Recientemente, implementé una ingenua DPLL Sat Solver en Haskell, adaptada de John Harrison's Handbook of Practical Logic and Automated Reasoning.Usando la Lógica Monad en Haskell
DPLL es una variedad de búsqueda de retroceso, por lo que quiero experimentar con el uso del Logic monad desde Oleg Kiselyov et al. Sin embargo, realmente no entiendo lo que necesito cambiar.
Aquí está el código que tengo.
- ¿Qué código debo cambiar para usar la mónada de Logic?
- Bonificación: ¿Hay algún beneficio concreto en el rendimiento al usar la mónada Logic?
{-# LANGUAGE MonadComprehensions #-}
module DPLL where
import Prelude hiding (foldr)
import Control.Monad (join,mplus,mzero,guard,msum)
import Data.Set.Monad (Set, (\\), member, partition, toList, foldr)
import Data.Maybe (listToMaybe)
-- "Literal" propositions are either true or false
data Lit p = T p | F p deriving (Show,Ord,Eq)
neg :: Lit p -> Lit p
neg (T p) = F p
neg (F p) = T p
-- We model DPLL like a sequent calculus
-- LHS: a set of assumptions/partial model (set of literals)
-- RHS: a set of goals
data Sequent p = (Set (Lit p)) :|-: Set (Set (Lit p)) deriving Show
{- --------------------------- Goal Reduction Rules -------------------------- -}
{- "Unit Propogation" takes literal x and A :|-: B to A,x :|-: B',
- where B' has no clauses with x,
- and all instances of -x are deleted -}
unitP :: Ord p => Lit p -> Sequent p -> Sequent p
unitP x (assms :|-: clauses) = (assms' :|-: clauses')
where
assms' = (return x) `mplus` assms
clauses_ = [ c | c <- clauses, not (x `member` c) ]
clauses' = [ [ u | u <- c, u /= neg x] | c <- clauses_ ]
{- Find literals that only occur positively or negatively
- and perform unit propogation on these -}
pureRule :: Ord p => Sequent p -> Maybe (Sequent p)
pureRule [email protected](_ :|-: clauses) =
let
sign (T _) = True
sign (F _) = False
-- Partition the positive and negative formulae
(positive,negative) = partition sign (join clauses)
-- Compute the literals that are purely positive/negative
purePositive = positive \\ (fmap neg negative)
pureNegative = negative \\ (fmap neg positive)
pure = purePositive `mplus` pureNegative
-- Unit Propagate the pure literals
sequent' = foldr unitP sequent pure
in if (pure /= mzero) then Just sequent'
else Nothing
{- Add any singleton clauses to the assumptions
- and simplify the clauses -}
oneRule :: Ord p => Sequent p -> Maybe (Sequent p)
oneRule [email protected](_ :|-: clauses) =
do
-- Extract literals that occur alone and choose one
let singletons = join [ c | c <- clauses, isSingle c ]
x <- (listToMaybe . toList) singletons
-- Return the new simplified problem
return $ unitP x sequent
where
isSingle c = case (toList c) of { [a] -> True ; _ -> False }
{- ------------------------------ DPLL Algorithm ----------------------------- -}
dpll :: Ord p => Set (Set (Lit p)) -> Maybe (Set (Lit p))
dpll goalClauses = dpll' $ mzero :|-: goalClauses
where
dpll' [email protected](assms :|-: clauses) = do
-- Fail early if falsum is a subgoal
guard $ not (mzero `member` clauses)
case (toList . join) $ clauses of
-- Return the assumptions if there are no subgoals left
[] -> return assms
-- Otherwise try various tactics for resolving goals
x:_ -> dpll' =<< msum [ pureRule sequent
, oneRule sequent
, return $ unitP x sequent
, return $ unitP (neg x) sequent ]
... todo, supongo. –
@DanielWagner: ¿De verdad? La parte que hace el backtracking es el 'msum' - en cierto modo pensé que solo necesitaba modificar' dpll'' ...? –