# Thing I Learned Today: Type Signatures in Haskell are Awesome

by Brian Shourd on December 5, 2012

My last post was about getting Hakyll set up to take over my blog, instead of wordpress. Good news - it’s done! At least, it’s working. I’m still using some example templates and css, which I’d like to swap out for some of my own designs.

One of the things that I need on my blog is a nice way to typeset mathematics. Since Hakyll is built on Pandoc, it can handle LaTeX conversions in a lot of ways. My personal choice is Mathjax, which looks awesome but has the drawback that it runs a fair chuck of javascript in the background.

The only thing left to do was work out exactly how I could get Hakyll to tell Pandoc to use the built-in Mathjax option. In addition, it would be nice if it only did it for pages that actually need it, so that I’m not making all my readers download a chunk of JS just to view a mathless static webpage.

A quick search revealed John Lenz’s blog, with a very helpful guide on doing exactly what I just said. It even has explicit code snippets accompanying it. Only problem - he didn’t say how to integrate his code snippets.

It turns out that this isn’t a problem at all. He basically included two code snippets:

pandocOptions :: WriterOptions
pandocOptions = defaultHakyllWriterOptions
{ writerHTMLMathMethod = MathJax ""
}
checkMathOption :: Page String -> Page String
checkMathOption page =
case getFieldMaybe "math" page of
Nothing -> setField "mathjax" "" page
Just _  -> setField "mathjax" "<script type=\"text/javascript\" src=\"http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML\" />" page

The type signatures immediately told me how to use the code. For the first, I needed to do a quick documentation search for the WriterOptions type in the Hakyll docs. It turned up a few functions, specifically the pageCompilerWith function, which is exactly what I needed. I just modified my page compiler:

match "posts/*" $do route$ setExtension ".html"
-       compile $pageCompiler + compile$ pageCompilerWith defaultHakyllParserState pandocOptions
>>> applyTemplateCompiler "templates/post.html"
>>> applyTemplateCompiler "templates/default.html"
>>> relativizeUrlsCompiler

For the second, I didn’t even need to consult the docs (but only because I had already glanced through them). Since it takes a Page to a Page, it’s the precursor to an arrow applied to a compiled page. I needed to make it an Arrow, then stick it in the compilation chain.

match "posts/*" $do route$ setExtension ".html"
compile \$ pageCompilerWith defaultHakyllParserState pandocOptions
+           >>> arr (checkMathOption)
>>> applyTemplateCompiler "templates/post.html"
>>> applyTemplateCompiler "templates/default.html"
>>> relativizeUrlsCompiler

How awesome was that? Because of the strong hints given by the type signatures, I didn’t even have to think at all!

Now of course, Haskell type signatures are great for lots of reasons, not just because it makes copying random code off the internet easier (in fact, lots of people might say that’s a bad thing), but I got a kick out of how this made me feel like an expert even though I’m still just learning.