Opening a specific literate haskell file causes emacs process to hang

emacs: GNU Emacs 27.2 - [Arch linux official package](https://archlinux.org/packages/extra/x86_64/emacs/)
haskell-mode: haskell-mode-20210429.117 - installed from melpa and updated today.

____

Following along Graham Hutton's emacs videos I tried to open up [his literate sudoku file](http://www.cs.nott.ac.uk/~pszgmh/sudoku.lhs) and emacs immediately becomes unresponsive and hangs - no screen refresh, no response to keyboard - even mashing ESC or `C-g` has no effect.

Looking at the active processes emacs is maxed out at 25% on my quad core machine. Seems to be stuck in an infinite loop

I've pared down my config to the simplest case but I still get the same result.

````
(use-package haskell-mode
:commands haskell-mode)
````

It's OK with a minimal literate file :

```
Some text at the start of the file.

> module Test where
> f v = 2 * v
```

I haven;t tried to find any other more complex literate files but I DO remember looking at a literate version of [the original by Richard Bird](https://www.cs.tufts.edu/~nr/cs257/archive/richard-bird/sudoku.pdf) some months ago and `haskell-mode` was handling that as expected.

Sorry I don't have firm dates or version numbers for when it stopped working.