[Rd] Proposed diff.character() method

[Arni Magnusson]

Dear R developers,

Recently, I was busy comparing different versions of several packages.
Tired of going back and forth between R and diff, I created a simple
file comparison function in R that I found quite useful. For an
efficient and familiar interface I called it diff.character() and ran
things like:

  diff("old/R/foo.R", "new/R/foo.R")

Before long, I found the need for a directory-wide comparison and
added support for:

  diff("old/R", "new/R")

I have now revisited and fine-polished this function to a point where
I'd like to humbly suggest that diff.character() could be incorporated
into the base package. See attached files and patch based on the
current SVN trunk. It can be tested quickly by sourcing diff.R, or by
building R.

The examples in diff.character.html are somewhat contrived, in the
absence of good example files to compare. You will probably have
better example files to compare from your own work.

Clearly, the functionality differs considerably from the default
diff() method that operates on a single x vector, but in the broad
sense, they're both about showing differences. For most programmers,
calling diff() on two files or directories is already a part of muscle
memory, both intuitive and efficient.

There are a couple of CRAN packages (diffobj, diffR) that can compare
files but not directories. They have package dependencies and return
objects that are more complex (S4, HTML) than the plain list returned
by diff.character().

This basic utility does by no means compete with Meld, Kompare, Emacs
ediff, or other feature-rich diff applications, and using setdiff() as
a basis for file comparison can be a somewhat simplistic approach.
Nevertheless, I think many users may find this a handy tool to quickly
compare scripts and data files. The method could be implemented
differently, with fewer or more features, and I'm happy to amend
according to the R Core Team decision.

In the past, I have proposed additions to core R, some rejected and
others accepted. This proposal fits a useful tool in a currently
vacant diff.character() method at a low cost, using relatively few
lines of base function calls and no compiled code. Its acceptance will
probably depend on whether members of the R Core Team and/or CRAN Team
might see it as a useful addition to their toolkit for interactive and
scripted workflows, including R and CRAN maintenance.

All the best,
Arni

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