[Rd] improve 'package not installed' load errors?

[Richard Cotton]

> A side question, which I do not know the answer to, is how users get themselves into this state.

I've fallen over this a few times.  It happens when you have multiple
R sessions running, and R tries to update Rcpp while it is loaded in
the other session.

For example, I'm working on one project, then I open another copy of R
to work on a different project.  Because I have update.packages in my
Rprofile, R occasionally tries to update Rcpp.  If that is loaded in
the first session, then a clean uninstall doesn't happen (the
directory and the dll are left).  Since the directory is still there,
update.packages thinks that the package exists, and I'm left with a
mangled copy of Rcpp that I need to manually remove.

On 24 October 2016 at 20:51, Kevin Ushey <kevinushey at gmail.com> wrote:
> Hi R-devel,
>
> One of the more common issues that new R users see, and become stumped
> by, is error messages during package load of the form:
>
>> library(ggplot2)
> Error in loadNamespace(j <- i[[1L]], c(lib.loc, .libPaths()),
> versionCheck = vI[[j]]) :
>   there is no package called 'Rcpp'
> Error: package or namespace load failed for 'ggplot2'
>
> Typically, error messages of this form are caused simply by one or
> more dependent packages (in this case, 'Rcpp') not being installed or
> available on the current library paths. (A side question, which I do
> not know the answer to, is how users get themselves into this state.)
>
> I believe it would be helpful for new users if the error message
> reported here was a bit more direct, e.g.
>
>> library(ggplot2)
> Error: 'ggplot2' depends on package 'Rcpp', but 'Rcpp' is not installed
> consider installing 'Rcpp' with install.packages("Rcpp")
>
> In other words, it might be helpful to avoid printing the
> 'loadNamespace()' call on error (since it's mostly just scary /
> uninformative), and check up-front that the package is installed
> before attempting to call 'loadNamespace()'. I'm sure a number of
> novice users will still just throw their hands up in the air and say
> "I don't know what to do", but I think this would help steer a number
> of users in the right direction.
>
> (The prescription to suggest installing a package from CRAN if
> available might be a step too far, but I think making it more clear
> that the error is due to a missing dependent package would help.)
>
> Any thoughts?
> Kevin
>
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-- 
Regards,
Richie

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