Parrot Babysteps 04 - Adding Command Line Arguments

Introduction

We have learned a reasonable amount so far. We know how to write fairly trivial applications using Parrot Intermediate Representation. We could probably write a simple formula calculator that gets input from the user, ensures that the content is valid, and presents the results of applying user input to the formula.

It would be nice to write more ambitious programs, though. It would be painful - maybe even impossible - to create a modern program using only the tools and opcodes we have learned so far.

We can start examining PMCs by writing a version of our hypotenuse calculator from a few steps ago that has command line arguments.

Command Line Arguments

How do we tell Parrot that our program accepts command line arguments, though? We need some way to show that our :main sub is ready to take parameters. Turns out that’s actually pretty easy.

.param directive

The .param directive is used at the start of a subroutine to indicate that the subroutine will accept a parameter and place it in the named variable.

.sub 'main' :main
    .param pmc argv
.end

argv is a ResizableStringArray: an ordered collection of strings.

We can use the elements opcode to find out how many arguments were passed to the file.

.sub 'main' :main
    .param pmc argv
    .local int argument_count
    .local string description

    argument_count = elements argv
    description = "I was called with "
    $S0 = argument_count
    description .= $S0
    description .= " arguments"

    say description
.end

Try it out.

$ parrot example-04-02.pir hey there
I was called with 3 arguments

Three? Let’s look at the arguments individually and see if we can figure this out.

shift

The shift opcode lets us pull the first item from an array. This shrinks the argv array by one as it shifts the rest of its contents over to fill the empty space, but it’s not a concern for us right now.

.sub 'main' :main
    .param pmc argv
    .local int argument_count
    .local string this_argument
    .local string description

  GET_ARG:
    argument_count = argv
    if argument_count <= 0 goto END
    this_argument = shift argv
    description = "This argument: " . this_argument
    say description
    goto GET_ARG

  END:

.end

We can use this code to look at our program arguments one at a time.

bash-3.2$ parrot example-04-03.pir hey there
This argument: example-04-03.pir
This argument: hey
This argument: there

Oh, right. The program name is the first argument. That is not unusual, especially in some lower level languages. I should have remembered.

Calculating a Hypotenuse

Let’s take what we’ve learned about handling the command line and apply it to our hypotenuse calculator.

.sub 'main' :main
    .param pmc argv
    .local int argument_count
    .local string program_name
    .local num a
    .local num b
    .local num c
    .local num a_squared
    .local num b_squared
    .local num c_squared
    .local string error_message

    program_name = shift argv
    argument_count = elements argv
    if argument_count != 2 goto BAD_ARG_COUNT
    a = shift argv
    b = shift argv
    a_squared = a * a
    b_squared = b * b
    say a_squared
    say b_squared
    c_squared = a_squared + b_squared
    c = sqrt c_squared
    say c
    goto END

  BAD_ARG_COUNT:
    error_message = "Exactly two arguments required"
    say error_message
    goto END

  END:
.end

First we shift the first item off of argv because we know for sure that it’s going to be the program name. Then we check to make sure that the user has provided us with two arguments that we can use for a and b. Not having two arguments is an error, so the program branches to displaying an error and quitting. When the argument count is right, the program shifts the arguments into a and b then uses them to calculate and display the hypotenuse.

Oh yeah - sooner or later you’re going to see this error message from Parrot:

error:imcc:syntax error, unexpected '=', expecting '(' ('=')
        in file 'example-04-04.pir' line 24

What did line 24 look like?

c_squared = a_squared + b_squared

It toook me a while to realize that I had never declared .local num c_squared. Unfortunately, Parrot’s error messages aren’t quite as descriptive as Perl’s. Perl has had a lot more time to figure out how to gently explain a user’s error to him, though.

bash-3.2$ parrot example-04-04.pir 12 23
144
529
25.9422435421457
bash-3.2$

That worked.

Parrot has many special opcodes for dealing with PMCs and soon we’ll be looking at more of them. I am especially interested in the opcodes that allow us to use arrays and other collection types.

Summary

Yes, this has been a very quick step. Maybe we didn’t learn a whole lot, but there’s a little bit of new stuff in there. We did learn how to add command line handling by using .param to tell Parrot that our :main method accepts parameters. We learned that for :main, the parameter is a particular PMC - something called a ResizableStringArray. The shift opcode removes the first item in a ResizableStringArray and lets us use it in a variable. We also saw that we can use the members opcode to get the number of members in an array.