elisp: Position Parse
34.6.2 Finding the Parse State for a Position
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For syntactic analysis, such as in indentation, often the useful thing
is to compute the syntactic state corresponding to a given buffer
position. This function does that conveniently.
-- Function: syntax-ppss &optional pos
This function returns the parser state that the parser would reach
at position POS starting from the beginning of the buffer. 
Parser State, for a description of the parser state.
The return value is the same as if you call the low-level parsing
function ‘parse-partial-sexp’ to parse from the beginning of the
buffer to POS (Low-Level Parsing). However, ‘syntax-ppss’
uses a cache to speed up the computation. Due to this
optimization, the second value (previous complete subexpression)
and sixth value (minimum parenthesis depth) in the returned parser
state are not meaningful.
This function has a side effect: it adds a buffer-local entry to
‘before-change-functions’ (Change Hooks) for
‘syntax-ppss-flush-cache’ (see below). This entry keeps the cache
consistent as the buffer is modified. However, the cache might not
be updated if ‘syntax-ppss’ is called while
‘before-change-functions’ is temporarily let-bound, or if the
buffer is modified without running the hook, such as when using
‘inhibit-modification-hooks’. In those cases, it is necessary to
call ‘syntax-ppss-flush-cache’ explicitly.
-- Function: syntax-ppss-flush-cache beg &rest ignored-args
This function flushes the cache used by ‘syntax-ppss’, starting at
position BEG. The remaining arguments, IGNORED-ARGS, are ignored;
this function accepts them so that it can be directly used on hooks
such as ‘before-change-functions’ (Change Hooks).
Info Catalog
elisp: Motion via Parsing
elisp: Parsing Expressions
elisp: Parser State