--- a/src/topic/__init__.py	Mon Mar 14 17:09:02 2016 +0000
+++ b/src/topic/__init__.py	Mon Mar 14 17:37:39 2016 +0000
@@ -38,6 +38,7 @@
 from . import constants
 from . import revset as topicrevset
 from . import destination
+from . import stack
 from . import topicmap
 from . import discovery
 
@@ -165,9 +166,15 @@
 @command('topics [TOPIC]', [
     ('', 'clear', False, 'clear active topic if any'),
     ('', 'change', '', 'revset of existing revisions to change topic'),
+    ('l', 'list', False, 'show the stack of changeset in the topic'),
 ])
-def topics(ui, repo, topic='', clear=False, change=None):
+def topics(ui, repo, topic='', clear=False, change=None, list=False):
     """View current topic, set current topic, or see all topics."""
+    if list:
+        if clear or change:
+            raise error.Abort(_("cannot use --clear or --change with --list"))
+        return stack.showstack(ui, repo, topic)
+
     if change:
         if not obsolete.isenabled(repo, obsolete.createmarkersopt):
             raise error.Abort(_('must have obsolete enabled to use --change'))

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/topic/stack.py	Mon Mar 14 17:37:39 2016 +0000
@@ -0,0 +1,20 @@
+# stack.py - code related to stack workflow
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+from mercurial.i18n import _
+from mercurial import error
+
+def _getstack(repo, topic):
+    # XXX need to exclude obsolete changesets
+    # XXX need sorting
+    return repo.revs("topic(%s)", topic)
+
+def showstack(ui, repo, topic):
+    if not topic:
+        topic = repo.currenttopic
+    if not topic:
+        raise error.Abort(_('no active topic to list'))
+    for r in _getstack(repo, topic):
+        # super crude initial version
+        ui.write(repo[r].description().splitlines()[0] + '\n')

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-topic-stack.t	Mon Mar 14 17:37:39 2016 +0000
@@ -0,0 +1,68 @@
+  $ . "$TESTDIR/testlib"
+
+Initial setup
+
+
+  $ cat << EOF >> $HGRCPATH
+  > [ui]
+  > logtemplate = {rev} {branch} \{{get(namespaces, "topics")}} {phase} {desc|firstline}\n
+  > EOF
+
+(new head warning seems buggy)
+  $ hg init main
+  $ cd main
+  $ echo aaa > aaa
+  $ hg add aaa
+  $ hg commit -m c_a
+  $ echo aaa > bbb
+  $ hg add bbb
+  $ hg commit -m c_b
+  $ hg topic foo
+  $ echo aaa > ccc
+  $ hg add ccc
+  $ hg commit -m c_c
+  $ echo aaa > ddd
+  $ hg add ddd
+  $ hg commit -m c_d
+  created new head
+  $ echo aaa > eee
+  $ hg add eee
+  $ hg commit -m c_e
+  created new head
+  $ echo aaa > fff
+  $ hg add fff
+  $ hg commit -m c_f
+  created new head
+  $ hg log -G
+  @  5 default {foo} draft c_f
+  |
+  o  4 default {foo} draft c_e
+  |
+  o  3 default {foo} draft c_d
+  |
+  o  2 default {foo} draft c_c
+  |
+  o  1 default {} draft c_b
+  |
+  o  0 default {} draft c_a
+  
+
+Simple test
+-----------
+
+hg topic -l list all changeset in the topic
+
+  $ hg topic
+   * foo
+  $ hg topic --list
+  c_c
+  c_d
+  c_e
+  c_f
+
+error case, nothing to list
+
+  $ hg topic --clear
+  $ hg topic --list
+  abort: no active topic to list
+  [255]

--- a/tests/test-topic.t	Mon Mar 14 17:09:02 2016 +0000
+++ b/tests/test-topic.t	Mon Mar 14 17:37:39 2016 +0000
@@ -14,8 +14,9 @@
   
   options:
   
-    --clear        clear active topic if any
-    --change VALUE revset of existing revisions to change topic
+      --clear        clear active topic if any
+      --change VALUE revset of existing revisions to change topic
+   -l --list         show the stack of changeset in the topic
   
   (some details hidden, use --verbose to show complete help)
   $ hg topics