Parsing Lisp (Hemlock User’s Manual)

8.7 Parsing Lisp

Lisp mode has a fairly complete knowledge of Lisp syntax, but since it does not use the reader, and must work incrementally, it can be confused by legal constructs. Lisp mode totally ignores the read-table, so user-defined read macros have no effect on the editor. In some cases, the values the Lisp Syntax character attribute can be changed to get a similar effect.

Lisp commands consistently treat semicolon (;) style comments as whitespace when parsing, so a Lisp command used in a comment will affect the next (or previous) form outside of the comment. Since #| ... |# comments are not recognized, they can used to comment out code, while still allowing Lisp editing commands to be used.

Strings are parsed similarly to symbols. When within a string, the next form is after the end of the string, and the previous form is the beginning of the string.

Hemlock Variable: Defun Parse Goal (initial value 2) ¶

Hemlock Variable: Maximum Lines Parsed (initial value 500) ¶

Hemlock Variable: Minimum Lines Parsed (initial value 50) ¶

In order to save time, Lisp mode does not parse the entire buffer every time a Lisp command is used. Instead, it uses a heuristic to guess the region of the buffer that is likely to be interesting. These variables control the heuristic.

Normally, parsing begins and ends on defun boundaries (an open parenthesis at the beginning of a line). Defun Parse Goal specifies the number of defuns before and after the point to parse. If this parses fewer lines than Minimum Lines Parsed, then parsing continues until this lower limit is reached. If we cannot find enough defuns within Maximum Lines Parsed lines then we stop on the farthest defun found, or at the point where we stopped if no defuns were found.

When the heuristic fails, and does not parse enough of the buffer, then commands usually act as though a syntax error was detected. If the parse starts in a bad place (such as in the middle of a string), then Lisp commands will be totally confused. Such problems can usually be eliminated by increasing the values of some of these variables.

Hemlock Variable: Parse Start Function (initial value start-of-parse-block) ¶
Hemlock Variable: Parse End Function (initial value end-of-parse-block) ¶: These variables determine the region of the buffer parsed. The values are functions that take a mark and move it to the start or end of the parse region. The default values implement the heuristic described above.