Copyright (c) Dan Doel BSD3 dan.doel@gmail.com experimental non-portable (multi-parameter type classes) Safe Haskell98

Description

Adapted from the paper /Backtracking, Interleaving, and Terminating Monad Transformers/, by Oleg Kiselyov, Chung-chieh Shan, Daniel P. Friedman, Amr Sabry (http://okmij.org/ftp/papers/LogicT.pdf)

Synopsis

# Documentation

Minimal implementation: msplit

Minimal complete definition

msplit

Methods

msplit :: m a -> m (Maybe (a, m a)) Source #

Attempts to split the computation, giving access to the first result. Satisfies the following laws:

msplit mzero                == return Nothing
msplit (return a mplus m) == return (Just (a, m))

interleave :: m a -> m a -> m a Source #

Fair disjunction. It is possible for a logical computation to have an infinite number of potential results, for instance:

odds = return 1 mplus liftM (2+) odds

Such computations can cause problems in some circumstances. Consider:

do x <- odds mplus return 2
if even x then return x else mzero

Such a computation may never consider the 'return 2', and will therefore never terminate. By contrast, interleave ensures fair consideration of both branches of a disjunction

(>>-) :: m a -> (a -> m b) -> m b infixl 1 Source #

Fair conjunction. Similarly to the previous function, consider the distributivity law for MonadPlus:

(mplus a b) >>= k = (a >>= k) mplus (b >>= k)

If 'a >>= k' can backtrack arbitrarily many tmes, (b >>= k) may never be considered. (>>-) takes similar care to consider both branches of a disjunctive computation.

ifte :: m a -> (a -> m b) -> m b -> m b Source #

Logical conditional. The equivalent of Prolog's soft-cut. If its first argument succeeds at all, then the results will be fed into the success branch. Otherwise, the failure branch is taken. satisfies the following laws:

ifte (return a) th el           == th a
ifte mzero th el                == el
ifte (return a mplus m) th el == th a mplus (m >>= th)

once :: m a -> m a Source #

Pruning. Selects one result out of many. Useful for when multiple results of a computation will be equivalent, or should be treated as such.

lnot :: m a -> m () Source #

Inverts a logic computation. If m succeeds with at least one value, lnot m fails. If m fails, then lnot m succeeds the value ().

Instances
 Source # Instance detailsDefined in Control.Monad.Logic.Class Methodsmsplit :: [a] -> [Maybe (a, [a])] Source #interleave :: [a] -> [a] -> [a] Source #(>>-) :: [a] -> (a -> [b]) -> [b] Source #ifte :: [a] -> (a -> [b]) -> [b] -> [b] Source #once :: [a] -> [a] Source #lnot :: [a] -> [()] Source # Monad m => MonadLogic (LogicT m) Source # Instance detailsDefined in Control.Monad.Logic Methodsmsplit :: LogicT m a -> LogicT m (Maybe (a, LogicT m a)) Source #interleave :: LogicT m a -> LogicT m a -> LogicT m a Source #(>>-) :: LogicT m a -> (a -> LogicT m b) -> LogicT m b Source #ifte :: LogicT m a -> (a -> LogicT m b) -> LogicT m b -> LogicT m b Source #once :: LogicT m a -> LogicT m a Source #lnot :: LogicT m a -> LogicT m () Source # MonadLogic m => MonadLogic (StateT s m) Source # See note on splitting above. Instance detailsDefined in Control.Monad.Logic.Class Methodsmsplit :: StateT s m a -> StateT s m (Maybe (a, StateT s m a)) Source #interleave :: StateT s m a -> StateT s m a -> StateT s m a Source #(>>-) :: StateT s m a -> (a -> StateT s m b) -> StateT s m b Source #ifte :: StateT s m a -> (a -> StateT s m b) -> StateT s m b -> StateT s m b Source #once :: StateT s m a -> StateT s m a Source #lnot :: StateT s m a -> StateT s m () Source # MonadLogic m => MonadLogic (StateT s m) Source # See note on splitting above. Instance detailsDefined in Control.Monad.Logic.Class Methodsmsplit :: StateT s m a -> StateT s m (Maybe (a, StateT s m a)) Source #interleave :: StateT s m a -> StateT s m a -> StateT s m a Source #(>>-) :: StateT s m a -> (a -> StateT s m b) -> StateT s m b Source #ifte :: StateT s m a -> (a -> StateT s m b) -> StateT s m b -> StateT s m b Source #once :: StateT s m a -> StateT s m a Source #lnot :: StateT s m a -> StateT s m () Source # MonadLogic m => MonadLogic (ReaderT e m) Source # Note that splitting a transformer does not allow you to provide different input to the monadic object returned. For instance, in:let Just (_, rm') = runReaderT (msplit rm) r in runReaderT rm' r'r' will be ignored, because r was already threaded through the computation. Instance detailsDefined in Control.Monad.Logic.Class Methodsmsplit :: ReaderT e m a -> ReaderT e m (Maybe (a, ReaderT e m a)) Source #interleave :: ReaderT e m a -> ReaderT e m a -> ReaderT e m a Source #(>>-) :: ReaderT e m a -> (a -> ReaderT e m b) -> ReaderT e m b Source #ifte :: ReaderT e m a -> (a -> ReaderT e m b) -> ReaderT e m b -> ReaderT e m b Source #once :: ReaderT e m a -> ReaderT e m a Source #lnot :: ReaderT e m a -> ReaderT e m () Source #

reflect :: MonadLogic m => Maybe (a, m a) -> m a Source #

The inverse of msplit. Satisfies the following law:

msplit m >>= reflect == m