Debra Cameron - Learning GNU Emacs, 3rd Edition

In addition to simply viewing the differences between files, you will sometimes want to resolve or merge them (especially if you've entered Ediff as the result of conflicts that occurred while checking in a file revision). Several commands help with this, and they generally require you to choose which buffer you want to work from. As you might have noticed in Figure 12-5, Ediff assigns each file or revision buffer a letter to identify it: A, Band sometimes Cif you are comparing three things. A number of Ediff commands work with these buffer identifiers—the letter Xis used to stand for these labels in the quick help window for most commands that use them.

To copy the version of the current difference found in one buffer to the other buffer, type the letter assigned to the buffer with the "right" version. For example, to copy A's version to B, type a. Ediff makes this change but keeps track of the old value in the buffer you changed. Following along in our example, if after changing buffer Blike this, you change your mind and want to restore its old state, you can type r b(for "restore buffer B"). These changes are kept track of on a difference-by-difference basis, so you don't have to change your mind right away; you can jump back to that difference and restore it at a later time, as long as you're still in the same Ediff session.

Of course, to make changes to a buffer, it cannot be in read-only mode. If you are comparing a current file with a historical revision, the buffer representing the older version is read-only because you can't change the past. If you want to avoid accidentally changing a file while browsing differences, you can cause its buffer to become read-only by typing Shiftand the buffer's letter label. ( Shift-bto make buffer Bread-only). This is a toggle, so doing it again makes a read-only buffer editable. If you do this to a buffer representing a historical revision, although Emacs will then let you edit the buffer, you're not actually affecting the revision within the version control system. So unless you're trying to confuse yourself, we'd suggest avoiding this practice.

If you're whipping through the buffers, making many changes by selecting appropriate versions to use within the Ediff control window, you may find yourself wanting to save one or the other of your difference buffers. While you can certainly click over to the difference window, move into that buffer, and use the standard C-scommand to save, Ediff offers a more convenient alternative. Simply press w(write) followed by the buffer's letter label to save that buffer without leaving the control window.

When you're done comparing the files, the quickest way to close the control window and get back to the "normal" Emacs world is to type qto quit your Ediff session. After confirming that you really want to do this, Ediff closes the control window and cleans up after itself. You can also suspend the session temporarily by typing zfor suspend. This closes the Ediff control window, but Ediff remembers that you were in the middle of a session, to which you can return later whenever you'd like. The easiest way to do this is to view the list of active Ediff sessions by choosing Tools → Ediff Miscellanea → List Ediff Sessions. When you actually quit an Ediff session, it no longer appears in this list.

If you've been cruising along in Ediff and suddenly find your commands aren't working, you've probably accidentally clicked on the differences window and are typing in one of the buffers directly, or perhaps you used an Ediff command that switched the window focus on you unexpectedly. Make sure to undo the stray characters you've typed into the comparison buffer, then click on the control window and start issuing commands again.

Of course, you may want to intentionally jump over to edit one of the buffers as you notice changes you'd like to make. You can do that at any time; just remember to switch back to the control window when you want move to other differences or use other Ediff commands. If, after editing one of the comparison buffers for a while, you'd like to return to viewing differences, starting with the difference nearest to your edit location, click in the control window and type gfollowed by the letter assigned to the buffer in which you're interested (as discussed earlier in "Making Changes").

If you've made substantial changes by editing the buffer directly, you may find that the difference region highlights have drifted out of synch with the actual location of differences. To fix this, once the control window has focus, type !to cause Ediff to recalculate and redisplay the differences.

If you've reconfigured the buffers you're looking at (perhaps you wanted to look up some help text, or engage in a side task, which Emacs certainly encourages) you can restore the window configuration for Ediff by clicking on the control window and typing C-l(recenter). This sets up the comparison window to display the files you're comparing and centers the current difference in each buffer. You may find that it also causes the comparison window to get keyboard focus, so be sure to click on the control window if necessary before you try to issue any Ediff commands.

As noted, there is a whole lot more to Ediff than we can discuss here. When you want to explore it, a good starting place is the built-in Ediff manual. You can get to this by typing E( Shift-e, the capitalization matters) in the Ediff control window. If you're not already inside Ediff, you can choose Tools → Ediff Miscellanea → Ediff Manual, or you can invoke Info, the Emacs documentation browser, by typing C-h i, and choose Ediff from the main menu of topics. (Typing mfor menu, followed by e d Enteris enough to complete "Ediff" and jump to its manual.)

For more task-specific help, you can click on any of the commands in the quick help window using your middle mouse button to get help describing what it does. (If you lack a three button mouse, you can click on the command with your regular mouse button and then press Enter.)

By now it should come as no surprise that you can change many details about the way that Ediff works so it better fits your way of thinking and working. After you've got a good grasp of the basics, you can use the Custom facility described in Chapter 10to tweak the way Ediff works by choosing Tools → Ediff Miscellanea → Customize Ediff. If the use of a separate operating system window (frame) for the control window is driving you batty, you can toggle that behavior right away by choosing Tools → Ediff Miscellanea → Toggle use of separate control buffer frame.

If Ediff is so powerful, why isn't it the default mode used by the vc-diffcommand? The most likely explanation is historical; vc-diffhas been around longer than Ediff, and it would have been disruptive to long-standing users of Emacs if a strange new interface was unexpectedly foisted on them. It seems people are writing patches to integrate Ediff more tightly with VC, but they are not (yet?) part of the Emacs distribution. If you're interested in the current state of any of these efforts, try a Google search for "vc ediff."