annotate mercurial/templates/map-cmdline.show @ 31765:264baeef3588

show: new extension for displaying various repository data

Currently, Mercurial has a number of commands to show information. And,
there are features coming down the pipe that will introduce more
commands for showing information.

Currently, when introducing a new class of data or a view that we
wish to expose to the user, the strategy is to introduce a new command
or overload an existing command, sometimes both. For example, there is
a desire to formalize the wip/smartlog/underway/mine functionality that
many have devised. There is also a desire to introduce a "topics"
concept. Others would like views of "the current stack." In the
current model, we'd need a new command for wip/smartlog/etc (that
behaves a lot like a pre-defined alias of `hg log`). For topics,
we'd likely overload `hg topic[s]` to both display and manipulate
topics.

Adding new commands for every pre-defined query doesn't scale well
and pollutes `hg help`. Overloading commands to perform read-only and
write operations is arguably an UX anti-pattern: while having all
functionality for a given concept in one command is nice, having a
single command doing multiple discrete operations is not. Furthermore,
a user may be surprised that a command they thought was read-only
actually changes something.

We discussed this at the Mercurial 4.0 Sprint in Paris and decided that
having a single command where we could hang pre-defined views of
various data would be a good idea. Having such a command would:

* Help prevent an explosion of new query-related commands
* Create a clear separation between read and write operations
  (mitigates footguns)
* Avoids overloading the meaning of commands that manipulate data
  (bookmark, tag, branch, etc) (while we can't take away the
  existing behavior for BC reasons, we now won't introduce this
  behavior on new commands)
* Allows users to discover informational views more easily by
  aggregating them in a single location
* Lowers the barrier to creating the new views (since the barrier
  to creating a top-level command is relatively high)

So, this commit introduces the `hg show` command via the "show"
extension. This command accepts a positional argument of the
"view" to show. New views can be registered with a decorator. To
prove it works, we implement the "bookmarks" view, which shows a
table of bookmarks and their associated nodes.

We introduce a new style to hold everything used by `hg show`.

For our initial bookmarks view, the output varies from `hg bookmarks`:

* Padding is performed in the template itself as opposed to Python
* Revision integers are not shown
* shortest() is used to display a 5 character node by default (as
  opposed to static 12 characters)

I chose to implement the "bookmarks" view first because it is simple
and shouldn't invite too much bikeshedding that detracts from the
evaluation of `hg show` itself. But there is an important point
to consider: we now have 2 ways to show a list of bookmarks. I'm not
a fan of introducing multiple ways to do very similar things. So it
might be worth discussing how we wish to tackle this issue for
bookmarks, tags, branches, MQ series, etc.

I also made the choice of explicitly declaring the default show
template not part of the standard BC guarantees. History has shown
that we make mistakes and poor choices with output formatting but
can't fix these mistakes later because random tools are parsing
output and we don't want to break these tools. Optimizing for human
consumption is one of my goals for `hg show`. So, by not covering
the formatting as part of BC, the barrier to future change is much
lower and humans benefit.

There are some improvements that can be made to formatting. For
example, we don't yet use label() in the templates. We obviously
want this for color. But I'm not sure if we should reuse the existing
log.* labels or invent new ones. I figure we can punt that to a
follow-up.

At the aforementioned Sprint, we discussed and discarded various
alternatives to `hg show`.

We considered making `hg log <view>` perform this behavior. The main
reason we can't do this is because a positional argument to `hg log`
can be a file path and if there is a conflict between a path name and
a view name, behavior is ambiguous. We could have introduced
`hg log --view` or similar, but we felt that required too much typing
(we don't want to require a command flag to show a view) and wasn't
very discoverable. Furthermore, `hg log` is optimized for showing
changelog data and there are things that `hg display` could display
that aren't changelog centric.

There were concerns about using "show" as the command name.

Some users already have a "show" alias that is similar to `hg export`.

There were also concerns that Git users adapted to `git show` would
be confused by `hg show`'s different behavior. The main difference
here is `git show` prints an `hg export` like view of the current
commit by default and `hg show` requires an argument. `git show`
can also display any Git object. `git show` does not support
displaying more complex views: just single objects. If we
implemented `hg show <hash>` or `hg show <identifier>`, `hg show`
would be a superset of `git show`. Although, I'm hesitant to do that
at this time because I view `hg show` as a higher-level querying
command and there are namespace collisions between valid identifiers
and registered views.

There is also a prefix collision with `hg showconfig`, which is an
alias of `hg config`.

We also considered `hg view`, but that is already used by the "hgk"
extension.

`hg display` was also proposed at one point. It has a prefix collision
with `hg diff`. General consensus was "show" or "view" are the best
verbs. And since "view" was taken, "show" was chosen.

There are a number of inline TODOs in this patch. Some of these
represent decisions yet to be made. Others represent features
requiring non-trivial complexity. Rather than bloat the patch or
invite additional bikeshedding, I figured I'd document future
enhancements via TODO so we can get a minimal implmentation landed.
Something is better than nothing.

# TODO add label() once we figure out which namespace the labels belong on.
showbookmarks = '{if(active, "*", " ")} {pad(bookmark, longestbookmarklen + 4)}{shortest(node, 5)}\n'