[Rd] R html help system [Was: How to document man/*.Rd pages with images?]

Simon Urbanek simon.urbanek at r-project.org
Thu May 12 15:13:20 CEST 2011

I just want to clarify the mechanics of the help system when using html.

R has a built-in HTTP server (aka Rhttpd) which transforms HTTP requests to function calls. It is not your usual web server, because it doesn't map URL paths to files, it just allows R functions to do anything with it -- something like CGI except that we are talking about functions and not files. Therefore you won't find any files and there is no file structure involved.

For the help system, the function handling the requests is tools:::httpd() - you can look at what it does. Basically, it generates pages according to the various paths it supports. As part of its path handling it allows certain paths to reference files, e.g. /library/myPackage/doc/randomStuffInPackagesDocDirectory.html will read the file doc/randomStuffInPackagesDocDirectory.html in myPackage.

Note that the whole point of the dynamic help *is* to generate content on the fly, because the content depends on the state of your current workspace -- packages loaded, classes defined, etc. so you cannot pre-generate pages as they won't have correct links - that's why we shifted from static html pages to the dynamic ones.

I'm curious about "If it’s a large picture this process nearly crashes my machine when trying to access the file via help" - do you have an example package that would illustrate the problem?


On May 11, 2011, at 7:14 PM, Sean Robert McGuffee wrote:

> Thanks everyone for your help,
> To summarize a resolution to my issue, it turns out that an image can be include in a documentation file via html by putting an image file in the inst/doc directory, for example inst/doc/myPic.png, and then pointing to it in the man/myHelpPage.Rd file, for example as follows:
> \if{html}{
> \out{<img src="../doc/myPic.png" alt="image ../doc/myPic.png should be here"/>}
> }\ifelse{latex}{}{}
> Note, this doesn’t mean that R’s help browser will view those images inside the properly generated html help files.
> Also, note that without the \out{} part, the text of the <img .../>  line would show up instead of the html commands. 
> I have some concerns incase anyone on the list is interested. If it’s a large picture this process nearly crashes my machine when trying to access the file via help—and I’m sure there must be some bug in that. I should note that the picture won’t actually display within R’s help console (at least on my machine--I’m on a mac with a binary version of R). To see that the html files are created properly, I have to copy a link to the help file and then point an actual browser such as firefox to the help file to see the page with the image. I’m not sure how R is running httpd or how that interacts with help. I’m not even sure about the basics of help. Is there a way to configure R to use an actual web browser by default instead of it’s slow one that doesn’t show images? It would also be nice if there were an address bar on R’s help browser. I mean, until I put a link to my help file inside another help file, there was no way for me to even get it’s address to copy and paste into firefox. It would also be nice if it didn’t almost crash and let me more easily get the link, but ideally it would be best not to have a semi-functional help browser. Furthermore, this brings up the point that I can’t find the files I’m browsing with the link. In this case, I get a link such as: 
> But I can’t find the MyPackage.html file anywhere on my computer. It’s there in the web browser, but seems to be only in existence via R’s httpd without actually existing on my file system. 
> Is it there and I can’t find it or is it encoded in R somehow? If it is there, where would it be? If I close R, I no longer have access to the page that R’s httpd is serving. It seems to me that it’s being created every time I use help—and I think that is extremely inefficient. I think firefox can handle file-type urls, so I if there is a way to get R to both generate these files and use firefox to browse them for help, I would very much like to know more about it. It would be much faster and useful than the status quo on my machine if this file were generated once at installation and remained as a file--and and using help simply pointed a web-browser to the file.
> Anyway, I suppose this is a tangent. The main point is that there is a way to provide help documentation with images—but even though it tries to view them correctly via help—R’s help browser displays broken images so I have the awkward need to copy and paste links into other web browsers.
> Regarding some feedback I’ve gotten about some user’s interests in help formatted as text, I think there are two things in this process that keep a text help user on track: (1) the conditional html part and (2) even if using a textual html browser, <img ... alt=”alternate text”/> take care of displaying images as text. I think though that the other way around, the users who require images in their help files are having less functionality via help in R. At least in this case, the best I could do was get R to generate the proper help pages in html, but R’s default html help browser (at least on my machine) doesn’t display the images (although they are there and can be displayed by the same link in firefox).  
> Sometimes it’s true what they say about a picture being worth a thousand words—I think in general this is true for complex things that need computer power to deal with, so I hope R can eventually support images in help files due to the usefulness of doing so in some cases.
> Thanks again,
> Sean

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