On Thursday 09 June 2011, 13:57:47, Sean Charles wrote:
Further to my recent attempts to scan a CSV file and build a map, I now have a foldl calling this with an empty map:
--forwardRoutes :: M.Map String [String] -> Record -> M.Map forwardRoutes map row = case lookup map (row!!0) of Nothing -> M.insert (row!!0) [(row !! 1)] map Just routes -> M.insert (row!!0) (row!!1):routes map
Needs parentheses Just routes -> M.insert (row!!0) ((row!!1):routes) map Without, it's parsed (M.insert (row!!0) (row!!1)) : (routes map) I'm not in the mood to figure out how the type inference reaches its conclusion, I can tell however, that it started inferring the type with the Just-branch.
With the type declaration commented out I get this: scread.hs:102:46: Occurs check: cannot construct the infinite type: a = M.Map [(a, b)] [[(a, b)]] Expected type: M.Map [(a, b)] [[(a, b)]] Inferred type: a In the third argument of `M.insert', namely `map' In the expression: M.insert (row !! 0) [(row !! 1)] map
and with it 'in#' i get this error instead... scread.hs:99:52: `M.Map' is not applied to enough type arguments Expected kind `?', but `M.Map' has kind `* -> * -> *' In the type signature for `forwardRoutes': forwardRoutes :: M.Map String [String] -> Record -> M.Map
You forgot the type arguments of Map in the result type. M.Map is a type constructor taking two arguments, but type arguments of (->) must be fully applied type constructors
Can anybody help me to understand both problems. I thought I had declared the type signature of forwardRoutes correctly but obv. not! LOL
I have seen information that says the "infinite error" message can be cured by supplying types so I did......
Thanks again. Sean