Re: [Haskell-beginners] Help with meaningful error message...

Think about using foldl [1] because it abstracts the general pattern of recursion you are attempting. So forward routes becomes something like: forwardRouter = foldl (\m row -> ...) Also you might want to move whatever's in "..." into a new function that updates the map. -deech [1] http://hackage.haskell.org/packages/archive/haskell98/latest/doc/html/List.h... On Tue, Jun 7, 2011 at 4:54 PM, 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

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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