Language server returns empty array when invoking `textDocument/completion`

[patrickt]

Hi there 👋

I’m using Emacs 27 and lsp-haskell for Haskell dev. Though many of haskell-language-server’s features work great, including in-buffer syntax checking and code actions, it runs into problems when attempting to invoke company-complete. As soon as I do so, Emacs goes unresponsive (despite these requests allegedly running asynchronously), only to return no results, displaying lsp: [] in the buffer. Examining lsp-log shows that textDocument/completion requests are returning no information:

[Trace - 01:35:22 PM] Received response 'textDocument/completion - (89)' in 678ms.
Result: null

In contrast, using hie-wrapper instead of haskell-language-server-wrapper returns all relevant completions.

I’m using GHC 8.8.3, and haskell-language-server off of the master branch.

I had a tough time deciding where to file this bug - haskell-lsp, company-lsp, or haskell-language-server. I settled on this repository because haskell-ide-engine handles this case correctly.

Here is the hie.yaml I’m using in this repository:

cradle:
  cabal:
    - path: "./src"
      component: "lib:tactics"
    - path: "./test"
      component: "tactics:test:tactics-test”

And here is the relevant section of my Emacs configuration, in case I’m doing something obviously wrong here:

(use-package lsp-mode
  :commands (lsp lsp-execute-code-action)
  :hook ((go-mode . lsp-deferred)
	 (lsp-mode . lsp-enable-which-key-integration)
	 (lsp-mode . lsp-diagnostics-modeline-mode))
  :bind ("C-c C-c" . #'lsp-execute-code-action)
  :custom
  (lsp-print-performance t)
  (lsp-log-io t)
  (lsp-diagnostics-modeline-scope :project))

(use-package lsp-ui
  :custom (lsp-ui-doc-delay 0.75)
  :after lsp-mode)

(use-package lsp-ivy
  :after (ivy lsp-mode))

(use-package company-lsp
  :custom (company-lsp-enable-snippet t)
  :after (company lsp-mode))

(use-package haskell-mode
  :bind (("C-c a c" . haskell-cabal-visit-file)))

(use-package lsp-haskell
  :hook (haskell-mode . lsp)
  :custom (lsp-haskell-process-path-hie  "hie-wrapper")
  )

Please let me know if there’s any more information I can provide, and thank you for your work on this project: it’s impossible to overstate the impact that powerful, reliable editor integration will have on all who learn and use Haskell.

[alanz]

FYI, haskell-language-server is still very early stage, and performance of the various operations is one of the areas that needs attention, and is being worked on.