Top-down Parsing - Programming Language Application

Programming Language Application

A compiler parses input from a programming language to assembly language or an internal representation by matching the incoming symbols to production rules. Production rules are commonly defined using Backus-Naur form. An LL parser is a type of parser that does top-down parsing by applying each production rule to the incoming symbols, working from the left-most symbol yielded on a production rule and then proceeding to the next production rule for each non-terminal symbol encountered. In this way the parsing starts on the Left of the result side (right side) of the production rule and evaluates non-terminals from the Left first and, thus, proceeds down the parse tree for each new non-terminal before continuing to the next symbol for a production rule.

For example:

would match and attempt to match next. Then would be tried. As one may expect, some languages are more ambiguous than others. For a non-ambiguous language in which all productions for a non-terminal produce distinct strings: the string produced by one production will not start with the same symbol as the string produced by another production. A non-ambiguous language may be parsed by an LL(1) grammar where the (1) signifies the parser reads ahead one token at a time. For an ambiguous language to be parsed by an LL parser, the parser must lookahead more than 1 symbol, e.g. LL(3).

The common solution to this problem is to use an LR parser, which is a type of shift-reduce parser, and does bottom-up parsing.