Editing Top-down parsing language (section)

== Generalized TDPL ==

A slight variation of TDPL, known as '''Generalized TDPL''' or GTDPL, greatly increases the apparent expressiveness of TDPL while retaining the same minimalist approach (though they are actually equivalent). In GTDPL, instead of TDPL's recursive rule form ''A'' → ''BC/D'', the rule form ''A'' → ''B[C,D]'' is used. This rule is interpreted as follows: When nonterminal ''A'' is invoked on some input string, it first recursively invokes ''B''.  If ''B'' succeeds, then ''A'' subsequently invokes ''C'' on the remainder of the input left unconsumed by ''B'', and returns the result of ''C'' to the original caller.  If ''B'' fails, on the other hand, then ''A'' invokes ''D'' on the original input string, and passes the result back to the caller.

The important difference between this rule form and the ''A'' → ''BC/D'' rule form used in TDPL is that ''C'' and ''D'' are never ''both'' invoked in the same call to ''A'': that is, the GTDPL rule acts more like a "pure" if/then/else construct using ''B'' as the condition.

In GTDPL it is straightforward to express interesting non-[[context-free language]]s such as the classic example {a<sup>''n''</sup>b<sup>''n''</sup>c<sup>''n''</sup>}.

A GTDPL grammar can be reduced to an equivalent TDPL grammar that recognizes the same language, although the process is not straightforward and may greatly increase the number of rules required.<ref name="Ford">Ford, Bryan. ''[http://pdos.csail.mit.edu/~baford/packrat/popl04/peg-popl04.pdf Parsing Expression Grammars: A Recognition-Based Syntactic Foundation]''</ref>
Also, both TDPL and GTDPL can be viewed as very restricted forms of [[parsing expression grammar]]s, all of which represent the same class of grammars.<ref name="Ford"/>