Editing Dangling else (section)

==Description==
In many [[programming language]]s, one may write conditionally executed code in two forms: the if-then form, or the if-then-else form. (The else clause is optional.):
 '''if''' a '''then''' s
 '''if''' b '''then''' s1 '''else''' s2

Ambiguous interpretation becomes possible when there are nested statements; specifically when an if-then-else form replaces the statement <code>s</code> inside the above if-then construct:

 '''if''' a '''then''' '''if''' b '''then''' s1 '''else''' s2
In this example, <code>s1</code> gets executed if and only if <code>a</code> is true ''and'' <code>b</code> is true. But what about <code>s2</code>? One person might be sure that <code>s2</code> gets executed whenever <code>a</code> is false (by attaching the ''else'' to the first ''if''), while another person might be sure that <code>s2</code> gets executed only when <code>a</code> is true ''and'' <code>b</code> is false (by attaching the ''else'' to the second ''if''). In other words, someone could interpret the previous statement as being equivalent to either of the following unambiguous statements:
 '''if''' a '''then''' { '''if''' b '''then''' s1 } '''else''' s2
 '''if''' a '''then''' { '''if''' b '''then''' s1 '''else''' s2 }

The dangling-else problem dates back to [[ALGOL 60]],<ref>{{cite journal |last1=Abrahams |first1=P. W. |title=A final solution to the Dangling else of ALGOL 60 and related languages |journal=Communications of the ACM |volume=9 |issue=9 |pages=679–682 |year=1966 |doi=10.1145/365813.365821 |doi-access=free |s2cid=6777841}}</ref> and subsequent languages have resolved it in various ways. In [[LR parser]]s, the dangling else is the archetypal example of a [[shift-reduce conflict]].