Re: [9fans] page as a presentation tool

["Russ Cox" <rsc@swtch.com>]

> I should say I am using whatever version is in the ports tree
> on FreeBSD.  Maybe there is a better version.  I have noticed
> two ways in which it differs.  One is that undo doesn't
> restore the dot.  This is pretty annoying since I like to
> use succesive refinement to select text and I make a lot of
> mistakes.  The other is the 'x' doesn't work quite as
> advertised in the paper.  The relevant passage is
>
> 	Sam uses a two-pass algorithm for making changes,
> 	and treats each file as a database against which
> 	transactions are registered. Changes are not made
> 	directly to the contents. Instead, when a command
> 	is started, a `mark' containing a sequence number
> 	is placed in the transcript Buffer, and each change
> 	made to the file, either an insertion or deletion or
> 	a change to the file name, is appended to the end of
> 	the transcript. When the command is complete, the
> 	transcript is rewound to the mark and applied to
> 	the contents.
>
> 	One reason for separating evaluation from application
> 	in this way is to simplify tracking the addresses of
> 	changes made in the middle of a long sequence. The
> 	two-pass algorithm also allows all changes to apply
> 	to the original data: no change can affect another
> 	change made in the same command. This is particularly
> 	important when evaluating an x command because it
> 	prevents regular expression matches from stumbling
> 	over changes made earlier in the execution.
>
> I haven't looked at the source to see whether UNIX sam also uses
> the two pass algorithm, but it is certainly stumbling over changes
> made earlier in the execution.

The one at http://pdos.lcs.mit.edu/~rsc/software/plan9
builds from the current Plan 9 sources, which are really
not very far from the Unix ones.  Undo definitely does
restore dot, though I wish samterm scrolled so you could
see it.  And x seems to work, though I can't imagine why
it would be too different from the Unix one.

Russ