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

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%{
=head1 SYNOPSIS

This grammar deals with the famous ambiguous PL/I phrase:

                if then=if then if=then

The (partial) solution uses C<YYExpect> in the lexical analyzer to predict the token
that fulfills the parser expectatives.

Compile it with:

               eyapp -C PLIConflictNested.eyp
               eyapp -P Assign.eyp

Run it with:

               ./PLIConflictNested.pm -t -c 'if if=then then then=if'

for more detail:

               ./PLIConflictNested.pm -d -t -c 'if if=then then then=if'

and also

               ./PLIConflictNested.pm -t -i -c 'if then=if then if=then

The problem arises again

Also try:

                ./PLIConflictNested.pm -t -c 'if then=if then if a=b then c=d'

=cut

%}

%strict

%token then =  %/then\b/
# %token if =%/if\b/ Assign%
%token if   =  {
                  my $pos = pos();
                  if (/\Gif\b/gc) {
                    if ($self->expects('if')) {
                       if (!$self->YYPreParse('Assign')) {
                         return ('if', 'if');
                       }
                    }
                  }
                  pos($_) = $pos;
               }
%token ID  = /([a-zA-Z_]\w*)/

%tree bypass
%%
stmt:
    ifstmt
  | assignstmt
;

ifstmt:
     %name IF
    if expr then stmt
;

assignstmt:
    %name ASSIGN
    id '=' expr
;

expr:
    %name EQ
    id '=' id
  | id
;

id:
    %name ID
    ID
;

%%