%{ 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 %conflict decORexp decl? ID:DEC : ID:EXP %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