%{ our $VERSION = '0.01'; use base q{DebugTail}; %} %strict %token ID INT NUM %right '=' %left '+' %tree bypass %expect 0 1 # 0 shift-reduce conflicts, 1 reduce-reduce conflict %% prog: %name EMPTY /* empty */ | %name PROG prog stmt ; stmt: %name EXP expr ';' | %name DECL decl ; expr: %name EXPID ID decexpconflict | %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 DECID ID decexpconflict | '(' declarator ')' ; decexpconflict: /* empty. Just to solve the reduce-reduce conflict */ { my $self = shift; if (${$self->input()} =~ m{^[)\s]*[;=]\s*}) { $self->YYSetReduce(')', 'DECID' ); } else { $self->YYSetReduce(')', 'EXPID' ); } } ; %% __PACKAGE__->lexer( sub { my $self = shift; for (${$self->input()}) { s{^(\s*)}{}; return ('', undef) unless $_; return ('INT', $1) if s{^(int\b)}{}; return ('ID', $1) if (s{^([a-zA-Z_]\w*)}{}); return ('NUM', $1) if s/^(\d+)//; return ($1, $1) if s/^(.)//; } return ('',undef); } ); unless (caller()) { my $prompt = 'Try first "int (x) = 2;" then "int (x) + 2;" '. '(press <CR><CTRL-D> to finish): '; __PACKAGE__->main($prompt) } #################################################### =head1 SYNOPSIS Compile it with eyapp -b '' Cplusplus Run it with: ./Cplusplus.pm -t try with inputs: 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++ grammarâthe 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