Control.Monad.Trans.Accum

The lazy AccumT monad transformer, which adds accumulation capabilities (such as declarations or document patches) to a given monad.

This monad transformer provides append-only accumulation during the computation. For more general access, use Control.Monad.Trans.State instead.

The Accum monad

type Accum w = AccumT w Identity

An accumulation monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

accum :: Monad m => (w -> (a, w)) -> AccumT w m a

Construct an accumulation computation from a (result, output) pair. (The inverse of runAccum.)

runAccum :: Accum w a -> w -> (a, w)

Unwrap an accumulation computation as a (result, output) pair. (The inverse of accum.)

execAccum :: Accum w a -> w -> w

Extract the output from an accumulation computation.

• execAccum m w = snd (runAccum m w)

evalAccum :: Monoid w => Accum w a -> w -> a

Evaluate an accumulation computation with the given initial output history and return the final value, discarding the final output.

• evalAccum m w = fst (runAccum m w)

mapAccum :: ((a, w) -> (b, w)) -> Accum w a -> Accum w b

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

• runAccum (mapAccum f m) = f . runAccum m

The AccumT monad transformer

newtype AccumT w m a

An accumulation monad parameterized by:

• w - the output to accumulate.
• m - The inner monad.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

This monad transformer is similar to both state and writer monad transformers. Thus it can be seen as

• a restricted append-only version of a state monad transformer or
• a writer monad transformer with the extra ability to read all previous output.

Constructors

AccumT (w -> m (a, w))

Instances

Monoid w => MonadTrans (AccumT w)

Defined in Control.Monad.Trans.Accum

Methods

lift :: Monad m => m a -> AccumT w m a

(Monoid w, Functor m, Monad m) => Monad (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

(>>=) :: AccumT w m a -> (a -> AccumT w m b) -> AccumT w m b

(>>) :: AccumT w m a -> AccumT w m b -> AccumT w m b

return :: a -> AccumT w m a

fail :: String -> AccumT w m a

Functor m => Functor (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

fmap :: (a -> b) -> AccumT w m a -> AccumT w m b

(<$) :: a -> AccumT w m b -> AccumT w m a

(Monoid w, Functor m, MonadFix m) => MonadFix (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

mfix :: (a -> AccumT w m a) -> AccumT w m a

(Monoid w, MonadFail m) => MonadFail (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

fail :: String -> AccumT w m a

(Monoid w, Functor m, Monad m) => Applicative (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

pure :: a -> AccumT w m a

(<*>) :: AccumT w m (a -> b) -> AccumT w m a -> AccumT w m b

liftA2 :: (a -> b -> c) -> AccumT w m a -> AccumT w m b -> AccumT w m c

(*>) :: AccumT w m a -> AccumT w m b -> AccumT w m b

(<*) :: AccumT w m a -> AccumT w m b -> AccumT w m a

(Monoid w, Functor m, MonadIO m) => MonadIO (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

liftIO :: IO a -> AccumT w m a

(Monoid w, Functor m, MonadPlus m) => Alternative (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

empty :: AccumT w m a

(<|>) :: AccumT w m a -> AccumT w m a -> AccumT w m a

some :: AccumT w m a -> AccumT w m [a]

many :: AccumT w m a -> AccumT w m [a]

(Monoid w, Functor m, MonadPlus m) => MonadPlus (AccumT w m)

Defined in Control.Monad.Trans.Accum

Methods

mzero :: AccumT w m a

mplus :: AccumT w m a -> AccumT w m a -> AccumT w m a

runAccumT :: AccumT w m a -> w -> m (a, w)

Unwrap an accumulation computation.

execAccumT :: Monad m => AccumT w m a -> w -> m w

Extract the output from an accumulation computation.

• execAccumT m w = liftM snd (runAccumT m w)

evalAccumT :: (Monad m, Monoid w) => AccumT w m a -> w -> m a

Evaluate an accumulation computation with the given initial output history and return the final value, discarding the final output.

• evalAccumT m w = liftM fst (runAccumT m w)

mapAccumT :: (m (a, w) -> n (b, w)) -> AccumT w m a -> AccumT w n b

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

• runAccumT (mapAccumT f m) = f . runAccumT m

Accum operations

look :: (Monoid w, Monad m) => AccumT w m w

look is an action that fetches all the previously accumulated output.

looks :: (Monoid w, Monad m) => (w -> a) -> AccumT w m a

look is an action that retrieves a function of the previously accumulated output.

add :: Monad m => w -> AccumT w m ()

add w is an action that produces the output w.

Lifting other operations

liftCallCC :: CallCC m (a, w) (b, w) -> CallCC (AccumT w m) a b

Uniform lifting of a callCC operation to the new monad. This version rolls back to the original output history on entering the continuation.

liftCallCC' :: CallCC m (a, w) (b, w) -> CallCC (AccumT w m) a b

In-situ lifting of a callCC operation to the new monad. This version uses the current output history on entering the continuation. It does not satisfy the uniformity property (see Control.Monad.Signatures).

liftCatch :: Catch e m (a, w) -> Catch e (AccumT w m) a

Lift a catchE operation to the new monad.

liftListen :: Monad m => Listen w m (a, s) -> Listen w (AccumT s m) a

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (a, s) -> Pass w (AccumT s m) a

Lift a pass operation to the new monad.

Monad transformations

readerToAccumT :: (Functor m, Monoid w) => ReaderT w m a -> AccumT w m a

Convert a read-only computation into an accumulation computation.

writerToAccumT :: WriterT w m a -> AccumT w m a

Convert a writer computation into an accumulation computation.

accumToStateT :: (Functor m, Monoid s) => AccumT s m a -> StateT s m a

Convert an accumulation (append-only) computation into a fully stateful computation.