Language Pragma causes hanging in haskell-lexeme-looking-at-token

[michzappa]

Adding a language pragma, for example:

{-# LANGUAGE ExistentialQuantification #-}

to my .hs files results in frequent pausing whenever emacs re-draws the buffer (after using a formatting package, for instance). Without the language pragma there is no delay for this action. I can C-g out of these delays usually, but sometimes it causes Emacs to crash. There are previous issues with the same context but different causes like #820.

This is the profiler report for the problematic action, :

       15521  86% - redisplay_internal (C function)
       15514  86%  - jit-lock-function
       15511  86%   - jit-lock-fontify-now
       15511  86%    - jit-lock--run-functions
       15511  86%     - run-hook-wrapped
       15511  86%      - #<compiled 0x19a2965dcd3c8bfd>
       15511  86%       - font-lock-fontify-region
       15511  86%        - font-lock-default-fontify-region
       15425  85%         - font-lock-fontify-syntactically-region
       15421  85%          - syntax-propertize
       14843  82%           - haskell-syntax-propertize
        9339  51%            - haskell-lexeme-looking-at-token
           4   0%               haskell-lexeme-looking-at-qidsym
           2   0%             + haskell-lexeme-looking-at-quasi-quote-literal
          82   0%         + font-lock-fontify-keywords-region
           2   0%  + eval
           2   0%    file-remote-p
        2102  11% + ...
         309   1% + command-execute
          26   0% + timer-event-handler
           4   0% + jit-lock-function

This is happening to me on emacs 27.2 and 28.1 (native-compiled), with haskell-mode pulled with straight.el at commit 4ec2aa3 (current HEAD). On NixOS, if it matters. My haskell-mode config is the following, but the additional options make no difference in this scenario.

(straight-use-package 'haskell-mode)
(add-hook 'haskell-mode-hook 'interactive-haskell-mode)
(add-hook 'haskell-mode-hook 'haskell-doc-mode)
(add-hook 'haskell-mode-hook
          #'(lambda () (setq-local format-all-formatters
                              '(("Haskell" stylish-haskell)))))

I am curious about this since it's such a fundamental feature of Haskell/haskell-mode and I haven't found any other issues.

Thank you!

[purcell]

Thanks for digging into this and providing the helpful specific information! Currently I think I'm the only "maintainer" (semi-)actively responding to issues but I am generally quite inactive atm and I will likely struggle to find time to tackle this.

[michzappa]

I just played around with a minimal version of my config and quickly found the offending customization: (add-hook 'prog-mode-hook #'(lambda () (modify-syntax-entry ?- "w"))). It probably breaks the assumptions made by this haskell-mode looking-at function for the syntax table and causes the looping.

Doesn't look like a problem with haskell-mode, and also doesn't seem to merit any documentation changes since I doubt this is a common configuration. I'll probably just add a workaround in my own config to remove this syntax change in haskell-mode.

Thanks!

[purcell]

Ah cool, thanks for following up with this info. It rings a bell w.r.t. past reported issues, I believe...

[flocks]

Hello @michzappa I'm running into what looks like the same issue, and I'm not sure if you finally found a way to work around it?

[flocks]

Ok, got it I guess you had

 (add-hook 'after-change-major-mode-hook (lambda () (modify-syntax-entry ?- "w")))

somewhere in your config (like I have). I confirm removing this line fix the behavior