Diff: custom diff via exec option?

[Reuben Thomas]

I would like to use an XML diff algorithm, but not have to re-invent
the wheel. What I'd like to do is have something like this:

diff --differ xmldiff -N [other neat diff options] tree1 tree2

then have diff essentially just do the equivalent of "cmp" on each
pair of files it ends up diffing, and then, if and only if the files
differ, feed the inputs to the program differ, which should work like
cmp (take two file names, return an exit code indicating difference or
similarity), but provide arbitrary textual output. If the program
specified decides that for its purposes the inputs don't differ, then
diff will act as if the files were, after all, the same. Otherwise, it
will include the differ program's output in its output. The first few
lines indicating the names of the files being compared can still be
generated by diff itself; indeed, should be, as in order to be able to
supply empty files to the differ program, it may sometimes want to use
/dev/null as an input to --differ, but of course that's not the
filename one wants to print in the output.

The example above could quite easily be extended to work with
--from-file, --to-file and other options.

Does anything like this exist? Would a patch that implements it be of interest?

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