# Copyright (C) 2008, Parrot Foundation.
# $Id: tutorial_episode_3.pod 48222 2010-07-29 22:21:45Z tcurtis $

=head1 Episode 3: Squaak Details and First Steps

=head2 Introduction

In the previous episodes we introduced the Parrot Compiler Tools (PCT).
Starting from a high-level overview, we quickly created our own little scripting
language called I<Squaak>, using a Perl script provided with Parrot. We
discussed the general structure of PCT-based compilers, and each of the default
four transformation phases.
This third episode is where the Fun begins. In this episode, we'll introduce
the full specification of Squaak. In this and following episodes, we'll
implement this specification step by step in small easy-to-digest increments.
So let's get started!

=head2 Squaak Grammar

Without further ado, here is the full grammar specification for Squaak. This
specification uses the following meta-syntax:

    statement   indicates a non-terminal, named "statement"
    {statement} indicates zero or more statements
    [step]      indicates an optional step
    'do'        indicates the keyword 'do'

Below is Squaak's grammar. The start symbol is C<program>.

    program              ::= {stat-or-def}

    stat-or-def          ::= statement
                           | sub-definition

    statement            ::= if-statement
                           | while-statement
                           | for-statement
                           | try-statement
                           | throw-statement
                           | variable-declaration
                           | assignment
                           | sub-call
                           | do-block

    block                ::= {statement}

    do-block             ::= 'do' block 'end'

    if-statement         ::= 'if' expression 'then' block
                             ['else' block]
                             'end'

    while-statement      ::= 'while' expression 'do'
                             block 'end'

    for-statement        ::= 'for' for-init ',' expression [step]
                             'do'
                             block
                             'end'

    step                 ::= ',' expression

    for-init             ::= 'var' identifier '=' expression

    try-statement        ::= 'try' block 'catch' identifier
                             block
                             'end'

    throw-statement      ::= 'throw' expression

    sub-definition       ::= 'sub' identifier parameters
                             block
                             'end'

    parameters           ::= '(' [identifier {',' identifier}] ')'

    variable-declaration ::= 'var' identifier ['=' expression]

    assignment           ::= primary '=' expression

    sub-call             ::= primary arguments

    primary              ::= identifier postfix-expression*

    postfix-expression   ::= key
                           | index
                           | member

    key                  ::= '{' expression '}'

    index                ::= '[' expression ']'

    member               ::= '.' identifier

    arguments            ::= '(' [expression {',' expression}] ')'

    expression           ::= expression {binary-op expression}
                           | unary-op expression
                           | '(' expression ')'
                           | term

    term                 ::= float-constant
                           | integer-constant
                           | string-constant
                           | array-constructor
                           | hash-constructor
                           | primary

    hash-constructor     ::= '{' [named-field {',' named-field}] '}'

    named-field          ::= string-constant '=>' expression

    array-constructor    ::= '[' [expression {',' expression} ] ']'

    binary-op            ::= '+'  | '-'  | '/'  | '*'  | '%'  | '..'
                           | 'and | 'or' | '>'  | '>=' | '<'  | '<='
                           | '==' | '!='

    unary-op             ::= 'not' | '-'

Gee, that's a lot, isn't it? Actually, this grammar is rather small compared to
"real world" languages such as C, not to mention Perl 6. No worries though, we
won't implement the whole thing at once, but in small steps. What's more, the
exercises section contains enough exercises for you to learn to use the PCT
yourself! The solutions to these exercises are in later episodes if you don't
want to take the time to solve them yourself.

=head2 Semantics

Most of the Squaak language is straightforward; the C<if-statement> executes
exactly as you would expect. When we discuss a grammar rule (for its
implementation), a semantic specification will be included. This is to avoid
writing a complete language manual since that's probably not what you're here
for.

=head2 Let's get started!

In the rest of this episode we will implement the basic parts of the grammar,
such as the basic data types and assignments. At the end of this episode,
you'll be able to assign simple values to (global) variables. It's not much but
it's a very important first step. Once these basics are in place, you'll notice
that adding a certain syntactic construct can be done in a matter of minutes.

First, open your editor and open the files F<src/Squaak/Grammar.pm> and
F<src/Squaak/Actions.pm>. The former implements the parser using Perl 6 rules
and the latter contains the parse actions, which are executed during the parsing
stage.

In the file Grammar.pm you'll see the top-level rule, named C<TOP>. It's
located at, ehm... the top. When the parser is invoked, it will start at this
rule.  A rule is nothing else than a method of the Grammar class.  When we
generated this language some default rules were defined. Now we're going to
make some small changes, just enough to get us started. Replace the
C<statement> rule with this rule:

    rule statement {
        <assignment>
    }

Replace the statementlist rule with this:

    rule statement_list {
        <stat_or_def>*
    }

When you work on the action methods later, you'll also want to replace $<statement> in the action
method with $<stat_or_def>

Add these rules:

    rule stat_or_def {
        <statement>
    }

    rule assignment {
        <primary> '=' <EXPR>
    }

    rule primary {
        <identifier>
    }

    token identifier {
        <!keyword> <ident>
    }

    token keyword {
        ['and'|'catch'|'do'   |'else' |'end' |'for' |'if'
        |'not'|'or'   |'sub'  |'throw'|'try' |'var'|'while']>>
    }

    token term:sym<primary> {
        <primary>
    }

Rename the token C<< term:sym<integer> >> to C<< term:sym<integer_constant> >> and
C<< term:sym<quote> >> to C<< term:sym<string_constant> >> (to better match our
language specification).

Add action methods for term:sym<integer_constant> and term:sym<string_constant>
to F<src/Squaak/Actions.pm>:

    method term:sym<integer_constant>($/) {
        make PAST::Val.new(:value($<integer>.ast), :returns<Integer>);
    }
    method term:sym<string_constant>($/) {
        my $past := $<quote>.ast;
        $past.returns('String');
        make $past;
    }
    method term:sym<primary>($/) {
        make $<primary>.ast;
    }

PAST::Val nodes are used the represent constant values.

Finally, remove the rules C<proto token statement_control>,
C<< rule statement_control:sym<say> >>, and C<< rule statement_control:sym<print> >>.

Phew, that was a lot of information! Let's have a closer look at some things
that may look unfamiliar. The first new thing is in the rule C<identifier>.
Instead of the C<rule> keyword, you see the keyword C<token>. In short, a token
doesn't skip whitespace between the different parts specified in the token,
while a rule does. For now, it's enough to remember to use a token if you want
to match a string that doesn't contain any whitespace (such as literal constants
and identifiers) and use a rule if your string does (and should) contain
whitespace (such as a an if-statement). We shall use the word C<rule> in a
general sense, which could refer to a token. For more information on rules and
tokens take a look at Synopsis 5 or look at Moritz's blog post on the subject
in the references.

In rule C<assignment>, the <EXPR> subrule is one that we haven't defined. The
EXPR rule is inherited from HLL::Grammar, and it initiates the grammar's
operator-precedence parser to parse an expression. For now, don't worry about
it. All you need to know is that it will give us one of our terms.

In token C<identifier> the first subrule is called an assertion. It asserts
that an C<identifier> does not match the rule keyword. In other words a keyword
cannot be used as an identifier. The second subrule is called C<ident> which is
a built-in rule in the class C<PCT::Grammar>, the parent class of this grammar.

In token C<keyword>, all keywords of Squaak are listed. At the end there's a
C<<< >> >>> marker, which indicates a word boundary. Without this marker, an
identifier such as "forloop" would wrongly be disqualified, because the part
"for" would match the rule keyword, and the part "loop" would match the rule
"ident". However, as the assertion <!keyword> is false (as "for" could be
matched), the string "forloop" cannot be matched as an identifier. The required
presence of the word boundary prevents this.

=head2 Testing the Parser

It is useful to test the parser before writing any action methods. This can save
you a lot of work; if you write the actions immediately after writing the
grammar rules, and only later find out that your parser must be updated, then
your action methods probably need to be updated as well.
In Episode 2 we saw the target command line option. In order to test the parser,
the "parse" target is especially helpful. When specifying this option, your
compiler will print the parse tree of your input string, or print a syntax
error. It is wise to test your parser with both correct and incorrect input, so
you know for sure your parser doesn't accept input that it shouldn't.

=head2 And... Action!

Now we have implemented the initial version of the Squaak grammar, it's time to
implement the parse actions we mentioned before. The actions are written in a
file called F<src/Squaak/Actions.pm>. If you look at the methods in this file,
here and there you'll see that the C<ast> method being called on the match object ($/) , or rather,
hash fields of it (like $<statement>).
The special make function can be used to set the ast to a value.
This means that each node in the parse tree (a Match object) can also hold its
PAST representation. Thus we use the make function to set the PAST
representation of the current node in the parse tree, and later use the C<ast>
method to retrieve the PAST representation from it.

In recap, the match object ($/) and any subrules of it (for instance
$<statement>) represent the parse tree; of course, $<statement>
represents only the parse tree what the $<statement> rule matched. So, any
action method has access to the parse tree that the equally named grammar rule
matched, as the match object is always passed as an argument. Calling the C<ast> method 
on a parse tree yields the PAST representation (obviously, this PAST
object should be set using the make function).

If you're following this tutorial, I highly advise you to get your feet wet, and
do the exercises. Remember, learning and not doing is not learning (or something
like that :-). This week's exercises are not that difficult, and after doing
them, you'll have implemented the first part of our little Squaak language.

=head2 What's next?

In this episode we introduced the full grammar of Squaak. We took the first
steps to implement this language. The first, and currently only, statement type
is assignments. We briefly touched on how to write the action methods that are
invoked during the parsing phase.
In the next episode, we shall take a closer look on the different PAST node
types, and implement some more parts of the Squaak language. Once we have all
basic parts in place, adding statement types will be rather straightforward.
In the mean time, if you have any questions or are stuck, don't hesitate to
leave a comment or contact me.

=head2 Exercises

This episode's exercises are simple enough to get started on implementing
Squaak.

=over 4

=item 1.

Rename the names of the action methods according to the name changes we made on
the grammar rules. So, "integer" becomes "integer_constant", and so on.

=item 2.

Look at the grammar rule for statement. A statement currently consists of an
assignment. Implement the action method "statement" to retrieve the result
object of this assignment and set it as statement's result object using the
special make function. Do the same for rule primary.

=item 3.

Write the action method for the rule identifier. As a result object of this
"match", a new PAST::Var node should be set, taking as name a string
representation of the match object ($/). For now, you can set the scope to
'package'. See "pdd26: ast" for details on PAST::Var nodes.

=item 4.

Write the action method for assignment. Retrieve the result objects for
"primary" and for "expression", and create a PAST::Op node that binds the
expression to the primary. (Check out pdd26 for PAST::Op node types, and find
out how you do such a binding).

=item 5.

Write the action method for stat_or_def. Simply retrieve the result object from statement and make
that the result object.

=item 6.

Run your compiler on a script or in interactive mode. Use the target option to
see what PIR is being generated on the input "x = 42".

=back

=head2 Some Notes

=over 4

=item * Help! I get the error message "no result object".

This means that the result object was not set properly (duh!).
Make sure there is an action method for that rule and that "make" is used to set
the appropriate PAST node. Note that not all rules have action methods, for
instance the C<keyword> rule (there's no point in that).

=item * While we're constructing parts of Squaak's grammar, we'll sometimes
make a shortcut, by forgetting about certain rules for a while. For instance,
you might have noticed we're ignoring float-constants right now. That's ok. When
we'll need them, these rules will be added.

=back

=head2 References

=over 4

=item * rules, regexes and tokens: http://perlgeek.de/blog-en/perl-5-to-6/07-rules.writeback#Named_Regexes_and_Grammars

=item * pdd26: ast

=item * synopsis 5: Rules

=item * docs/pct/*.pod

=back

=head2 Solutions to the exercises

By now, you may have finished the PCT tutorial. If you felt too lazy to do the
exercises or if you want to see what solution I had in mind, here are the
solutions to the exercises in Episode 3 (Episode 1's exercise was discussed at
the end of Episode 2, and the latter didn't have any coding assignments).

=over 4

=item 1

Rename the names of the action methods according to the name changes we made
on the grammar rules. So, "integer" becomes "integer_constant", and so on.

I assume you don't need any help with this.

=item 2

Look at the grammar rule for statement. A statement currently consists of an
assignment. Implement the action method "statement" to retrieve the result
object of this assignment and set it as statement's result object using the
special make function. Do the same for rule primary.

 method statement($/) {
     make $<assignment>.ast;
 }

 method primary($/) {
     make $<identifier>.ast;
 }

=item 3

Write the action method for the rule identifier. As a result object of this
"match", a new C<PAST::Var> node should be set, taking as name a string
representation of the match object ($/). For now, you can set the scope to
'package'. See "pdd26: ast" for details on C<PAST::Var> nodes.

 method identifier($/) {
     make PAST::Var.new( :name(~$/), :scope('package'), :node($/) );
 }

=item 4

Write the action method for assignment. Retrieve the result objects for
"primary" and for "expression", and create a C<PAST::Op> node that binds the
expression to the primary. (Check out pdd26 for C<PAST::Op> node types, and
find out how you do such a binding).

 method assignment($/) {
     my $lhs := $<primary>.ast;
     my $rhs := $<EXPR>.ast;
     $lhs.lvalue(1);
     make PAST::Op.new( $lhs, $rhs, :pasttype('bind'), :node($/) );
 }

Note that we set the lvalue flag on $lhs. See PDD26 for details on this flag.

=item 5

Write the action method for stat_or_def. Simply retrieve the result object from statement and make
that the result object.

 method stat_or_def($/) {
     make $<statement>.ast;
 }

=item 6

Run your compiler on a script or in interactive mode. Use the target option to
see what PIR is being generated on the input "x = 42".

 .namespace.sub "_block10"
   new $P11, "Integer"
   assign $P11, 42
   set_global "x", $P11
   .return ($P11)
 .end

The first two lines of code in the sub create an object to store the number 42,
the third line stores this number as "x". The PAST compiler will always
generate an instruction to return the result of the last statement, in this
case C<$P11>.

=back


=cut

