Copyright | (c) Tom Harding 2020 |
---|---|

License | MIT |

Safe Haskell | None |

Language | Haskell2010 |

`MoriarT`

is a monad transformer implementing the `MonadCell`

class with
provenance and backtracking search. In other words, it can search large
parameter spaces using different parameter configurations, looking for
contradicting sets of parameters to prune out parts of the search tree. It does
this by keeping track of which cells influence which results, and considering
any influencers on a failure to be contradictory.

In other words: if a write to cell `A`

fails, and the write was based on values
from cells `B`

and `C`

, any search branch in which `B`

and `C`

have these
current values will be *pruned* from the search, and we won't try them.

(In practice, this isn't strictly true: we just abort any branch that ever
produces any cell with any provenance that contains those values for `B`

and
`C`

. This is a "lazier" strategy, and doesn't involve evaluating the search
space up front).

## Synopsis

- newtype MoriarT (m :: Type -> Type) (x :: Type) = MoriarT {}
- runAll :: Monad m => MoriarT m x -> m [x]
- runOne :: Monad m => MoriarT m x -> m (Maybe x)
- solve :: (PrimMonad m, EqR f, Merge (f x), Typeable x) => Config (MoriarT m) (f x) -> (forall m'. MonadCell m' => [Prop m' (f x)] -> Prop m' (f Bool)) -> MoriarT m [f x]
- unsafeRead :: PrimMonad m => Cell (MoriarT m) x -> MoriarT m x

# Documentation

newtype MoriarT (m :: Type -> Type) (x :: Type) Source #

The constraint-solving monad transformer. We implement the current
computation context with `MonadReader`

, and the current "no goods" list with
`MonadState`

.

This transformer exposes its internals through the `MonadReader`

,
`MonadState`

, `MonadLogic`

, and `MonadIO`

interfaces, and should therefore
*not* be used directly. The reason is simply that misuse of any of these
will break the computation, so the library provides Control.Monad.Holmes
and Control.Monad.Watson, who do their best to thwart `MoriarT`

.

#### Instances

MonadTrans MoriarT Source # | |

Defined in Control.Monad.MoriarT | |

Monad (MoriarT m) Source # | |

Functor (MoriarT m) Source # | |

Applicative (MoriarT m) Source # | |

MonadIO m => MonadIO (MoriarT m) Source # | |

Defined in Control.Monad.MoriarT | |

Alternative (MoriarT m) Source # | |

MonadPlus (MoriarT m) Source # | |

Monad m => MonadLogic (MoriarT m) Source # | |

Defined in Control.Monad.MoriarT | |

PrimMonad m => PrimMonad (MoriarT m) Source # | |

PrimMonad m => MonadCell (MoriarT m) Source # | |

Defined in Control.Monad.MoriarT discard :: MoriarT m x Source # fill :: x -> MoriarT m (Cell (MoriarT m) x) Source # watch :: Cell (MoriarT m) x -> (x -> MoriarT m ()) -> MoriarT m () Source # with :: Cell (MoriarT m) x -> (x -> MoriarT m ()) -> MoriarT m () Source # write :: Merge x => Cell (MoriarT m) x -> x -> MoriarT m () Source # | |

Semigroup x => Semigroup (MoriarT m x) Source # | |

Monoid x => Monoid (MoriarT m x) Source # | |

type PrimState (MoriarT m) Source # | |

Defined in Control.Monad.MoriarT | |

newtype Cell (MoriarT m) content Source # | |

runAll :: Monad m => MoriarT m x -> m [x] Source #

Run a `MoriarT`

computation and return the list of **all** valid branches'
results, in the order in which they were discovered.

runOne :: Monad m => MoriarT m x -> m (Maybe x) Source #

Run a `MoriarT`

computation and return the *first* valid branch's result.

solve :: (PrimMonad m, EqR f, Merge (f x), Typeable x) => Config (MoriarT m) (f x) -> (forall m'. MonadCell m' => [Prop m' (f x)] -> Prop m' (f Bool)) -> MoriarT m [f x] Source #

unsafeRead :: PrimMonad m => Cell (MoriarT m) x -> MoriarT m x Source #

Unsafely read from a cell. This operation is unsafe because it doesn't
factor this cell into the provenance of any subsequent writes. If this value
ends up causing a contradiction, we may end up removing branches of the
search tree that are totally valid! This operation is safe as long as it is
the **very last thing** you do in a computation, and its value is **never**
used to influence any writes in any way.