repl

Whenever I used to get the Scala REPL warning message, "There were deprecation warnings, re-run with -deprecation", I used to have to restart the REPL with the -deprecation flag to see the warning message, like this:

$ scala -deprecation

That works, but sadly, you had to lose your entire REPL session to run it.

Use :warnings

Thanks to the Scala gods, this is no longer necessary in Scala 2.10 (and newer). Now, when you get the REPL deprecation warning message, like this:

In a little bit of accidental Scala REPL fun this afternoon, I was testing a recursive algorithm like the following simple factorial algorithm, and forgot a return statement:

def factorial(n: Int):Int = {
  if (n == 1) n    // forgot 'return' here
  factorial(n - 1)
}

When I pasted that code into the Scala REPL and then tried to test it as shown below, it didn't come back for a while, until it failed with this error message:

When you're working in the Scala REPL and want to see what methods are available on a class/object, you can create an instance of an object, follow that with the "." character, and then press the [Tab] key. This process, known as "tab completion" in the REPL, gives you a preliminary list of methods that can be called on the object.

Here's what this looks like when we try it on an Int object:

Scala REPL FAQ: How do I add a Jar file to the Scala REPL classpath? (The Scala REPL is the interactive command line you get if you just type "scala" at your command line.)

To add a new jar file to the Scala REPL classpath (interactive command line classpath), use the ":cp" command at the command line, like this:

scala> :cp myjar.jar

After you do this, you should see a reply from the REPL like this: