Parsing command line arguments as an Array[String] in Scala 3?

Question

Okay, so I've been trying to learn Scala from a textbook, but it was written before Scala 3 was released so a lot of the example code doesn't compile.

Here's how the book introduces command line arguments:

println(args(0).toDouble+args(1).toDouble)

and my best attempt at converting this to something Scala 3 compatible:

@main def main(args: Array[String]) = {  
  println(args(0).toDouble+args(1).toDouble)
}

and here's what the compiler tells me:

[error] ./add.scala:1:1
[error] No given instance of type scala.util.CommandLineParser.FromString[Array[String]] was found for parameter fs of method parseArgument in object CommandLineParser
[error] @main def main(args: Array[String]) = {
[error] ^

How do I fix this? There's another question like this one already posted, but the answers seem to all be written in prior versions of Scala. As for the official release guide, it recommends using something to do with CLP, which I don't understand.

CoreyOConnor · Accepted Answer · 2024-12-31 18:01:02Z

5

@main will pass all un-parsed args to a String* parameter if there is one. In the case there are no parsed args, this means:

@main def main(args: String*) = {  
  println(args(0).toDouble+args(1).toDouble)
}

answered Dec 31, 2024 at 18:01

CoreyOConnor

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Gaël J · Accepted Answer · 2025-01-01 07:29:36Z

If you're using only Scala 3, the @main annotation is the way to go but then the parameters should be defined as regular method parameters:

// ❌ Instead of:
@main def main(args: Array[String])

// ✅ Do:
@main def main(arg0: Double, arg1: Double)

One thing to note is that parameters can be typed, they don't have to be Strings only anymore. Conversion is handled for you automatically*.

You can thus write:

@main def main(arg0: Double, arg1: Double) = { 
  println(arg0 + arg1) 
}

*: only simple types are supported by default, but this can be extended by providing instances of scala.util.CommandLineParser.FromString. Note that this is the error you were initially getting: the compiler was saying "I don't know how to convert from String to Array[String]"

EDIT: as CoreyOConnor mentions in another answer, it's also possible to get all remaining parameters as a String* (varargs) if you don't explicitly type some parameters.

If you're cross-compiling to Scala 2 or just you prefer the Scala 2 way, closer to Java's, then remove the @main annotation and be sure to include the method in an object.

object MyProgram {
  def main(args: Array[String]) = ???
}

Documentation: https://docs.scala-lang.org/scala3/book/methods-main-methods.html

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