Re: Syntax tables for multiple modes [was: bug#22983: syntax-ppss returns wrong result.]

[Stefan Monnier <monnier@iro.umontreal.ca>]

> No, no. Context should not have absolute positions in it. That would ruin the
> whole thing. It should contain information about the nesting of language
> constructs sufficient to be able to indent first line of an inner span without
> any other positional knowledge.

As explained, the SMIE indentation has no such "summary of context".
The only context it could use is either a position or the complete text
before that position.

> For example, if current line is directly part of IF block, most
> languages don't care what precedes IF head at all, only the offset
> of IF.

Yes, there are simple cases we know how to handle.  The problem is the
general case, along with the work to modify the existing indentation
codes to be able to generate and use that data.

> So an entry in the context data structure might look like (IF
> . IF-OFFSET). If line number within the block is important then you
> can pass (IF IF-OFFSET . RELATIVE-LINE). My hunch is that for most
> languages you would be able to reduce indentation to a small number of
> block-continuation constructs (less than 10).

Consider

    x = a << b + c * d

the code after this line can be indented in various different ways:

                 * e
or
             + e
or
        == e
or
  ; e

And of course, in this example, I put all relevant operators, but in
practice they'll generally be on different lines.  And SMIE (which
supports that kinds of indentation, e.g. in sm-c-mode) doesn't
pre-compute that context: it's only when it sees the "+" at the
beginning of line that it moves back over higher-precedence operators to
find the matching alignment spot.

In theory, SMIE could try to create the kind of context you're thinking
of, but that would amount to a complete rewrite (and it would likely be
very difficult if not impossible to make it work with existing
smie-rules-functions, so it'd break backward compatibility).


        Stefan