Typechecking code in a Hakyll blog

After years of programming in Haskell, my natural ability to scrutinize my code for type errors (and even more mundane things, like parse errors and spelling errors) has atrophied. Usually my friends ghc and ghcid take care of this for me so I can think about other things.

Recovering this workflow when writing blog posts took a little time.

Literate Haskell

Hakyll handles Literate Haskell files already - you don’t have to do any work. Just create a file in posts/ with the .lhs file extension.

The format is described well in the Haskell Wiki. The only features I’ve used so far are

Code blocks

Most of my code blocks are long, so I prefer the latex-like command over bird-tracks

\begin{code}
example = do
  x <- getTheX
  useX x
\end{code}

Hidden imports

Of course we need to import some modules if ghc is to typecheck our .lhs file. Since the imports are often not important to the explanation in the blog, I hide them.

Imaginary functions that I will later use in the blog post but which aren’t important to the topic at hand can be defined here, too.

\long\def\ignore#1{}

\ignore{
\begin{code}
{-# LANGUAGE ScopedTypeVariables, OverloadedStrings, TypeApplications #-}
import qualified Servant.Server as Servant
import Servant.API ((:<|>), (:>))
import qualified Database.PostgreSQL.Simple as PG

processInputs :: Inputs -> IO Outputs
processInputs = undefined
\end{code}

nix-shell

A separate nix-shell is available for every post, in shells.nix. At the top of this file is a record that maps post names to the names of Haskell packages that I need to have in scope. There’s only one entry at the time of writing, because I only have one Literate Haskell blog post:

let
  shellPkgs =
    { config-phases = ["generic-lens" "lens" "postgresql-simple" "katip"]; };

Later in the file, these lists are turned into derivations, so that I can say

nix-shell ./shells.nix -A config-phases \
  --run 'ghcid --command "ghci posts/2019-10-17-config-phase.lhs"'

and now ghcid is recompiling my post and showing me the type errors every time the post is saved.