monad (ReaderT stateType m) where
(MkReaderT f) >>= k =
MkReaderT (\st => do v <- f st
let MkReaderT kv = k v
kv st)
public export
implementation MonadTrans (ReaderT stateType) where
lift x = MkReaderT (\_ => x)
public export
implementation Monad m => MonadReader stateType (ReaderT stateType m) where
ask = MkReaderT (\st => pure st)
local f (MkReaderT action) = MkReaderT (action . f)
public export
implementation HasIO m => HasIO (ReaderT stateType m) where
liftIO f = MkReaderT (\_ => liftIO f)
public export
implementation (Monad f, Alternative f) => Alternative (ReaderT stateType f) where
empty = lift empty
(MkReaderT f) <|> (MkReaderT g) = MkReaderT (\st => f st <|> g st)
||| Evaluate a function in the context held by this computation
public export
asks : MonadReader stateType m => (stateType -> a) -> m a
asks f = ask >>= pure . f
||| Unwrap and apply a ReaderT monad computation
public export
%inline
runReaderT : stateType -> ReaderT stateType m a -> m a
runReaderT s action = runReaderT' action s
||| The Reader monad. The ReaderT transformer applied to the Identity monad.
public export
Reader : (stateType : Type) -> (a : Type) -> Type
Reader s a = ReaderT s Identity a
||| Unwrap and apply a Reader monad computation
public export
runReader : stateType -> Reader stateType a -> a
runReader s = runIdentity . runReaderT s