# Copyright (C) 2008, Parrot Foundation.
# $Id: tutorial_episode_9.pod 36833 2009-02-17 20:09:26Z allison $

=head1 Episode 9: Wrap up and Conclusion

Welcome to the final Episode of the Parrot Compiler Tools Tutorial!
Let's review the previous episodes, and summarize this tutorial.

=head2 Review

In Episode 1, we introduced the Parrot Compiler Tools (PCT), gave a high-level
feature overview of Squaak, the case study language that we are implementing in
this tutorial, and we generated a language shell that we use as a foundation to
implement Squaak.

Episode 2 discussed the general structure of PCT-based compilers. After this,
we described each of the four default compilation stages: parse phase,
parse tree to PAST, PAST to POST and POST to PIR. We also added a command line
banner and command line prompt to the interactive language shell.

In Episode 3, we introduced the full grammar of the Squaak language. After this,
we started implementing the first bits, after which we were able to generate
code for (simple) assignments.

In Episode 4 we discussed the construction of Parrot Abstract Syntax Tree nodes
in more detail, after which we implemented the if-statement and throw-statement.

Episode 5 focused on variable declarations and variable scope. We implemented
the necessary infrastructure to handle global and local variables correctly. In
Episode 6 we continued the discussion of scope, but now in the context of
subroutines. After this we implemented subroutine invocation.

Episode 7 extended our grammar to handle complex expressions that allows us to
use arithmetic and other operators. We discussed how to use PCT's built-in
support for handling operator precedence.

In the previous episode, Episode 8, we discussed the grammar and action methods
for handling the aggregate data types of Squaak: arrays and hashes. We also
touched on the topic of argument passing by reference and by value.

If you followed the tutorial and did the exercises, your implementation should
be complete. Although a lot of the implementation was discussed, some parts were
left as the proverbial exercise to the reader. This is to stimulate you to get
your hands dirty and figure out things for yourself, while the text contained
enough hints (in my opinion) to solve the given problems. Sure enough, this
approach requires you to spend more time and think for yourself, but I think
you're reading all this stuff to learn something. The extra time spent is well
worth it, in my opinion.

Now it's time to see what we can do with this language. Squaak is more than just
the average calculator example, which is often provided in beginners'
discussions on parsers; it's a complete programming language.

=head2 What's Next?

This is the last episode of the Parrot Compiler Tools tutorial. We showed how we
implemented a complete language for the Parrot virtual machine in only a few
hundred lines of source code. Surely, this must be the proof that the PCT really
is an effective toolkit for implementing languages. At the moment of writing,
the PCT still lacks efficient support for certain language constructs.
Therefore, we focused on the parts that are easy to build with the PCT. Once the
PCT is feature complete, there's bound to be another tutorial on advanced
features. Think of object-oriented programming, closures, coroutines, and
advanced control-flow such as return statements. Most of them can be done
already, but are too complex for this tutorial's level.

=head2 The Game of Life

You might have noticed that Squaak looks a bit like Lua, although it does differ
in some points. This is not entirely accidental. In the distribution of the Lua
source code, there's an example called "life.lua", which implements Conway's
"Game of Life". This is a nice demonstration program, and it's easy to port it
to Squaak. Its implementation is shown below. Run it, and enjoy!

 ## John Conway's Game of Life
 ## Implementation based on life.lua, found in Lua's distribution.
 ##
 var width          = 40    # width of "board"
 var height         = 20    # height of "board"
 var generation     = 1     # generation counter
 var numgenerations = 50    # how often should we evolve?

 ## initialize board to all zeroes
 sub initboard(board)
     for var y = 0, height do
         for var x = 0, width do
             board[y][x] = 0
         end
     end
 end

 ## spawn new life in board, at position (left, top),
 ## the life data is stored in shapedata, and shape width and
 ## height are specified.
 sub spawn(board, left, top, shapew, shapeh, shapedata)
     for var y = 0, shapeh - 1 do
         for var x = 0, shapew - 1 do
             board[top + y][left + x] = shapedata[y * shapew + x]
         end
     end
 end

 ## calculate the next generation.
 sub evolve(thisgen, nextgen)
     var ym1 = height - 1
     var y   = height
     var yp1 = 1
     var yi  = height

     while yi > 0 do
         var xm1 = width-1
         var x   = width
         var xp1 = 1
         var xi  = width

         while xi > 0 do
             var sum = thisgen[ym1][xm1]
                     + thisgen[ym1][x]
                     + thisgen[ym1][xp1]
                     + thisgen[y][xm1]
                     + thisgen[y][xp1]
                     + thisgen[yp1][xm1]
                     + thisgen[yp1][x]
                     + thisgen[yp1][xp1]

             nextgen[y][x] = sum==2 and thisgen[y][x] or sum==3
             xm1 = x
             x   = xp1
             xp1 = xp1 + 1
             xi  = xi - 1
         end

         ym1 = y
         y   = yp1
         yp1 = yp1 + 1
         yi  = yi - 1
     end
 end

 ## display thisgen to stdout.
 sub display(thisgen)
     var line = ""
     for var y = 0, height do
         for var x = 0, width do
             if thisgen[y][x] == 0 then
                 line = line .. "-"
             else
                 line = line .. "O"
             end
         end
         line = line .. "\n"
     end
     print(line, "\nLife - generation: ", generation)
 end

 ## main program
 sub main()
     var heart   = [1,0,1,1,0,1,1,1,1]
     var glider  = [0,0,1,1,0,1,0,1,1]
     var explode = [0,1,0,1,1,1,1,0,1,0,1,0]
     var thisgen = []
     initboard(thisgen)
     var nextgen = []
     initboard(nextgen)
     spawn(thisgen,3,5,3,3,heart)
     spawn(thisgen,5,4,3,3,glider)
     spawn(thisgen,25,10,3,4,explode)
     while generation <= numgenerations do
         evolve(thisgen, nextgen)
         display(thisgen)
         generation = generation + 1

         ## prevent switching nextgen and thisgen around,
         ## just call evolve with arguments switched.
         evolve(nextgen, thisgen)
         display(nextgen)
         generation = generation + 1
     end
 end

 ## start here.
 main()

Note the use of a subroutine "print". Check out the file src/builtins/say.pir,
and rename the sub "say" (which was generated by the language shell creation
script) to "print".

=head2 Exercises

Squaak was designed to be a simple language, offering enough features to get
some work done, but at the same time keeping it simple. Of course, after reading
this tutorial, You are an expert too ;-) If you feel like adding more features,
here are some suggestions.

=over 4

=item *

Implement prefix and postfix increment/decrement operators, allowing you to
write "generation++" instead of "generation = generation + 1".

=item *

Implement augmenting assign operators, such as "+=" and friends.

=item *

Extend the grammar to allow multiple variable declarations in one statement,
allowing you to write "var x = 1, y, z = 3". Of course, the initialization part
should still be optional. How do you make sure that the identifier and
initialization expression are kept together?

=item *

Implement a mechanism (such as an "import" statement) to include or load
another Squaak file, so Squaak programs can be split into multiple files.
The PCT does not have any support for this, so you'll need to write a bit of
PIR to do this.

=item *

Improve the for-statement, to allow for a negative step. Note that the loop
condition becomes more complex when doing so.

=back

Note that these are suggestions, and I did not implement them myself, so I
won't have a solution for you at the end.

=head2 Final words and Acknowledgments

By now, you should have got a good impression of the PCT and you should be able
to work on other languages targeting Parrot. Currently, work has been done on
ECMAScript, Python, Ruby and of course Perl 6. Most of them are not complete
yet (hint, hint).

I hope you enjoyed reading this tutorial and learned enough to feel confident
about working on other (existing) languages targeting Parrot. The Perl 6
implementation can still use more contributors!

Many thanks to all who read this tutorial and provided me with hints, tips and
feedback! Thank I<You> for reading this!

=cut

