Unless in Haskell

In Haskell, unless is a predefined function in the Control.Monad module. It is used to perform a certain action only if a given Boolean expression is False. The syntax of the unless function is as follows:

unless :: Monad m => Bool -> m () -> m ()

Here, Monad is a typeclass, and m is a type variable that represents a monadic type. The Bool argument is the Boolean expression to be tested, and the second argument is the action to be performed if the Boolean expression is False. The m () part represents the monadic action, which can be thought of as a function that takes no arguments and returns no value.

Here's an example that demonstrates the use of the unless function in Haskell:

import Control.Monad
main = do

    putStrLn "Enter a number:"

    input <- getLine

    unless (null input) $ do

        let num = read input

        putStrLn ("The number is " ++ show num)

In this example, the program prompts the user to enter a number. If the user enters a non-empty string, the program converts the string to a number and prints it. If the user enters an empty string, nothing happens. This behavior is achieved using the unless function. The null input expression returns True if the input string is empty, so the action inside the $ symbol is only executed if the input string is not empty.