I have been playing around with literate Haskell recently while working on some exercises in Types and Programming Languages. The book uses OCaml for the implementations, but I decided to do them in Haskell. I wanted a way to write a blog post in the same format as my other posts, i.e. Markdown and processed by Jekyll. But I also did not want to copy and paste a bunch of code examples from the implementations in order to talk about them.

Hence literate Haskell. I just write the code in a way that both the Jekyll processor can turn in to HTML and which GHC can turn into executable bytes. It wasn't too hard to do this either, although it was not 100% trivial. I wrote two new Jekyll plugins, one for doing a little bit a preprocessing to the .lhs file to turn it into a Markdown file, and another to load up the blog post as a module and execute commands via GHC in a subprocess while the blog is being generated.

A simple function

Just to show some stuff in this post without going into all of the TAPL stuff that I want to get to later, here are some function definitions in Haskell

myfact 0 = 1
myfact n = n * myfact (n-1)

myfact' n = myfacttail n 1
myfacttail 0 n = n
myfacttail n m = myfacttail (n-1) m*n

You should probably know what these are.

Code execution

We can also easily execute code during the blog generation pass and embed the results directly in the page. Without further ado:

λ> :t myfact
myfact :: (Eq a, Num a) => a -> a
λ> myfact 10
3628800
λ> :t myfact'
myfact' :: (Eq a, Num a) => a -> a
λ> myfact' 10
3628800

That is pretty cool in my opinion. The output here is actually probably not that cool, but if you look at the source and then at the output, the fact that all I had to do was run jekyll build in order to get all of this put together is pretty neat. You can see the plugins in the git repo for this blog on the source branch.

Edit: this post no longer true

I changed this blog to no longer use jekyll and as I never really ended up using this feature in future posts, I decided not to port it over. So instead I have just embedded the actual output. I will put a link here to the old code at some point if someone is still interested.