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In my vimrc I have <C-H>, <C-J>, <C-K> and <C-L> mapped to navigate between split windows in normal mode. When I use diff mode I only ever have windows in vertical splits, so I'd like to re-purpose <C-J> and <C-K> to jump to the previous/next diff.

:help diff mentions &diff when using vimdiff to determine whether you are in diff mode. It looks like this also gets set when you start a diff from an existing vim session, so I assume I should leverage this somehow to re-set the mappings, but I'm not exactly sure how. Based on my 2 second test just before posting this, I tried checking &diff in my vimrc and performing the mapping as needed, but as I suspected the vimrc doesn't get read in again when going into diff mode from an existing vim session.

What would be the best way to detect going to/from diff mode and re-setting the mappings accordingly?

19

You can create an expression mapping that checks &diff. This is done using the <expr> keyword.

Example

nnoremap <expr> <C-J> &diff ? ']c' : '<C-W>j'

With this, each time you type <C-J>, the expression &diff ? ']c' : '<C-W>h' is evaluated, and the result (a string) is used for your map. So, in diff-mode, <C-J> will be like ]c (jump to start of next change); otherwise, it will be like <C-W>j (move to window below).

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    Would the &diff flag only be detected if vim starts in diff mode, say vim -d ... or also if you open a buffer in diff mode from within Vim already (say :diffthis or equivalent)? – gented Nov 4 '20 at 10:57
  • @gented, I think this command, as it stands, operates at the moment it is run, and never again (unless you reaload the source file where the command is). Probably this is a command you would put in a more complex logic, if you want it to behave dynamically. – Enlico Jan 29 at 21:54
  • The <expr> modifier makes it so that the expression is evaluated every time the mapping is invoked. The result of the expression is a string, which is interpreted as keystrokes. – tommcdo Jan 31 at 15:06
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I use this feature to change the highligting during diffs.

if &diff

    " Your setting you want to set when using diff mode.
    "

endif
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    You can execute this block regardless of the value of &diff. The Diff* syntax groups are only used for diff. Furthermore, there's no guarantee that this will be re-evaluated when diff mode is entered interactively. – tommcdo Mar 23 '15 at 20:57
  • @tommcdo: I removed the example stuff. But the re-evaluation stuff I was unaware of (mainly because I don't switch modes, I ether run as vim or as vimdiff). – Martin York Mar 23 '15 at 21:15

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