Dangling else

The dangling else is a problem in computer programming in which a seemingly well-defined statement can become ambiguous. In many programming languages one may write conditionally executed code in two forms: the if-then form, and the if-then-else form:

if a then s
if a then s1 else s2

This gives rise to an ambiguity in interpretation whenever an if-then form appears as s1 in an if-then-else form:^[1]

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
      or
if a then (if b then s else s2)

Avoiding ambiguity while keeping the syntax

This is a problem that often comes up in compiler construction. The convention when dealing with the dangling else is to attach the else to the nearby if statement,^[2] allowing for unambiguous context-free grammars, in particular. Programming languages like Pascal^[3] and C^[4] 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.

Depending on the compiler construction approach, one may take different corrective actions to avoid ambiguity:

• If the compiler is the product of a 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.^[2]
• If the compiler is the product of a Pruning and Deep Pruning LR generator, one can issue directives than prune away the ambiguities completely.^[1]
• If the compiler is the product of a programmer instead of a parser generator, the programmer may use a non-ambiguous context-free grammar. Alternatively, one may rely on a non-context-free grammar or a parsing expression grammar.

Programmers using C and languages with similar syntax sometimes standardize a practice of using braces to clearly define the intent of the statement. Similarly, some programmers use logical and ternary operators to avoid ambiguity.

Avoiding ambiguity by changing the syntax

The problem can also be solved by making explicit the link between an else and its if, within the syntax. This usually helps avoid human errors.^[5] Possible solutions are:

• Having an "end if" statement. Examples of such languages are ALGOL 68, Ada, Eiffel, PL/SQL, REALbasic, and Modula-2.

• Disallowing the statement following a then to be an if itself (it may however be a pair of statement brackets containing nothing but an if-then-close). This approach is followed by ALGOL 60^[6] and Python.^[7]

• Requiring braces (parenthesizing) when an "else" follows an "if".^[8]

• Requiring every "if" to be paired with an "else". This is necessary in a language which does not allow mutable variables.^[9]

References