> -----Original Message-----
> From: Duncan Murdoch [mailto:murdoch.dun...@gmail.com]
> Sent: Thursday, April 12, 2012 12:27 PM
> To: Ken Williams
> Cc: r-devel@r-project.org
> Subject: Re: [Rd] [patch] giving library() a 'version' argument
>
> I haven't tested it, but according to the documentation in Writing R
> Extensions, the dependencies are enforced at the time library() is called.
Oh, I hadn't suspected that. I can look into testing that, if it's true then
of course that changes this all. I probably won't be able to do that for a few
days because I'll be traveling though.
I've never noticed a package failing to load at runtime because its
prereq-version dependency wasn't met though.
> [...]
> But a single line at the top of the script would fix this:
>
> stopifnot(packageVersion("foo") == "3.14")
For the most common use case, that would look more like:
stopifnot(compareVersion(packageVersion("foo"), "3.14") < 0)
which gets less declarative, and I'd argue less clear about exactly what it's
trying to enforce.
And I can see myself (& presumably others) getting that comparison operator
backwards a lot, having to look it up each time or copy-paste it from other
code.
And then that still doesn't add nice error messages, that would be yet more
code.
*And*, it doesn't actually behave correctly if the package is already loaded by
other code, because it might have been loaded from a different location than
the one that would be found in the packageVersion() call. (Or am I maybe wrong
about what packageVersion() does in that case? I don't think the docs specify
that behavior.)
For prior art on this whole concept, a useful precedent is the 'use()' function
in Perl, which accepts a version argument, even though there is also robust
version checking at installation/testing time.
>
> Another problem with putting this into library() is that packages aren't
> always loaded by library(): there is require(), and there are implicit
> loads triggered by dependencies of other packages.
That's not really a problem. If someone wants to enforce a runtime dependency,
they stick the enforcement line into their code, and it will correctly stop if
the criterion is not met.
-Ken
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