company-lsp

Company completion backend for lsp-mode.

It provides features that are not available by using company-capf + lsp-mode:

• Support trigger characters. For example typing . will trigger completion for TypeScript.
• Use completion item's label as completion labels and replace it with its insertText if available.
• Fetch completion candidates asynchronously (Thanks @sebastiencs).
• Apply text edits specified by completion candidates after completion.
• Do not cache the completion candidates if they are incomplete.
• Expand snippets on completion (requires yasnippet).

Usage

Company-lsp is available on MELPA. To install it, first setup MELPA, then M-x package-install <RET> company-lsp.

After installing company-lsp, simply add company-lsp to company-backends:

(require 'company-lsp)
(push 'company-lsp company-backends)

Customization

• company-lsp-cache-candidates: Can be set to 'auto, t, or nil.

  When set to 'auto, company-lsp caches the completion. It sends incremental completion requests to the server if and only if the cached results are incomplete. The candidate list may not be sorted or filtered as the server would for cached completion results.

  When set to t, company-mode caches the completion. It won't send incremental completion requests to the server.

  When set to nil, results are not cached at all. The candidates are always sorted and filtered by the server. Use this option if the server handles caching for incremental completion or sorting/matching provided by the server is critical.

• company-lsp-async: When set to non-nil, fetch completion candidates asynchronously.

• company-lsp-enable-snippet: Set it to non-nil if you want to enable snippet expansion on completion. Set it to nil to disable this feature.

• company-lsp-enable-recompletion: If set to non-nil, when company-lsp finishes completion, it checks if the current point is before any completion trigger characters. If yes, it re-triggers another completion request.

  This is useful in cases such as std is completed as std:: in C++."

Defining completion snippet for a certain language

If the language server for that language doesn't support returning snippets, you can customize the variable company-lsp--snippet-functions do define snippets for candidates. company-lsp--snippet-functions is an alist of (LANGUAGE-ID . SNIPPET-FUNCTION).

LANGUAGE-ID is the language ID defined by the lsp client. Currently there is no good way knowing it other than guessing or reading the code of the lsp client for the language. For example if you use lsp-rust, it's defined as following in lsp-rust.el:

(lsp-define-stdio-client lsp-rust "rust" #'lsp-rust--get-root nil
			 :command-fn #'lsp-rust--rls-command
			 :initialize #'lsp-rust--initialize-client)

The language ID is the second parameter, "rust".

SNIPPET-FUNCTION is a function that transforms a hash table representing the CompletionItem message to a snippet string or nil. Below is an example on how to extact a snippet for Rust function parameters:

(defun company-lsp--rust-completion-snippet (item)
  "Function providing snippet with the rust language.
It parses the function's signature in ITEM (a CompletionItem)
to expand its arguments."
  (-when-let* ((kind (gethash "kind" item))
               (is-function (= kind 3)))
    (let* ((detail (gethash "detail" item))
           (snippet (when (and detail (s-matches? "^\\(pub \\)?\\(unsafe \\)?fn " detail))
                      (-some--> (substring detail (1+ (s-index-of "(" detail)) (s-index-of ")" detail))
                                (replace-regexp-in-string "^[^,]*self\\(, \\)?" "" it)
                                (s-split ", " it)
                                (mapconcat (lambda (x) (format "${%s}" x)) it ", ")))))
      (concat "(" (or snippet "$1") ")$0"))))