Question

I know almost nothing about haskell and I’m reading lots of tutorials and code samples to understand Haskell‘s IO, but it is still not clear to me.

All examples are like:

main = do  
    putStrLn "Hello, what's your name?"  
    name <- getLine  
    putStrLn ("Hey " ++ name ++ ", welcome !") 

They read a string from IO and there are many other examples that do some customization on input that I don’t understand.

My question is simply: How to read Int, Float, Double or Char like using scanf in C with a format string?

And can anyone help me with a clear explanation to IO customization or tricks?

Hint: if the code comes with a main function that would
be great – it would make testing easier.

I have very good coding experience in C, C++, Java, C#, Matlab and PHP.

Answer 1

isturdy’s answer is valid, but in case you want something a bit simpler, the easiest approach is to do it step-by-step:

main = do
    i' <- getLine -- some integral input; it will be received as a String
    d' <- getLine -- some floating input; also will be received as a String
    let i = read i' :: Int -- i is an Int now
        d = read d' :: Double -- d is a Double now
    putStr "" -- so that GHC doesn't complain

The type of getLine is IO String – it reads input as a String. You can then change the type of this string with read (and indicating what type you want). As for a scanf-like experience, you can try something like this:

main = do
    inp <- getLine -- the input is an Int, a String and a Double, like: 4 derp 2.7
    let list = words inp -- divide the input String into a list of Strings
        val = read (list !! 0) :: Int -- val equals 4, its type is Int
        str = list !! 1 -- str is a String ("derp"), no additional action required
        dbl = read (list !! 2) :: Double -- dbl equals 2.7 and is a Double
    putStr ""