Code action to add existing, unused pragma

[alanz]

I have a file with pragmas at the top

{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE ExplicitNamespaces #-}

DeriveAnyClass is unused, and hlint reports this. But I get a code action to add it.

Your environment

Output of haskell-language-server --probe-tools or haskell-language-server-wrapper --probe-tools:

haskell-language-server version: 0.9.0.0 (GHC: 8.8.4) (PATH: /Users/alanzimm/.cabal/store/ghc-8.8.4/hskll-lngg-srvr-0.9.0.0-a8c1156a/bin/haskell-language-server)
Tool versions found on the $PATH
cabal:		3.2.0.0
stack:		2.5.1
ghc:		8.8.4

Which lsp-client do you use: emacs lsp-mode

HLS at a065cd6

[berberman]

This is a known bug: #1165 (comment)

[Anton-Latukha]

If blocked upstream - what is the upstream report link?

[jneira]

the linked #1165 is from this repo, maybe it was moved from ghcide?

[Anton-Latukha]

Link probably should be: https://gitlab.haskell.org/ghc/ghc/-/issues/18516, but I need to ask people in the thread to be sure.

[jneira]

Oh yeah, structured errors in ghc will help a lot to fix this (and other issues for sure), as described in the linked comment in the merged pr.
We have an issue in the repo from the actual implementors of that ghc feature: #2014

---

The relevant comment in the pr is:

• Ideally, we'd have errors-as-values proposal ghc-proposals/ghc-proposals#306 so that we'd only have to search for pragma names in the right field(s?) of the structured GHC error, instead of having to search the entire error message. Even better, the structured GHC error could contain a field with suggested pragmas 🙂.

• In the meantime, there is another alternative: instead of matching just the pragma names, which might trigger false positives, we could use some regexps like below (copied from my Emacs config):

 (or (and " -X\\([A-Z][A-Za-z]+\\)"
          (not "\\([A-Z][A-Za-z]+\\) is deprecated"))
     "[Uu]se \\([A-Z][A-Za-z]+\\)"
     "Using \\([A-Z][A-Za-z]+\\) might"
     "You need \\([A-Z][A-Za-z]+\\)"
     "Try \\(?:enabling \\)?\\([A-Z][A-Za-z]+\\)"
     "intended to use \\([A-Z][A-Za-z]+\\)"
     "Enable \\(?:the \\)?\\([A-Z][A-Za-z]+\\)"
     "need \\([A-Z][A-Za-z]+\\) turned on"
     "extension '\\([A-Z][A-Za-z]+\\)'")

(I'm sure the regexp can be optimised, it's just something that grew organically.) However, this is rather ad-hoc and will get out of date when an error message is added/changed in GHC.