[Haskell-beginners] Infinite type ... or 'not enough type arguments'...
daniel.is.fischer at googlemail.com
Thu Jun 9 14:27:14 CEST 2011
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
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
> With the type declaration commented out I get this:
> 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...
> `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.
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