% Pattern Matching of Lazy Non-Deterministic Data % Sebastian Fischer (sebf@informatik.uni-kiel.de) > {-# LANGUAGE > RankNTypes, > TypeFamilies, > FlexibleContexts, > FlexibleInstances, > ExistentialQuantification > #-} > > module Data.LazyNondet.Matching ( > > Match, match, withHNF, caseOf, caseOf_ > > ) where > > import Data.Data > import Data.LazyNondet.Types > import Data.LazyNondet.Combinators > > import Control.Monad.State > import Control.Monad.Constraint > import Control.Monad.Constraint.Choice > > withHNF :: (Monad m, MonadSolve cs m m) > => Nondet cs m a > -> (HeadNormalForm cs m -> cs -> Nondet cs m b) > -> cs -> Nondet cs m b > withHNF x b cs = Typed (do > (hnf,cs') <- runStateT (solve (untyped x)) cs > untyped (b hnf cs')) The `withHNF` operation can be used for pattern matching and solves constraints associated to the head constructor of a non-deterministic value. An updated constraint store is passed to the computation of the branch function. Collected constraints are kept attached to the computed value by using an appropriate instance of `MonadSolve` that does not eliminate them. > class WithUntyped a > where > type C a > type M a :: * -> * > type T a > > withUntyped :: a -> [Untyped (C a) (M a)] -> Nondet (C a) (M a) (T a) We repeat the definition of the type class `With` because the current implementation of GHC does not allow equality constraints in super-class constraints. We would prefer to define this class as follows: class (With [Untyped cs m] a, m ~ Mon [Untyped cs m] a) => WithUnique a where withUnique :: a -> [Untyped cs m] -> Nondet cs m (Typ [Untyped cs m] a) withUnique = with So it is just a copy of the type class `With` where the argument type is specialized to use the same monad. > instance WithUntyped (Nondet cs m a) > where > type C (Nondet cs m a) = cs > type M (Nondet cs m a) = m > type T (Nondet cs m a) = a > > withUntyped = const > > instance (WithUntyped a, cs ~ C a, m ~ M a) > => WithUntyped (Nondet cs m b -> a) > where > type C (Nondet cs m b -> a) = C a > type M (Nondet cs m b -> a) = M a > type T (Nondet cs m b -> a) = T a > > withUntyped alt (x:xs) = withUntyped (alt (Typed x)) xs > withUntyped _ _ = error "LazyNondet.withUntyped: too few arguments" These instances define the overloaded function `withUntyped` that has all of the following types at the same time: withUntyped :: Nondet cs m a -> [Untyped cs m] -> Nondet cs m a withUntyped :: (Nondet cs m a -> Nondet cs m b) -> [Untyped cs m] -> Nondet cs m b withUntyped :: (Nondet cs m a -> Nondet cs m b -> Nondet cs m c) -> [Untyped cs m] -> Nondet cs m c ... If the function given as first argument has n arguments, then the application of `withUntyped` to this function consumes n elements of the list of untyped values and yields the result of applying the given function to typed versions of these values. > newtype Match a cs m b = Match { unMatch :: (ConIndex, cs -> Branch cs m b) } > data Branch cs m a = > forall t . (WithUntyped t, cs ~ C t, m ~ M t, a ~ T t) => Branch t > > match :: (ConsRep a, WithUntyped b) > => a -> (C b -> b) -> Match t (C b) (M b) (T b) > match c alt = Match (constrIndex (consRep c), Branch . alt) The operation `match` is used to build destructor functions for non-deterministic values that can be used with `caseOf`. > caseOf :: (MonadSolve cs m m, MonadConstr Choice m) > => Nondet cs m a -> [Match a cs m b] -> cs -> Nondet cs m b > caseOf x bs = caseOf_ x bs failure > > caseOf_ :: (MonadSolve cs m m, MonadConstr Choice m) > => Nondet cs m a -> [Match a cs m b] -> Nondet cs m b > -> cs -> Nondet cs m b > caseOf_ x bs def = > withHNF x $ \hnf cs -> > case hnf of > FreeVar _ y -> caseOf_ (Typed y) bs def cs > Delayed res -> caseOf_ (Typed (res cs)) bs def cs > Cons _ idx args -> > maybe def (\b -> branch (b cs) args) (lookup idx (map unMatch bs)) > > branch :: Branch cs m a -> [Untyped cs m] -> Nondet cs m a > branch (Branch alt) = withUntyped alt We provide operations `caseOf_` and `caseOf` (with and without a default alternative) for more convenient pattern matching. The untyped values are hidden so functional-logic code does not need to match on the `Cons` constructor explicitly. However, using this combinator causes an additional slowdown because of the list lookup. It remains to be checked how big the slowdown of using `caseOf` is compared to using `withHNF` directly.