[Haskell-beginners] Expected / Inferred mis-match and confused!

[Daniel Fischer]

On Thursday 09 June 2011, 18:26:18, Sean Charles wrote:
> Here are the snippets of my code currently hurting my coding ulcer...
> 
> routes :: [Record] -> IO ()
> routes legodata = do
>    let all = foldl forwardRoutes M.empty legodata -- [Record]
>    return ()
> 
> 
> forwardRoutes :: M.Map String [String] -> Record -> M.Map String
> [String]
> forwardRoutes map row =
>    let key = row !! 0 in
>    let val = row !! 1 in
>    case lookup key map of

Ah, that needs to be M.lookup, without qualifying the call, it refers to 
Prelude.lookup :: Eq a => a -> [(a,b)] -> Maybe b

>      Nothing     -> M.insert key [val] map


>      Just routes -> M.insert key val:routes map

This needs parentheses, M.insert key (val:routes) map

> 
> I have spent a long time searching and reading and trying to fathom it 
> out from first principles but I cannot for the life of me figure out 
> where '[(Field, b)]' is coming from!

Prelude.lookup vs. Data.Map.lookup

> I can smell Field in there because 
> Record is typed as [Field], but a tuple? And what is 'b' ?
> 
> The 'inferred type' matches what I thought it should be in the error 
> message and I am right (I think) in stating that the 'expected type' is 
> what it actually got from the code at compile time?!
> 
> Help!
> 
> :)