eyapp-debugging-tutorial/Cplusplus1.eyp at master · ULL-ESIT-PL/eyapp-debugging-tutorial · GitHub

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%{
our $VERSION = '0.01';
%}

%strict

%token INT = /(int\b)/
%token ID  = /([a-zA-Z_]\w*)/
%token NUM = /(\d+)/

%right '='
%left '+'

%tree bypass

%expect 0 1  # 0 shift-reduce conflicts, 1 reduce-reduce conflict

%conflict decORexp
      {
        if (m{\G
                (?=
                    [)\s]* # ) ) ... )
                    [;=]
                )
             }gxc) {
          $self->YYSetReduce(')', 'ID:DEC' );
        }
        else {
          $self->YYSetReduce(')', 'ID:EXP' );
        }
      }

%%
prog:
    %name EMPTY
    /* empty */
  | %name PROG
    prog stmt
;

stmt:
    %name EXP
    expr ';'
  | %name DECL
    decl
;

expr:
    %name ID:EXP
    ID                            %PREC decORexp
  | %name NUM
    NUM
  | %name TYPECAST
    INT '(' expr ')' /* typecast */
  | %name PLUS
    expr '+' expr
  | %name ASSIGN
    expr '=' expr
;

decl:
    %name DECLARATOR
    INT declarator ';'
  | %name DECLARATORINIT
    INT declarator '=' expr ';'
;

declarator:
    %name ID:DEC
    ID                            %PREC decORexp
  | '(' declarator ')'
;

%%


####################################################

=head1 SYNOPSIS

Compile it with

   eyapp -C Cplusplus1

Run it with:

   ./Cplusplus1.pm -t -i -c ' int (x) + 2; int (x) = 2;'

the result will be the generated abstract sytax tree

=head1  C++ Ambiguities

This grammar models a problematic part of the C++ grammar—the ambiguity between certain
declarations and statements. For example,

     int (x) = y+z;

parses as either an expr or a stmt.


Eyapp detects this as a reduce/reduce conflict:

  State 17 contains 1 reduce/reduce conflict

  State 17:

	expr -> ID .	(Rule 5)
	declarator -> ID .	(Rule 11)

	')'	[reduce using rule 11 (declarator)]
	$default	reduce using rule 5 (expr)

The C++ rule is: take it as a declaration if it looks as a declaration,
otherwise is an expression.

This Eyapp parser solves the problem by dynamically changing the parser.

=head1 SEE ALSO

=over 2

=item * The file C<Cplusplus2.eyp> in C<examples/debuggintut>

=item * L<http://www.gnu.org/software/bison/manual/html_mono/bison.html#GLR-Parsers>

=item * L<http://en.wikipedia.org/wiki/Significantly_Prettier_and_Easier_C%2B%2B_Syntax>

=item * L<http://www.csse.monash.edu.au/~damian/papers/PS/ModestProposal.ps>

=item * L<http://www.nobugs.org/developer/parsingcpp/>

=item * Edward Willink's "Meta-Compilation for C++" PhD thesis at L<http://www.computing.surrey.ac.uk/Research/CSRG/fog/FogThesis.pdf>

=back

=cut