Scala 3/scala/scala.annotation/MainAnnotation

MainAnnotation

scala.annotation.MainAnnotation

See theMainAnnotation companion object

trait MainAnnotation[Parser[_], Result] extends StaticAnnotation

MainAnnotation provides the functionality for a compiler-generated main class. It links a compiler-generated main method (call it compiler-main) to a user written main method (user-main). The protocol of calls from compiler-main is as follows:

create a command with the command line arguments,
for each parameter of user-main, a call to command.argGetter, or command.varargGetter if is a final varargs parameter,
a call to command.run with the closure of user-main applied to all arguments.

Example:

/** Sum all the numbers
 *
 *  @param first Fist number to sum
 *  @param rest The rest of the numbers to sum
 */
@myMain def sum(first: Int, second: Int = 0, rest: Int*): Int = first + second + rest.sum

generates

object foo {
  def main(args: Array[String]): Unit = {
    val mainAnnot = new myMain()
    val info = new Info(
      name = "foo.main",
      documentation = "Sum all the numbers",
      parameters = Seq(
        new Parameter("first", "scala.Int", hasDefault=false, isVarargs=false, "Fist number to sum"),
        new Parameter("rest", "scala.Int" , hasDefault=false, isVarargs=true, "The rest of the numbers to sum")
      )
    )
    val mainArgsOpt = mainAnnot.command(info, args)
    if mainArgsOpt.isDefined then
      val mainArgs = mainArgsOpt.get
      val args0 = mainAnnot.argGetter[Int](info.parameters(0), mainArgs(0), None) // using parser Int
      val args1 = mainAnnot.argGetter[Int](info.parameters(1), mainArgs(1), Some(() => sum$default$1())) // using parser Int
      val args2 = mainAnnot.varargGetter[Int](info.parameters(2), mainArgs.drop(2)) // using parser Int
      mainAnnot.run(() => sum(args0(), args1(), args2()*))
  }
}

Value parameters

Parser: The class used for argument string parsing and arguments into a T
Result: The required result type of the main method. If this type is Any or Unit, any type will be accepted.

Attributes

Companion: object
Experimental: true
Source: MainAnnotation.scala
Graph
Supertypes: trait StaticAnnotation

class Annotation

class Object

trait Matchable

class Any
Known subtypes: class newMain

Members list

Value members

Abstract methods

The getter for the idxth argument of type T

Value parameters

defaultArgument: Optional lambda to instantiate the default argument
idx: The index of the argument

Attributes

Source: MainAnnotation.scala

Process the command arguments before parsing them.

Return Some of the sequence of arguments that will be parsed to be passed to the main method. This sequence needs to have the same length as the number of parameters of the main method (i.e. info.parameters.size). If there is a varags parameter, then the sequence must be at least of length info.parameters.size - 1.

Returns None if the arguments are invalid and parsing and run should be stopped.