Control.Monad.Trans.Accum

Copyright	(c) Nickolay Kudasov 2016
License	BSD-style (see the file LICENSE)
Maintainer	[email protected]
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

The Accum monad
The AccumT monad transformer
Accum operations
Lifting other operations
Monad transformations

Description

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 Source

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 Source

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

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

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

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

Extract the output from an accumulation computation.

```
execAccum m w = snd (runAccum m w)
```

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

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 Source

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 Source

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

Instances details

Monoid w => MonadTrans (AccumT w)
Instance details Defined in Control.Monad.Trans.Accum Methods lift :: Monad m => m a -> AccumT w m a Source
(Monoid w, Functor m, Monad m) => Monad (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods (>>=) :: AccumT w m a -> (a -> AccumT w m b) -> AccumT w m b Source (>>) :: AccumT w m a -> AccumT w m b -> AccumT w m b Source return :: a -> AccumT w m a Source
Functor m => Functor (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods fmap :: (a -> b) -> AccumT w m a -> AccumT w m b Source (<$) :: a -> AccumT w m b -> AccumT w m a Source
(Monoid w, Functor m, MonadFix m) => MonadFix (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods mfix :: (a -> AccumT w m a) -> AccumT w m a Source
(Monoid w, MonadFail m) => MonadFail (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods fail :: String -> AccumT w m a Source
(Monoid w, Functor m, Monad m) => Applicative (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods pure :: a -> AccumT w m a Source (<>) :: AccumT w m (a -> b) -> AccumT w m a -> AccumT w m b Source liftA2 :: (a -> b -> c) -> AccumT w m a -> AccumT w m b -> AccumT w m c Source (>) :: AccumT w m a -> AccumT w m b -> AccumT w m b Source (<*) :: AccumT w m a -> AccumT w m b -> AccumT w m a Source
(Monoid w, Functor m, MonadIO m) => MonadIO (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods liftIO :: IO a -> AccumT w m a Source
(Monoid w, Functor m, MonadPlus m) => Alternative (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods empty :: AccumT w m a Source (<\|>) :: AccumT w m a -> AccumT w m a -> AccumT w m a Source some :: AccumT w m a -> AccumT w m [a] Source many :: AccumT w m a -> AccumT w m [a] Source
(Monoid w, Functor m, MonadPlus m) => MonadPlus (AccumT w m)
Instance details Defined in Control.Monad.Trans.Accum Methods mzero :: AccumT w m a Source mplus :: AccumT w m a -> AccumT w m a -> AccumT w m a Source

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

Unwrap an accumulation computation.

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

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 Source

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 Source

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 Source

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

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

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

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

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 Source

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 Source

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 Source

Lift a catchE operation to the new monad.

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

Lift a listen operation to the new monad.

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

Lift a pass operation to the new monad.