Re: [Haskell-beginners] function to read file and return list of contents not compiling

Thanks. I played around some more with return and came up with this which compiles fine: readData :: String -> IO [String] readData classNameP = do let fileName = classNameP ++ ".txt" contents <- readFile fileName return $ lines contents Geoffrey On Thu, Feb 19, 2015 at 12:25 PM, Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:

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 wrote:

...

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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-- Regards

Sumit Sahrawat

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