Control.Break

Example usage:

import Control.Break
import Control.Monad.State
import Prelude hiding (break)

example :: State Int ()
example = loop (do
    n <- lift get                -- Inside a `loop`, wrap commands in `lift`
    if n < 10
        then lift (put (n + 1))  -- You keep looping by default
        else break () )          -- Use `break` to exit from the `loop`

The loop command runs the given command repeatedly until the command breaks from the loop using break:

>>> execState example 0
10

For some effects (like State), you can omit lift:

example :: State Int ()
example = loop (do
    n <- get
    if n < 10
        then put (n + 1)
        else break () )

The loop will return whatever value you supply to break:

example :: State Int Bool
example = loop (do
    n <- get
    if n < 10
        then put (n + 1)
        else break True )

>>> runState example 0
(True,10)

Break

data Break r m a

For the most common use cases you will:

• build Break commands using lift or break
• combine Break commands using do notation
• consume Break commands using loop

The meaning of the type parameters:

• r: the argument type of break and the return type of the loop
• m: the base Monad that you are running in a loop
• a: the return type of a Break command (not the same as the return value of the loop)

Instances

MonadReader d m => MonadReader d (Break r m)

Methods

ask :: Break r m d

local :: (d -> d) -> Break r m a -> Break r m a

reader :: (d -> a) -> Break r m a

MonadState s m => MonadState s (Break r m)

Methods

get :: Break r m s

put :: s -> Break r m ()

state :: (s -> (a, s)) -> Break r m a

MonadWriter w m => MonadWriter w (Break r m)

Methods

writer :: (a, w) -> Break r m a

tell :: w -> Break r m ()

listen :: Break r m a -> Break r m (a, w)

pass :: Break r m (a, w -> w) -> Break r m a

MonadTrans (Break r)

Methods

lift :: Monad m => m a -> Break r m a

Monad m => Monad (Break r m)

Methods

(>>=) :: Break r m a -> (a -> Break r m b) -> Break r m b

(>>) :: Break r m a -> Break r m b -> Break r m b

return :: a -> Break r m a

fail :: String -> Break r m a

Functor m => Functor (Break r m)

Methods

fmap :: (a -> b) -> Break r m a -> Break r m b

(<$) :: a -> Break r m b -> Break r m a

Monad m => Applicative (Break r m)

Methods

pure :: a -> Break r m a

(<*>) :: Break r m (a -> b) -> Break r m a -> Break r m b

(*>) :: Break r m a -> Break r m b -> Break r m b

(<*) :: Break r m a -> Break r m b -> Break r m a

MonadIO m => MonadIO (Break r m)

Methods

liftIO :: IO a -> Break r m a

MonadCont m => MonadCont (Break r m)

Methods

callCC :: ((a -> Break r m b) -> Break r m a) -> Break r m a

loop :: Monad m => Break r m () -> m r

(loop m) runs the action 'm' repeatedly until you break from the loop

break :: Monad m => r -> Break r m a

break from a loop

The argument you supply to break is the return value of the loop

Re-exports

lift :: MonadTrans t => forall m a. Monad m => m a -> t m a

Lift a computation from the argument monad to the constructed monad.