%{
my $INT = '(int)\b';
my $ID = '([a-zA-Z_][a-zA-Z_0-9]*)';
my $NUM = '(\d+)';
%}
%token NUM = /$NUM/
%token INT = /$INT/
%token ID = /$ID/
%right '='
%left '+'
%conflict decORexp {
if ($self->YYIs('decl')) { $self->YYSetReduce('ID:DEC' ); }
else { $self->YYSetReduce('ID:EXP' ); }
}
%explorer decORexp decl
%expect-rr 1 # expect 1 reduce-reduce conflict
%tree bypass
%%
prog:
%name EMPTY
/* empty */
| %name PROG
prog %decORexp? 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 CplusplusNested
the compile it again, using 'decl' as start symbol:
eyapp -S decl -P CplusplusNested.eyp
Run it with:
./CplusplusNested.pm -t -nos -i
or
./CplusplusNested.pm -t -i -c 'int (x) + 2; int (z) = 4;'
try with inputs:
int (x) = 2;
int (x) + 2;
the output will be a description of the generated abstract syntax 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++ disambiguation 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