How to implement comments that always start at the beginning of a line

How to implement comments that always start at the beginning of a line

[Philipp]

Hi,

I'm writing my own Emacs major mode for a language in with all comments
start with an starisk at the beginning of a line. Unfortunately, I haven't
found a way to implement this in the syntax table. 

I tried to do a two-character comment sequence, with the first character
being a newline and the 2nd character an asterisk. This would neglect
comments in the first line of a buffer, but it would be better than nothing.
The lines I used were

		(modify-syntax-entry ?\n ">1" form-mode-syntax-table)
		(modify-syntax-entry ?* ".2" form-mode-syntax-table)

but that didn't work the way I expected. Does anybody have an idea how to
solve this?

Thanks,
Philipp
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Re: How to implement comments that always start at the beginning of a line

[rgb]

Philipp wrote:
> Hi,
>
> I'm writing my own Emacs major mode for a language in with all comments
> start with an starisk at the beginning of a line. Unfortunately, I haven't
> found a way to implement this in the syntax table.
>
> I tried to do a two-character comment sequence, with the first character
> being a newline and the 2nd character an asterisk. This would neglect
> comments in the first line of a buffer, but it would be better than nothing.
> The lines I used were
>
> 		(modify-syntax-entry ?\n ">1" form-mode-syntax-table)
> 		(modify-syntax-entry ?* ".2" form-mode-syntax-table)
>
> but that didn't work the way I expected. Does anybody have an idea how to
> solve this?

You probably found that it only recognized every other comment line
or something basically similar.  It's because the newline character
can't be both part of a comment starter and a comment ender at the
same time.

The only way to get exactly what you want is to use
font-lock-syntactic-keywords to assign comment starter syntax
specifically to to the characters you want.

Modified version of code ripped from ddl-mode...

(defvar ddl-font-lock-syntactic-keywords
 `(("^\\*" (0 "<")))
 "A list of regexp's or functions.  Used to add syntax-table properties
to characters that can't be set by the syntax-table alone.")

... Later used in the "Further item elements" section of
font-lock-defaults.

  ;; I use font-lock-syntactic-keywords to set some properties
  ;; and I don't want them ignored.
  (set (make-local-variable 'parse-sexp-lookup-properties) t)
  (set (make-local-variable 'font-lock-defaults)
       '(ddl-font-lock-keywords
         ;; keywords-only means no strings or comments get fontified
         nil
         ;; case-fold (ignore case)
         t
         ;; syntax-alist, nothing needs overriding
         nil
         ;; syntax-begin - move outside syntactic block
         nil
         ;; Further elements - set font-lock-syntactic-keywords
         (font-lock-syntactic-keywords .
ddl-font-lock-syntactic-keywords)))

Re: How to implement comments that always start at the beginning of a line

[Stefan Monnier]

> I'm writing my own Emacs major mode for a language in with all comments
> start with an starisk at the beginning of a line. Unfortunately, I haven't
> found a way to implement this in the syntax table. 

> I tried to do a two-character comment sequence, with the first character
> being a newline and the 2nd character an asterisk. This would neglect
> comments in the first line of a buffer, but it would be better than nothing.
> The lines I used were

> 		(modify-syntax-entry ?\n ">1" form-mode-syntax-table)
> 		(modify-syntax-entry ?* ".2" form-mode-syntax-table)

Firt, the syntax of the string passed as second argument treats the frst two
chars specially, so the "1" and "2" need to be placed further:

 (modify-syntax-entry ?\n "> 1" form-mode-syntax-table)
 (modify-syntax-entry ?*  ". 2" form-mode-syntax-table)

but this likely won't work because this kind of mixing a close-comment with
a begin-comment in the same char-sequence was not expected by the (latest)
coder of the comment-handling code (i.e. myself).  And even if I had
expected it, it's not clear how I'd have interpreted it: /*/ in c-mode
(and -- in snmp-mode) either closes or opens a comment, but not both.

> but that didn't work the way I expected. Does anybody have an idea how to
> solve this?

font-lock-syntactic-keywords is the onl thing that springs to my mind.
See fortran.el for an example.



        Stefan