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Re: perl mode color highlighting working badly

From: martin rudalics
Subject: Re: perl mode color highlighting working badly
Date: Wed, 02 Nov 2005 13:54:24 +0100
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

Half-baken ideas ahead - don't pay too much attention.

>>I believe that `jit-lock-after-change'
>>should contain something like
>>        (setq font-lock-syntactically-fontified
>>              (min font-lock-syntactically-fontified start))
> Note that jit-lock is supposed to be a generic framework used by packages
> such as font-lock (and glasses-mode).  So adding font-lock specific code to
> it is ugly.

`jit-lock-after-change' already contains that (pretty useless) check for
`font-lock-multiline'.  Though, `font-lock-turn-on-thing-lock' could put
this in a separate after-change-hook that got executed for jit-lock-mode

> Let's say you have already fontified the whole buffer,
> and the following things happen:
> - someone modifies text between positions 50 and 60.

An after-change-hook would have set `font-lock-syntactically-fontified'
to something <= 50 here ...

> - font-lock-fontify-region is called on 50..60.

... which would set `font-lock-syntactically-fontified' to 60 ...

>   The syntax-ppss state
>   at position 60 may be different from what it used to be because
>   syntactic-keywords may have added some syntax-table properties.
> - font-lock-fontify-region is called on 100..200.
>   If font-lock-syntactic-keywords uses syntax-ppss, that means that the
>   syntax-table properties placed on the text between 60-100 may depend on the
>   syntax-ppss state at position 60.  So in order to get the correct
>   syntax-ppss at position 100, we should re-apply syntactic-keywords between
>   60 and 100.

... which would get done automatically with the after-change-hook since
`font-lock-syntactically-fontified' is 60 and that's where both the
syntactic-keywords pass and the syntactic pass would start.

Note that with `font-lock-syntax-props-depend-on-themselves' you have
the following problem:

1. Jump to a position X such that some portion of the buffer before X is
   not keyword-fontified yet.  `font-lock-syntactically-fontified' will
   be around X.  Essentially the configuration that started this thread.

2. Jump or scroll to some unfontified position Y < X.
   `font-lock-syntactically-fontified' will now be around Y.

3. Jump or scroll to some unfontified position Z > X.  You will have to
   syntactically/syntactic-keyword refontify the buffer from Y to Z
   although you have not modified a thing.  And you also syntactically
   refontify text that has been keyword-fontified before.

With an after-change-hook you wouldn't suffer this.

By the way, wouldn't it make sense to define `syntax-begin-function' for
perl-mode?  Something like a "well-formed" `sub' starting in column zero
- ignoring any syntax-table properties.  I believe it would be less
costly to run both `font-lock-fontify-syntactic-keywords-region' and
`font-lock-fontify-syntactically-region' from such a position.

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