bug#31726: haskell-mode fails to build with (at least) Emacs 26.1

[Jack Hill]

I'm running guix (GNU Guix) 19b662ea7c53a28270a783f10ca033d8c9d2a954, and 
have /gnu/store/s32nba6rxx9x2kibsn6wsm1ixzhv64nl-emacs-26.1 installed. 
When I then try to install haskell-mode with

  guix package -i haskell-mode

the derivation build fails with

  haskell-process.el:160:1:Error: Unused lexical variable 
‘move-point-in-windows’

This is the same error as a failed build on Hydra[0].

I have opened an upstream bug report[1], but have not yet done more 
troubleshooting.

Best,
Jack

[0] https://hydra.gnu.org/build/2766429/nixlog/1
[1] https://github.com/haskell/haskell-mode/issues/1601