Copyright  (c) Dan Doel 

License  BSD3 
Maintainer  dan.doel@gmail.com 
Stability  experimental 
Portability  nonportable (multiparameter type classes) 
Safe Haskell  Safe 
Language  Haskell98 
A backtracking, logic programming monad.
Adapted from the paper /Backtracking, Interleaving, and Terminating Monad Transformers/, by Oleg Kiselyov, Chungchieh Shan, Daniel P. Friedman, Amr Sabry (http://okmij.org/ftp/papers/LogicT.pdf)
Synopsis
 class MonadPlus m => MonadLogic m where
 reflect :: MonadLogic m => Maybe (a, m a) > m a
Documentation
class MonadPlus m => MonadLogic m where Source #
Minimal implementation: msplit
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 softcut. 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)
Pruning. Selects one result out of many. Useful for when multiple results of a computation will be equivalent, or should be treated as such.
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
MonadLogic [] Source #  
Monad m => MonadLogic (LogicT m) Source #  
Defined in Control.Monad.Logic msplit :: 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 #  
MonadLogic m => MonadLogic (StateT s m) Source #  See note on splitting above. 
Defined in Control.Monad.Logic.Class msplit :: 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 #  
MonadLogic m => MonadLogic (StateT s m) Source #  See note on splitting above. 
Defined in Control.Monad.Logic.Class msplit :: 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 #  
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'

Defined in Control.Monad.Logic.Class msplit :: 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 # 
reflect :: MonadLogic m => Maybe (a, m a) > m a Source #
The inverse of msplit. Satisfies the following law:
msplit m >>= reflect == m