Using Images in Haddock Documentation on Hackage

A useful but little used feature of Haddock is the ability to include inline images in Haddock pages. Here are a few examples. You can use images for diagrams or for inserting mathematics into your documentation in a readable way. In order to use an image, you just put <<path-to-image>> in a Haddock comment. The image can be of any format that browsers support: PNG, JPEG, SVG, whatever.

Until the most recent (1.18) version of Cabal, using this feature was kind of a pain because there was no way to ensure that your image files ended up in a reasonable place in the documentation tree. That meant either hosting the images somewhere other than Hackage, or inserting the image data directly into your Haddock comments, which is painful in the extreme.

Now though, Cabal has a new extra-doc-files option you can specify in the Cabal file for your package, which lists files that should be copied from your source tree into the documentation tarball. That means you can get your image files into just the right place with no pain at all.

As an example, in the arb-fft package I use a couple of SVG images to render some equations in the documentation. In the source tree there’s a directory called doc-formulae that contains a couple of bits of LaTeX and a Makefile that processes them and uses dvisvgm to turn the resulting DVI output into SVG files. The Cabal file contains a line saying extra-doc-files: doc-formulae/*.svg which ensures that the SVG images end up in the Haddock documentation tarball. I can then refer to them in Haddock comments as something like <<doc-formulae/fft-formula.svg>>, which is really convenient.

This should now work for any new uploads to Hackage after a fix last night by Duncan Coutts.