[dsp1234]

Details about the "incremental" mode are listed in the documentation PDF[0] at section 9.2

Here are the first couple of paragraphs:

"In this API, control is inverted. The parser does not have access to the lexer. Instead, when the parser needs the next token, it stops and returns its current state to the user. The user is then responsible for obtaining this token (typically by invoking the lexer) and resuming the parser from that state. The directory demos/calc-incremental contains a demo that illustrates the use of the incremental API.

This API is “incremental” in the sense that the user has access to a sequence of the intermediate states of the parser. Assuming that semantic values are immutable, a parser state is a persistent data structure: it can be stored and used multiple times, if desired. This enables applications such as “live parsing”, where a buffer is continuously parsed while it is being edited. The parser can be re-started in the middle of the buffer whenever the user edits a character. Because two successive parser states share most of their data in memory, a list of n successive parser states occupies only O(n) space in memory."

There does not appear to be a specific mention of having the partial AST available.

[0] - linked from their front page and available at http://gallium.inria.fr/~fpottier/menhir/manual.pdf