Dangling else

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The dangling else is a well-known problem in computer programming in which a seemingly well-defined grammar can become ambiguous. In many programming languages you can write code like this:

if a then if b then s1 else s2

which can be understood in two ways. Either as

if a then
  if b then
    s1
  else
    s2

or as

if a then
  if b then
    s1
else
  s2

This is a problem that often comes up in compiler construction. It can be solved either at the implementation level, by telling the parser the right way to solve the ambiguity, or at the grammar level by using a Parsing expression grammar or equivalent.

[edit] External links

• http://www.scit.wlv.ac.uk/cbook/chap5.dangling.else.html