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 -- Solver.hs
 -- cleaned up with stylish-haskell (hackage)
 -- and https://github.com/tibbe/haskell-style-guide
 -- also, flatten is 'concat', I think
 
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE MultiParamTypeClasses  #-}
 {-# LANGUAGE ScopedTypeVariables    #-}
 
 module Solver
     (Solver,State,Transition)
   where
 
 class (Show t,Eq t) => Transition t where
     transition :: State s => s -> t -> s
 
 class (Show s,Eq s) => State s where
     getPossibleTransitions :: Transition t => s -> [t]
     isStateValid :: s -> Bool
     isGoalState  :: s -> Bool
 
 class (State s, Transition t) => Solver s t | s -> t where
     getPossibleNextStates :: s -> [s]
     getPossibleNextStates state =
        filter isStateValid (map transitionFunction possibleTransitions)
       where
        transitionFunction  = transition state :: t -> s
        possibleTransitions = getPossibleTransitions state
 
     isStateVisited        :: [s] -> s -> Bool
     isStateVisited visitedStates state =
        any (== state) visitedStates
 
     getNextFringeStates :: [s] -> [[s]]
     getNextFringeStates = map getPossibleNextStates