ktp-forked-repos/emacs-pl

This is a parsing library in the spirit of Haskell's parsec. For example:

(pl-parse
  (delete-region (pl-str "<xml>" :beg)
                 (pl-until
                   (pl-str "</xml>" :end))))

There are a few parsers, whose job is to inspect whatever is at the current buffer position, and return zero or more details regarding what was found:

pl-ch                Match a single character
pl-str               Match a string
pl-re                Match a regular expression
pl-num               Match an integer or floating-point number

Other possibilities include: inspecting text properties, overlays, etc.

If the parser succeeds, it returns the object matched (a string by default), and advances point to the next position after the match. Keywords may be given to return other details:

:beg                 Beginning of the match
:end                 End of the match
:group N             A particular regexp group
:props               All properties within the matched region
:nil                 Return `nil` (same as using `ignore`)

If a parser fails, it throws the exception failed. This is caught by the macro pl-try, which returns nil upon encountering the exception. This makes it possible to build certain combinators out of these few parts:

pl-or                Return result from first successful parser
pl-and               Return last result, if all parsers succeed
pl-until             If the parse fails, advance cursor position by
                     one character and try again.  Keywords can
                     change the advance amount.

For other constructs, such as returning the result of every parser as a list, just combine parsers with regular Lisp forms (pl-parse is just a synonym for pl-try):

(pl-parse
  (list (pl-str "Hello") (pl-str "World")))

Note that even though a parse may fail, and thus return no value, any side-effects that occur during the course of the parse will of course be retained. This can be used to good effect, by continuing an action for as long as a parse succeeds:

(pl-parse
  (while t
     (delete-region (pl-str "<xml>" :beg)
                    (pl-until
                      (pl-str "</xml>" :end)))))

This will delete blocks demarcated by <xml> and </xml>, for as long as such blocks continue to occur contiguously to one another.