# Contstuff

### From HaskellWiki

## Contents |

## 1 Introduction

The contstuff library implements a number of monad transformers and monads, which make heavy use of continuation passing style (CPS). This makes them both fast and flexible. Please note that this is neither a CPS tutorial nor a monad transformer tutorial. You should understand these concepts, before attempting to use *contstuff*.

## 2 Basics

### 2.1 ContT

The *ContT* monad transformer is the simplest of all CPS-based monads. It essentially gives you access to the current continuation, which means that it lets you label certain points of execution and reuse these points later in interesting ways. With *ContT* you get an elegant encoding of computations, which support:

- abortion (premature termination),
- resumption (start a computation at a certain spot),
- branches (aka
*goto*), - result accumulation,
- etc.

*ContT*. If you don't use them, then

*ContT*behaves like the identity monad. A computation of type

*ContT*computation you can use

runContT :: (a -> m r) -> ContT r m a -> m r evalContT :: Applicative m => ContT r m r -> m r

### 2.2 Abortion

Let's have a look at a small example:

testComp1 :: ContT () IO () testComp1 = forever $ do txt <- io getLine case txt of "info" -> io $ putStrLn "This is a test computation." "quit" -> abort () _ -> return ()

*ContT*. First of all,

*ContT*is a monad transformer, so you can for example lift

*IO*actions to a CPS computation. The

*IO*monad. You can also use the more generic

*ContT*. Each

*ContT*subcomputation receives a continuation, which is a function, to which the subcomputation is supposed to pass the result. However, the subcomputation may choose not to call the continuation at all, in which case the entire computation finishes with a final result. The

### 2.3 Resumption and branches

You can capture the current continuation using the commonlabelCC :: a -> ContT r m (a, Label (ContT r m) a) goto :: Label (ContT r m) a -> a -> ContT r m b

These slightly complicated looking functions are actually very simple to use:

testComp2 :: ContT r IO () testComp2 = do (i, again) <- labelCC 0 io (print i) when (i < 10) $ goto again (i+1) io (putStrLn $ "Final result: " ++ show i)

Labels are first class values in *contstuff*. This means you can carry them around. They are only limited in that they can't be carried outside of a *ContT* computation.

### 2.4 Lifting

As noted earlier there are three lifting functions, which you can use to access monads in lower layers of the transformer stack:

lift :: (Transformer t, Monad m) => m a -> t m a base :: (LiftBase m a) => Base m a -> m a io :: (Base m a ~ IO a, LiftBase m a) => Base m a -> m a

*IO*.

### 2.5 Accumulating results

*ContT*does not require the underlying functor to be a monad. Whenever the underlying functor is an

testComp3 :: Num a => ContT r [] (a, a) testComp3 = do x <- pure 10 <|> pure 20 y <- pure (x+1) <|> pure (x-1) return (x, y)

*contstuff*library implements a convenience function

listA :: (Alternative f) => [a] -> f a

testComp3' :: Num a => ContT r [] (a, a) testComp3' = do x <- listA [10, 20] y <- listA [x+1, x-1] return (x, y)

testComp4 :: Num a => ContT (a, a) [] (a, a) testComp4 = do x <- listA [10, 20] when (x == 10) (abort (10, 10)) y <- listA [x+1, x-1] return (x, y)