How to read an Int in one line of code?

Question

I'm just starting to learn Haskell after first getting my hands dirty in functional programming with python.

First of all, I know what I will ask is bad practice is virtually every programming language known to man, and I promise to not use this in any serious purpose other than purely educational. That said...

One of the things I liked best was the elegance of writing a function using just one line of code while still keeping the code readable(things like C/C++ style where you just place all the statements on one line separated by semicolons don't count, obviously)

I have looked trough questions and tried at least a few different haskell tutorials and documentations and some things have gotten close, but I could not find something that lets me read a usable Int in one line of code.

For the sake of the question let's say I want something like reading an Int for the keyboard and printing "Hello World!" that many times(shamelessly stolen from HackerRank).

The things I have tried using getLine, readLn, read, looked into >>= instead of do notation, only to remain stuck with IO operations stubbornly giving me an error every time I did not put them in a separate statement.

Some of the things I have tried:

main :: IO ()
main = readLn :: IO Int >>= \n -> mapM_ putStrLn $ replicate n "Hello World!"

gives:

parse error on input ‘\’

Then

main = readLn >>= \n -> read n :: Int >>= \n -> mapM_ putStrLn $ replicate n "Hello World!"

Gives the same error.

Same with:

main = getLine >>= \n -> read n :: Int >>= \n -> mapM_ putStrLn $ replicate n "Hello World!"

What exactly am I not understanding? Thanks in advance!

Answer 1

You're mixing type declarations with expressions. It should be:

main = (readLn :: IO Int) >>= \n -> mapM_ putStrLn $ replicate n "Hello World!"

Your problem was that without bracketing, the compiler continues to parse as if it were reading a type declaration, and so it ran across \ and got stuck.

However, this type declaration is inferred (since replicate :: Int -> a -> [a]) and thus unnecessary. Remove it!

main = readLn >>= \n -> mapM_ putStrLn $ replicate n "Hello World!"

In general however, it's much better to avoid this entirely and just use do-notation. This kind of 'golfing' in Haskell is very fun, but not the way to make clear, maintainable code.

import Control.Monad (replicateM_)

main = do
    n <- readLn :: IO Int                   -- Type optional here.
    replicateM_ n $ putStrLn "Hello World!" -- A more concise expression.