diff -r 15707e58fc3d -r 264baeef3588 hgext/show.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext/show.py	Fri Mar 24 19:19:00 2017 -0700
@@ -0,0 +1,137 @@
+# show.py - Extension implementing `hg show`
+#
+# Copyright 2017 Gregory Szorc <gregory.szorc@gmail.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""unified command to show various repository information (EXPERIMENTAL)
+
+This extension provides the :hg:`show` command, which provides a central
+command for displaying commonly-accessed repository data and views of that
+data.
+"""
+
+from __future__ import absolute_import
+
+from mercurial.i18n import _
+from mercurial import (
+    cmdutil,
+    commands,
+    error,
+    registrar,
+)
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
+cmdtable = {}
+command = cmdutil.command(cmdtable)
+
+class showcmdfunc(registrar._funcregistrarbase):
+    """Register a function to be invoked for an `hg show <thing>`."""
+
+    # Used by _formatdoc().
+    _docformat = '%s -- %s'
+
+    def _extrasetup(self, name, func, fmtopic=None):
+        """Called with decorator arguments to register a show view.
+
+        ``name`` is the sub-command name.
+
+        ``func`` is the function being decorated.
+
+        ``fmtopic`` is the topic in the style that will be rendered for
+        this view.
+        """
+        func._fmtopic = fmtopic
+
+showview = showcmdfunc()
+
+@command('show', commands.formatteropts, _('VIEW'))
+def show(ui, repo, view=None, template=None):
+    """show various repository information
+
+    A requested view of repository data is displayed.
+
+    If no view is requested, the list of available views is shown and the
+    command aborts.
+
+    .. note::
+
+       There are no backwards compatibility guarantees for the output of this
+       command. Output may change in any future Mercurial release.
+
+       Consumers wanting stable command output should specify a template via
+       ``-T/--template``.
+
+    List of available views:
+
+    """
+    if ui.plain() and not template:
+        raise error.Abort(_('"hg show" cannot be used in plain mode because '
+                            'output is not stable'),
+                          hint=_('unset HGPLAIN and invoke with -T/--template '
+                                 'to control output'))
+
+    views = showview._table
+
+    if not view:
+        ui.pager('show')
+        # TODO consider using formatter here so available views can be
+        # rendered to custom format.
+        ui.write(_('available views:\n'))
+        ui.write('\n')
+
+        for name, func in sorted(views.items()):
+            ui.write(('%s\n') % func.__doc__)
+
+        ui.write('\n')
+        raise error.Abort(_('no view requested'),
+                          hint=_('use "hg show VIEW" to choose a view'))
+
+    # TODO use same logic as dispatch to perform prefix matching.
+    if view not in views:
+        raise error.Abort(_('unknown view: %s') % view,
+                          hint=_('run "hg show" to see available views'))
+
+    template = template or 'show'
+    fmtopic = 'show%s' % views[view]._fmtopic
+
+    ui.pager('show')
+    with ui.formatter(fmtopic, {'template': template}) as fm:
+        return views[view](ui, repo, fm)
+
+@showview('bookmarks', fmtopic='bookmarks')
+def showbookmarks(ui, repo, fm):
+    """bookmarks and their associated changeset"""
+    marks = repo._bookmarks
+    if not len(marks):
+        # TODO json output is corrupted; consider using formatter
+        ui.write(_('(no bookmarks set)\n'))
+        return
+
+    active = repo._activebookmark
+    longestname = max(len(b) for b in marks)
+    # TODO consider exposing longest shortest(node).
+
+    for bm, node in sorted(marks.items()):
+        fm.startitem()
+        fm.context(ctx=repo[node])
+        fm.write('bookmark', '%s', bm)
+        fm.write('node', fm.hexfunc(node), fm.hexfunc(node))
+        fm.data(active=bm == active,
+                longestbookmarklen=longestname)
+
+# Adjust the docstring of the show command so it shows all registered views.
+# This is a bit hacky because it runs at the end of module load. When moved
+# into core or when another extension wants to provide a view, we'll need
+# to do this more robustly.
+# TODO make this more robust.
+longest = max(map(len, showview._table.keys()))
+for key in sorted(showview._table.keys()):
+    cmdtable['show'][0].__doc__ += ' %s   %s\n' % (
+        key.ljust(longest), showview._table[key]._origdoc)