Editing Dangling else (section)

===Avoiding the conflict in LR parsers===
The above example could be rewritten in the following way to remove the ambiguity :
<pre>
statement: open_statement
         | closed_statement
         ;

open_statement: IF '(' expression ')' statement
              | IF '(' expression ')' closed_statement ELSE open_statement
              ;

closed_statement: non_if_statement
                | IF '(' expression ')' closed_statement ELSE closed_statement
                ;

non_if_statement: ...
                ;
</pre>

Any other statement-related grammar rules may also have to be duplicated in this way if they may directly or indirectly end with a <code>statement</code> or <code>selection-statement</code> non-terminal.

However, we give grammar that includes both of if and while statements.

<pre>
statement: open_statement
         | closed_statement
         ;

open_statement: IF '(' expression ')' statement
              | IF '(' expression ')' closed_statement ELSE open_statement
              | WHILE '(' expression ')' open_statement
              ;

closed_statement: simple_statement
                | IF '(' expression ')' closed_statement ELSE closed_statement
                | WHILE '(' expression ')' closed_statement
                ;

simple_statement: ...
                ;
</pre>

Finally, we give the grammar that forbids ambiguous IF statements.

<pre>
statement: open_statement
         | closed_statement
         ;

open_statement: IF '(' expression ')' statement
              | IF '(' expression ')' closed_statement ELSE open_statement
              | WHILE '(' expression ')' open_statement
              ;

closed_statement: simple_statement
                | IF '(' expression ')' closed_statement ELSE closed_statement
                | WHILE '(' expression ')' closed_statement
                ;

simple_statement: ...
                ;
</pre>

With this grammar the statement <code>if (a) if (b) c else d</code> can only be parsed one way, because the other interpretation (<code>if (a) {if (b) c} else d</code>) is produced as
<pre>
statement
open_statement
IF '(' expression ')' closed_statement ELSE open_statement
'if' '(' 'a' ')' closed_statement 'else' 'd'
</pre>
and then the parsing fails trying to match <code>closed_statement</code> to "if (b) c". An attempt with <code>closed_statement</code> fails in the same way. The other parse, <code>if (a) {if (b) c else d}</code>) succeeds:
<pre>
statement
open_statement
IF '(' expression ')' statement
IF '(' expression ')' closed_statement
IF '(' a ')' (IF '(' expression ')' closed_statement ELSE closed_statement)
IF '(' a ')' (IF '(' b ')' c ELSE 'd')
</pre>