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Re: A vision for multiple major modes: some design notes

From: Phillip Lord
Subject: Re: A vision for multiple major modes: some design notes
Date: Wed, 20 Apr 2016 23:27:34 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.92 (gnu/linux)

A few comments, rather than an in-depth analysis, am afraid.

Alan Mackenzie <address@hidden> writes:
> (iv) Islands.
>   o - An island will be delimited in two complementary ways:
>     * - It will be enclosed syntactically by characters with "open island" and
>       "close island" syntax (see section (v)).  Both of these syntactic
>       markers will include a flag "chain" indicating whether there is a
>       previous/next island in the chain.  The cdr of the syntax value will be
>       the island chain to which the island belongs.
>     * - It will be covered by the text property `island', whose value will be
>       the pertinent island or island chain (see section (ii)) (not yet
>       decided).  Note that if islands are enclosed inside other islands, the
>       value is the innermost island.  There is the possibility of using an
>       interval tree independent of the one for text properties to increase
>       performance.

When you say "complementary" do you mean alternative or simultaneous?
I.e. will an island always be enclosed by syntax markers and always have
a text property. Or can it have either?

I'm still not understanding how the chain of islands is set up. Is this
entirely the super modes responsibility? The use of "syntax" suggests
that the islands can be detected *purely* syntactically. But, there are
many places where this is not true: consider org-mode:

#+begin_src emacs-lisp
(message "hello world")

We cannot assume that "+end_src" is the end of a island.

Also, how will the regexp engine work when it spans an island? I ask
because, if we use the regexp engine to match delimiters, the which
syntax do we use, if there are multiple modes in the buffer.

>   o - An island might be represented by a C or Lisp structure, it might not
>     (not yet decided).  This structure would hold the containing chain,
>     markers pointing to the start and end of the chain, and the previous and
>     next islands in the chain.
> (v) Syntax, etc.
>   o - Two new syntax classes, "open island" and "close island" will be
>     introduced.  These will be designated by the characters "{" and "}".  
> Their
>     "matching character" slots will contain the island's chain.  There will be
>     an extra flag "chain" (denoted by "i") indicating whether there is a
>     previous/next island in the chain.
>   o - `scan-lists', `scan-sexps', etc. will treat a "foreign" island as
>     whitespace, much as they do comments.  They will also treat as whitespace
>     the gap between two islands in a chain.

Difficult to say, but this might produce some counter intuitive
behaviour. So, for example, consider some text like so:

=== Example

(here is some lisp)

;; This is a long and tedious piece of documentation in my lisp program.
(here is some more lisp)

=== End Example

Now moving backward a paragraph will have a significant difference in
behaviour -- on the "(" of "here is some more lisp", we move to "(here
is some lisp), while on the char before, we move the "This is a long".
Good, bad, expected? Don't know.

>   o - The (currently 11 element) parser state will be enhanced to support
>     islands as follows:
>     * - A twelfth element will be introduced.  This will contain an
>       association list whose elements will have the form (island-chain
>       . 12-element parse state); each element will contain the suspended state
>       of parsing in the island chain which is the car of the element.  An
>       element with a car of nil will represent the suspended parsing state of
>       the buffer outside of islands.
>     * - Elements 12, 13, .... will be island chains of the enclosing islands,
>       elt 12 being that of the innermost enclosing island, etc.  An element
>       with a value of nil indicates being outside all islands.
>   o - `parse-partial-sexp' will create and use an enhanced parser state as
>     described above.  Note that a two character construct (such as a C comment
>     opener) can not enclose an island, and special handling will be required
>     to exclude this.  The syntax table in use will change as the current
>     position passes between islands.
>   o - `syntax-ppss' will do the right thing with the extended parser state.
>     Alternatively, `syntax-ppss' will have an independent 12-element state in
>     each island chain, where elt. 11 is always nil.  Its cache mechanism will
>     be enhanced such that buffer changes outside of an island chain need not
>     invalidate the stored cache pertaining to the chain.
>   o - The facilities in this section are active even when `in-islands' is
>     nil.
> (vi) Regexps.
>   o - The regexp engine will be enhanced such that the regexps "\\s-", "\\s ",
>     and "[[:space:]] will match an entire island.
>   o - The gap between two islands in a chain will also be matched by the above
>     regexps.
>   o - This treatment of an island, and a gap between two islands, as WS will
>     occur only when `in-islands' is non-nil.
>   o - When `in-islands' is nil, there will be no reliable way of scanning over
>     an island by regexps, since it is a potentially nested structure, and FSMs
>     don't recognise arbitrarily nested structures.
> (vii) Variables.
>   o - Island chain local variable bindings will come into existence.  These
>     bindings depend on the island point is in.  There will be lower level
>     routines that will have "position" parameters as an alternative to using
>     point.
>   o - All variables which are currently buffer local will become chain local
>     except for those whose symbols are given a non-nil `entire-buffer'
>     property.  There will be no new functions like
>     `make-chain-local-variable'.

What is the default-value of a chain local variable, if the variable is
also buffer-local?

Will we need functions for setting all chains in a certain mode in a
single buffer?


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