Just to document some of the pieces we specifically want to evaluate and/or change before merging:

• currently , temp files are placed in the current directory, with *.u.tmp extensions to avoid being caught by the typechecker. We could still put them in /tmp/<tmpdir>/*.u since they are temp files and we may not want to clutter the project directory either, while also enabling syntax highlighting for unison editors. (There may not be a lot of unison editors yet, but the people doing a unison merge are more likely to have one.)

• upon more digging, I'm not sure if MERGETOOL as an environment variable is a standard thing? (It is a standard thing as a git config variable.) If it's not, we should probably just switch to UCM_MERGETOOL to match our other variables.

Also want to put out a note at release time about our longer-term plans for merges in Unison.

Side note: apparently git allows configuring a separate environment variable for a gui mergetool than for the tui one. (--gui option). I wonder what the point of that is.

Ok, putting files in a temp dir and switching to UCM_MERGETOOL sounds good to me.

I'm about to test out vscode as a mergetool (supposedly the magic string is

code --wait --merge $REMOTE $LOCAL $BASE $MERGED

), and maybe a few others, to make sure we've made the right assumptions about $MERGED.

Looking pretty great:

In initial testing, a couple things jumped out:

• we want some feedback if the command fails; I think these 1 2 worked out well, though I'm not sure what kind of info we get back from the "shell" call?
• it looks like tools (vscode at least) expect merged.u to exist. I think we want it to be another copy of the lca file.
• the alice and bob temp file names become part of the interface:
  
  could we put any randomness just in the directory part of the name and not in the filename, to clean this up? Maybe with withTempDirectory

@mitchellwrosen I wanna try to get this merged for use in the knn demo work—

Besides the merge conflict, where did we land on merged.u, I think we were deciding between "a copy of lca" vs "has git diff markers for 2-3 versions"?

Decent workaround is vscode's "compare new untitled text files" command, and then copying in the two versions from the scratch file

vs

@aryairani I believe we landed on the latter, that merged.u is supposed to contain git merge markers

@aryairani Ok, I think this is ready for review again

Merging for now to be able to test it with other new features already in trunk, even though I think we should tweak the auto-load behavior after writing the files, and also we want to make a pass to limit the amount of time merge will work without a progress indicator, before doing a new release.