%{
=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 PL_I_conflictNested.eyp

Run it with:

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

for more detail:

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

and also

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

The problem arises again

Also try:

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

=cut

%}

%strict

%token ID 

# Adaptative lexer
%lexer {
      m{\G\s*(\#.*)?}gc;

      m{\G([a-zA-Z_]\w*)}gc and do {
        my $id = $1;

        return ('if',   'if')   if ($id eq 'if')   && $self->YYExpects('if') 
               && !$self->YYPreParse('Assign');     #, 0xF # debug ); 

        return ('then', 'then') if ($id eq 'then') && $self->YYExpects('then');
        
        return ('ID', $id);
      };

      m{\G(.)}gc and return ($1, $1);

      return('',undef);
  }

%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
;

%%