Can't find the error in my Haskell code

问题

I tried to translate a (working !) solution of the cabbage-goat-wolf puzzle from Scala to Haskell, but the code throws and error when calling head in findSolutions because the solution list is empty, so the problem seems to be somewhere in loop. findMoves seems to work fine.

import Data.Maybe(fromMaybe)

data Item = Farmer | Cabbage | Goat | Wolf deriving (Eq, Show)

type Position = ([Item], [Item]) 

validPos :: Position -> Bool
validPos p = valid (fst p) && valid (snd p) where   
   valid list = elem Farmer list || notElem Goat list || 
                (notElem Cabbage list && notElem Wolf list) 

findMoves :: Position -> [Position]
findMoves (left,right) = filter validPos moves where
    moves | elem Farmer left = map (\item -> (delItem item left, addItem item right)) left 
          | otherwise = map (\item -> (addItem item left, delItem item right)) right
    delItem item = filter (\i ->  notElem i [item, Farmer]) 
    addItem Farmer list = Farmer:list      
    addItem item list = Farmer:item:list      

findSolution :: Position -> Position -> [Position]
findSolution from to = head $ loop [[from]] where
    loop pps = do
          (p:ps) <- pps
          let moves = filter (\x -> notElem x (p:ps)) $ findMoves p
          if elem to moves then return $ reverse (to:p:ps)
                           else loop $ map (:p:ps) moves  

solve :: [Position]
solve = let all = [Farmer, Cabbage, Goat, Wolf]
        in findSolution (all,[]) ([],all)

Of course I would also appreciate hints concerning improvements not related to the actual error.

[Update]

Just for the record, I followed the suggestion to use a Set. Here is the working code:

import Data.Set

data Item = Farmer | Cabbage | Goat | Wolf deriving (Eq, Ord, Show)

type Position = (Set Item, Set Item)

validPos :: Position -> Bool
validPos p = valid (fst p) && valid (snd p) where
   valid set = or [Farmer `member` set, Goat `notMember` set, 
                   Cabbage `notMember` set && Wolf `notMember` set]

findMoves :: Position -> [Position]
findMoves (left,right) = elems $ Data.Set.filter validPos moves where
    moves | Farmer `member` left = Data.Set.map (move delItem addItem) left
          | otherwise = Data.Set.map (move addItem delItem) right
    move f1 f2 item = (f1 item left, f2 item right)
    delItem item = delete Farmer . delete item 
    addItem item = insert Farmer . insert item 

findSolution :: Position -> Position -> [Position]
findSolution from to = head $ loop [[from]] where
    loop pps = do
          ps <- pps
          let moves = Prelude.filter (\x -> notElem x ps) $ findMoves $ head ps
          if to `elem` moves then return $ reverse $ to:ps
                             else loop $ fmap (:ps) moves

solve :: [Position]
solve = let all = fromList [Farmer, Cabbage, Goat, Wolf]
        in findSolution (all, empty) (empty, all)

The call to head in findSolution could be made safer, and a better way to print out the solution should be used, but apart from that I'm quite happy with it.

[Update 2]

I think the former representations of the positions were suboptimal for this kind of problem. I switched to the following data model, which made moving etc slightly more verbose, but much more readable:

data Place = Here | There deriving (Eq, Show)

data Pos = Pos { cabbage :: Place
               , goat :: Place
               , wolf :: Place
               , farmer :: Place 
               } deriving (Eq, Show)

回答1:

The problem is that [Farmer,Goat,Cabbage,Wolf] is not the same that [Farmer,Cabbage,Goat,Wolf] and you don't check it when use elem and notElem. One solution is always sort the list of elements, e.g in the function findMoves you can use:

import Data.List(ord)
import Control.Arrow((***))

data Item = Farmer | Cabbage | Goat | Wolf deriving (Eq, Show, Ord)

findMoves (left,right) = map (sort***sort) $ filter validPos moves where
-- ....

solve = let all = sort [Farmer, Cabbage, Goat, Wolf]
-- ....

Or you can use a set of Item instead a list of Item.

来源：https://stackoverflow.com/questions/6580208/cant-find-the-error-in-my-haskell-code