32.1.2 Motivation of LHS terminals

Recall that a terminal symbol is a symbol that may appear in the input. Traditionally, all LHS symbols, as well as the start symbol, must be non-terminals. By default, this is Marpa’s behavior.

In a departure from tradition, Marpa had a feature that allowed the user to eliminate the distinction between terminals and non-terminals. This feature is now deprecated.

When LHS terminals are in use, a terminal can appear on the LHS of one or more rules, and can be be the start symbol. Note however, that terminals can never be zero length.

The basis of the LHS terminals feature was that, while sharp division between terminals and non-terminals was a useful simplification for proving theorems, it was not essential in practice. In the UNIX “toolkit” tradition, the practice has been to include even awkward, dangerous tools with no known use, in the toolkit. The philosophy was that empowering the user who discovers new techniques is more important than playing nanny to the toolkit’s users.

LHS symbols could be used to bypass, or “short circuit”, the rules on whose LHS they occur. Short circuiting rules, it was thought, might prove helpful in debugging, or have other applications.

But, a decade after the release of Libmarpa, no uses for LHS symbols have emerged. And they do introduce many new corner cases into the code and complicate the API documentation.