Haskell get values from IO domain

Question

After reading the Haskell books I am kind of confused (or I simply forgot) how to get a value from the IO domain, into the 'Haskell world' to parse it, like so:

fGetSeq = do
  input <- sequence [getLine, getLine, getLine]
  fTest input
  mapM_ print input

fTest =  map (read :: String -> Int)

Obviously compiler complains. Couldn't match [] with IO. Is there a simple rule of thumb for passing values between 'worlds' or is it just my bad by omitting typesigs?

Answer 1

get a value from the IO domain, into the 'Haskell world'

You use the bind operator: (>>=) :: Monad m => m a -> (a -> m b) -> m b.

If m = IO it looks like: (>>=) :: IO a -> (a -> IO b) -> IO b.

As you can see, the function with type a -> IO b addresses the a without IO.

So given a value in the IO monad, e.g. getLine :: IO String:

getInt :: IO Int
getInt = getLine >>= (\s -> return (read s))

Here, s :: String, read :: String -> Int, and return :: Int -> IO Int.

You can rewrite this using a do-block:

getInt :: IO Int
getInt = do
  s <- getLine
  return (read s)

Or use the standard library function that does exactly this:

getInt :: IO Int
getInt = readLn

As for your example, you can immediately fix it using a let-binding:

foo :: IO ()
foo = do
  input <- sequence [getLine, getLine, getLine]
  let ints = bar input
  mapM_ print ints

bar :: [String] -> [Int]
bar = map read

Or you can restructure it to use getInt as defined above:

foo :: IO ()
foo = sequence [getInt, getInt, getInt] >>= mapM_ print