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Better parse-partial-sexp; multiple major modes (was: Idea for syntax-pp

From: Daniel Colascione
Subject: Better parse-partial-sexp; multiple major modes (was: Idea for syntax-ppss)
Date: Sun, 31 Aug 2008 04:37:53 -0400

On Jul 27, 2008, at 10:50 AM, Alan Mackenzie wrote:
What I think really needs doing is to make this function bulletproof: It should work on narrowed buffers, it should give reliable elements 2 and 6, its cache should be cleared when functions like `modify-syntax- entry'
are called or parse-sexp-lookup-properties is changed, and the cache
should be bound to nil on `with-syntax-table'.  I actually think it
could be useful to maintain several parallel caches, each for a
different syntax-table (or an equivalence class of syntax tables). And
so on.  Basically, I would like `(syntax-ppss)' to tell me with 100%
reliability, no ifs, no buts, whether I am at top-level, in a comment,
or in a string.

Surely, such a creature would have to live on the C side of things, if only for practical reasons. (With the proliferation of with-this and inhibit-that options available to Lisp, I don't see how one can easily and robustly catch all buffer modification. Not to mention that no matter which of before-change-functions and after-change-functions you used, you could still race against other functions using the same facility.)

If this perfectly caching parse-partial-sexp lives in C anyway, why not just call it parse-partial-sexp? Optimize parse-partial-sexp for the case of start being 1 or (point-min). syntax-ppss becomes a simple wrapper. Not only would it be possible to robustly catch all buffer and context modification, but by optimizing the existing function, all existing users would automatically win. I'd offer to write a patch, but I don't know the core well enough to know how to "easily and robustly catch all buffer modification".

Also, Lennart is asking for it to work nicely with multiple major modes. Surely this would be a Good Thing. Files containing several major modes
are commonplace (awk or sed embedded within a shell script, html
embedded within php, ....).

After several attempts at using and understanding multiple major mode facilities, I'm convinced the only way forward is core support for the concept. Lennart's done a fine job with what's in Emacs currently. But anything involving multiple major modes today is a quivering mound of hacks. All the work Lennart's had to do to get modes playing nice with each other is a testament to that.

Maybe a core solution could be something like this: in a given buffer, each character has a chunk-name character property. You'd buffer- locally map chunk names to major modes. For each chunk name, create a buffer containing just the text assigned to that chunk. Make the major- mode the major mode for the chunk buffer, and let that major-mode handle fontification, keybindings, and so on. In the main buffer, assemble the various bits from the chunk-buffers and allow the user to navigate the combined buffer normally.

Keybindings with point at a particular character would just be the keybindings present in that character's chunk-buffer. If you need special keybindings common across all chunk buffers, just bind the key in all the chunk buffers. If a given chunk needs placeholder text to represent text of some other chunk, it should be possible add it to that chunk buffer without affecting any of the others.

Anyway, this scheme is:

1) Robust - no messing around with variables, no tweaking fontification
2) Backwards compatible - a major-mode doesn't need to know it's being used this way 3) Versatile - you can compose arbitrary modes this way, even recursively
4) Conceptually simple (I hope)

Any thoughts?

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