pony-echoarg.md

Echo Arg

This is a followup to the "Hello World" walkthrough.

Code

actor Main
  new create(env: Env) =>
    try
      env.out.print(env.args(1))
    else
      env.out.print("oops")
    end

Breakdown

The first two lines of this program are the same as the "Hello World" program. To recap, they represent the entry point of the program by defining an actor called Main with a constructor called create that accepts a single argument of type Env. This is required for all Pony programs.

What is different from the "Hello World" program is the body of the create constructor. The program is still printing a string, but is now considering outside input when deciding what to print.

If you're familiar with languages that have exceptions, you can probably figure out what is going on based on the syntax, but it may seem strange that printing would require this construct.

Try

The try keyword opens a block of code that may throw an exception. The else keyword closes that block, and opens a new block that will run only if an exception is thrown in the previous block. The end keyword closes the else block.

Args

As mentioned in the "Hello World" walkthrough, the Env type provides access to things that are normally globally available. In this case, the program needs access to the string arguments passed into the porgram at the command line. Pony stores these arguments in the args field of Env as a string array, which has the type Array[String].

The expression env.args(1) is showing off some of Pony's syntactic sugar. The expression expands to env.args.apply(1). That is, the method apply is called on the string array args of the env variable, with the argument 1. There is more too this syntax, but for now, it can be assumed that if a variable is being called like a method, then under the hood, the apply method is actually being called.

Exception

So, where does the exception come from? An exception is thrown if the index provided for an array lookup is out of bounds. In many languages, this consitutes a runtime exception and is not required to be handled. However, Pony checks this at compile time.

The compiler will either require the array lookup to occur in a try block, or for the programmer to indicate that the error should be propogated. In this case, the program is required to handle the exception because create constructor on Main cannot be marked to propogate the exception.

The else block will handle the exception and print oops to the screen. This will happen if no args are passed to the program. If an else block was not provided, the exception would still be handled, but be silently ignored.