[R-pkg-devel] URL syntax causes R CMD build failure - a fix

[J C Nash]

I'm posting this in case it helps some other developers getting build failure.

Recently package nlsr that I maintain got a message that it failed to build on
some platforms. The exact source of the problem is still to be illuminated,
but seems to be in knitr::render and/or pandoc or an unfortunate interaction.
An update to pandoc triggered a failure to process a vignette that had been
happily processed for several years. The error messages are unhelpful, at least
to me,

    Error at "nlsr-devdoc.knit.md" (line 5419, column 1):
    unexpected end of input
    Error: pandoc document conversion failed with error 64
    Execution halted

Unfortunately, adding "keep_md: TRUE" (you need upper case TRUE to save it when
there is no error of this type), did not save the intermediate file in this
case. However, searching for "pandoc error 64" presented one web page where the author
used brute force search of his document by removing / replacing sections to find
the line(s) that caused trouble. This is a little tedious, but effective. In my
case, the offending line turned out to be a copied and pasted URL

https://en.wikipedia.org/wiki/Levenberg%E2%80%93Marquardt_algorithm

The coded characters can be replaced by a hyphen, to give,

https://en.wikipedia.org/wiki/Levenberg-Marquardt_algorithm

and this, when pasted in Mozilla Firefox at least, will go to the appropriate
wikipedia page.

I'd be interested in hearing from others who have had similar difficulties. I
suspect this is relatively rare, and causing some sort of infelicity in the
output of knitr::render that then trips up some versions of pandoc, that may,
for instance, be now applying stricter rules to URL syntax.

Best,

John Nash