Editing Ambiguous grammar (section)

=== Dangling else ===
{{main|Dangling else}}
A common example of ambiguity in computer programming languages is the [[dangling else]] problem. In many languages, the <code>else</code> in an [[Conditional (computer programming)#If–then(–else)|If–then(–else)]] statement is optional, which results in nested conditionals having multiple ways of being recognized in terms of the context-free grammar.

Concretely, in many languages one may write conditionals in two valid forms: the if-then form, and the if-then-else form – in effect, making the else clause optional.

In a grammar containing the rules{{efn|The following example uses [[Pascal (programming language)|Pascal]] syntax.}}

 Statement → '''if''' Condition '''then''' Statement |
             '''if''' Condition '''then''' Statement '''else''' Statement |
             ...
 Condition → ...

some ambiguous phrase structures can appear. The expression
 '''if''' a '''then''' '''if''' b '''then''' s '''else''' s2
can be parsed as either
 '''if''' a '''then''' '''begin''' '''if''' b '''then''' s '''end''' '''else''' s2
or as
 '''if''' a '''then''' '''begin''' '''if''' b '''then''' s '''else''' s2 '''end'''
depending on whether the <code>else</code> is associated with the first <code>if</code> or second <code>if</code>.

This is resolved in various ways in different languages. Sometimes the grammar is modified so that it is unambiguous, such as by requiring an <code>endif</code> statement or making <code>else</code> mandatory. In other cases the grammar is left ambiguous, but the ambiguity is resolved by making the overall phrase grammar context-sensitive, such as by associating an <code>else</code> with the nearest <code>if</code>. In this latter case the grammar is unambiguous, but the context-free grammar is ambiguous.{{clarify|reason=There is no such thing as an 'overall phrase grammar'. The 'nearest-if' rule can possibly be implemented by using a slightly modified, but still context-free grammar.|date=January 2017}}