On 12/04/2012 1:46 PM, Ken Williams wrote:
> -----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)
The compareVersion call doesn't need to be explicit, i.e. you'll get the
same result from
stopifnot(packageVersion("foo")< "3.14")
which looks pretty clear to me. It works in some quick tests,
recognizing that rgl version 0.92.879 is bigger than 0.92.100 but less
than 0.92.1000.
Duncan Murdoch
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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