Data.List.Cut

A ListT-like monad transformer equipped with the cut operator known from Prolog.

The CutListT transformer

data CutListT' m a

Constructors

CCons a (m (CutListT' m a))

CNil

CCut

Instances

Functor m => Functor (CutListT' m)

Foldable m => Foldable (CutListT' m)

Traversable m => Traversable (CutListT' m)

newtype CutListT m a

A monad transformer that behaves like the list transformer, but it allows Prolog's cut operator.

Constructors

CutListT

Fields

unCutListT :: m (CutListT' m a)

Instances

MonadTrans CutListT

Monad m => Monad (CutListT m)

Functor m => Functor (CutListT m)

(Functor m, Monad m) => Applicative (CutListT m)

Foldable m => Foldable (CutListT m)

Traversable m => Traversable (CutListT m)

(Functor m, Monad m) => Alternative (CutListT m)

(Functor m, Monad m) => MonadPlus (CutListT m)

(Functor m, Monad m) => Monoid (CutListT m a)

(Functor m, Monad m) => MonoidRZero (CutListT m a)

retract :: Monad m => CutListT m a -> m ()

Ignore the elements on the list and combine the monadic computations.

The CutList monad

type CutList = CutListT Identity

List with Prolog's cut operator.

cutToList :: CutList a -> [a]

Convert to a regular list.

cutFromList :: [a] -> CutList a

Convert from a regular list

Control functions

cut :: (Functor m, Monad m) => CutListT m ()

Discard yet uninspected choices.

cutFail :: (Functor m, Monad m) => CutListT m ()

Discard the uninspected choices and fail the current branch of computation. Equal to cut >> mzero.

scope :: (Functor m, Monad m) => CutListT m a -> CutListT m a

Delimit the scope of cuts in the argument.

Examples

takeWhile using lists with cut

We implement the functon takeWhile using cuts:

takeWhile :: (a -> Bool) -> [a] -> [a]
takeWhile p xs = toList $ do
  x <- fromList xs
  when (not $ p x) cutFail
  return x