On Tuesday 07 June 2011, 23:54:18, Sean Charles wrote:
On 07/06/11 22:50, aditya siram wrote:
I think the parse error is here:
forwardRoutes m [] = m forwardRoutes m (row:rows) =
case M.lookup m (row !! 0) of
Just route -> M.update m (row !! 1):route Nothing -> M.insert from []
forwardRoutes m rows<----------------- you probably want to remove this line
If I do that, how does my recursive function call itself again?
I think it is badly broken in the syntax! In the last five minutes I have progressed to ...
forwardRoutes :: M.Map String [String] -> [Record] -> M.Map forwardRoutes m [] = m forwardRoutes m (row:rows) = case M.lookup m (row !! 0) of Just route -> M.update m (row !! 1):route Nothing -> M.insert (row!!0) [] forwardRoutes m rows
but I think it ought to be...
forwardRoutes :: M.Map String [String] -> [Record] -> M.Map forwardRoutes m [] = m forwardRoutes m (row:rows) = case M.lookup m (row !! 0) of Just route -> forwardRoutes (M.update m (row !! 1):route) rows Nothing -> forwardRoutes (M.insert (row!!0) []) rows
Much closer. Now you have a recursion and it can be parsed. The error will now arise in type-checking. You've flipped the arguments in the lookup. In the `Just' case, (M.update m (row !! 1):route) is parsed as (M.update m (row !! 1)) : route which is a list, not a map. You'll need to parenthesize thus: ((row !! 1) : route) However, Prelude> :t Data.Map.update Data.Map.update :: Ord k => (a -> Maybe a) -> k -> Data.Map.Map k a -> Data.Map.Map k a so there's more to change in that branch. It seems that if (row !! 0) is a key in the map, you want to modify the associated value by prepending (row !! 1) to it. That would be achieved by (M.update (\_ -> Just ((row !! 1):route)) (row !! 0) m) But you don't use the value associated to the key here [you could, but you've already looked that up before], so update is probably the wrong function here, try (M.insert (row !! 0) ((row !! 1) : route) m) In the `Nothing' branch, you've omitted the map in which to insert. forwardRoutes m (row@(h:t):rows) = case M.lookup h m of Just route -> forwardRoutes (M.insert h (head t:route) m) rows Nothing -> forwardRoutes (M.insert h [] m) rows would do what I think you want. But it'd better be written as a fold, forwardRoutes m rows = Data.List.foldl' update1 m rows -- you can leave off the arguments if you wish Now, update1 is the single-row update function, e.g. update1 m [] = m -- empty row, do nothing update1 m (x:xs) = case M.lookup x m of Nothing -> M.insert x [] m Just route | null xs -> ? -- what then? | otherwise -> M.insert x (head xs:route) m