Monday, March 31, 2008

Solutions to the Exercises in Episode 3

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).

1. Rename the names of the action methods according to the name changes we made on the grammar rules. So, "integer" becomes "integer_constant", "value" becomes "expression", and so on.
I assume you don't need any help with this.
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> );
Note that at this point, the rule statement doesn't define different #= keys for each type of statement, so we don't declare a parameter $key. This will be changed later.
method primary($/) {
make $( $<identifier> );
3. Write the action method for the rule identifier. As a result object of this "match", a new PAST::Var node should be set, taking as name a string representation of the match object ($/). For now, you can set the scope to 'package'. See "pdd26: ast" for details on PAST::Var nodes.
method identifier($/) {
make :name(~$/),
:node($/) );
4. Write the action method for assignment. Retrieve the result objects for "primary" and for "expression", and create a PAST::Op node that binds the expression to the primary. (Check out pdd26 for PAST::Op node types, and find out how you do such a binding).
method assignment($/) {
my $lhs := $( $<primary> );
my $rhs := $( $<expression> );
make $lhs, $rhs,
:node($/) );
Note that we set the lvalue flag on $lhs. See PDD26 for details on this flag.

5. 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".
.sub "_block10"
new $P11, "Integer"
assign $P11, 42
set_global "x", $P11
.return ($P11)
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 $P11.