The type should be IO [String]. You do IO, and it results in a list of
Strings.
Also, take a look at the following types:
contents :: String
lines :: String -> [String]
Which means that,
lines contents :: [String]
But because you're dealing with a monad (IO in this case), this will not
typecheck. You can convert a pure value to a monadic value using the return
function:
return :: Monad m => a -> m a
Not specific to IO, but all monads.
Hope this helps.
On 19 February 2015 at 22:47, Geoffrey Bays
Haskellers:
I want to write a function that takes a class name and reads from a file and then returns the list of String of the file lines. I have experimented with various return types, but GHC does not like any of them. Here is my function:
readData :: String -> IO String -- what type here?? readData classNameP = do let fileName = classNameP ++ ".txt" contents <- readFile fileName lines contents
Not to complain, but this would not be difficult in any other language I have tried, so I could use some explanation. I suspect it has to do with understanding the IO Monad.
Many Thanks
Geoffrey
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