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 Anybody! LOL :)

-deech

On Tue, Jun 7, 2011 at 4:44 PM, Sean Charles wrote:

...

OK, I am losing enough hair and accruing follicular damage more than I can take tonight, what is wrong, what have I done that is stupid and wrong etc...

called: let routes = forwardRoutes M.empty csvdata -- [Record] from Text.CSV

implemented:

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

error: scread.hs:101:2: parse error on input `forwardRoutes'

Um yeah, thanks, that's really helping me figure it out.

All I wanted to do was build a map of keys and values where each value is a list that gradually accumulates(!) every row that ends at (!!1) from the same location (!!0) of my spreadsheet. I started off zipping with 'repeat []' and stuff but I just want to know what I have done to annoy the compiler enough to want to taunt me with that message.

And, on the verge of posting I think the above code won't work anyway because I am not passing the new Map from 'insert' through.......

ARGH! :(

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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