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Concept
Dangling else
Summary
The dangling else is a problem in programming of parser generators in which an optional else clause in an if–then(–else) statement results in nested conditionals being ambiguous. Formally, the reference context-free grammar of the language is ambiguous, meaning there is more than one correct parse tree.
In many programming languages one may write conditionally executed code in two forms: the if-then form, and the if-then-else form – the else clause is optional:
if a then s
if b then s1 else s2
This gives rise to an ambiguity in interpretation when there are nested statements, specifically whenever an if-then form appears as s1 in an if-then-else form:
if a then if b then s else s2
In this example, s is unambiguously executed when a is true and b is true, but one may interpret s2 as being executed when a is false (thus attaching the else to the first if) or when a is true and b is false (thus attaching the else to the second if). In other words, one may see the previous statement as either of the following expressions:
if a then (if b then s) else s2
if a then (if b then s else s2)
The dangling else problem dates to ALGOL 60, and has been resolved in various ways in subsequent languages. In LR parsers, the dangling else is the archetypal example of a shift-reduce conflict.
This is a problem that often comes up in compiler construction, especially scannerless parsing. The convention when dealing with the dangling else is to attach the else to the nearby if statement, allowing for unambiguous context-free grammars, in particular. Programming languages like Pascal, C and Java follow this convention, so there is no ambiguity in the semantics of the language, though the use of a parser generator may lead to ambiguous grammars. In these cases alternative grouping is accomplished by explicit blocks, such as begin...end in Pascal and {...} in C.
Depending on the compiler construction approach, one may take different corrective actions to avoid ambiguity:
If the parser is produced by an SLR, LR(1) or LALR LR parser generator, the programmer will often rely on the generated parser feature of preferring shift over reduce whenever there is a conflict.
Official source
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