Using underscores in a Scala for loop (for comprehension)

By Alvin Alexander. Last updated: June 4, 2016

I was looking through this code on Github, which was shared at the 2014 Lambda Conference in Boulder, Colorado this summer, and ran across this for comprehension:

def gameLoop: IO[Unit] = for {
    input <- getLine
    _     <- putStrLn("So, you want to " + input + ", do you?")
    _     <- if (input == "quit") IO(Unit) else gameLoop
} yield Unit

The use of the underscore in the for comprehension (or for loop) is what I thought was interesting. As the author states in his README file, “The underscore in the notation _ <- putStrLn(...) just means we don’t care about the value of Unit that will be produced by putStrLn.”

If it helps to know what’s going on here, as the name implies, this gameLoop is the main loop for a simple RPG game the author was creating at the Lambda Conference. Because putStrLn returns Unit, and because we don’t care about the IO(Unit) potentially returned by the third line in the for comprehension, the underscore is used on the left side of the statements/expressions.

scala

for loop

unit

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for comprehension

underscore

Using underscores in a Scala for loop (for comprehension)

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