Add Flow Language Server support (#603)

* Add support for JavaScript Flow LSP

This language server is considered an "add-on", that is, it will be
enabled simultaneously with another JavaScript/TypeScript server if
both are present and the buffer meets the conditions of a Flow source
file. The server used is the official one from https://flow.org, which
is supposedly the one that will be better supported now and in the
future.

In order to fine grain the decision of when to enable this server, a
new `activation-fn` has been added to the `lsp-client` structure. If
present, this function overrides `major-modes` and `remote?`, in case
that any of them are also present when registering a new language
client.

In particular, the new Flow client considers that the file is a Flow
file if the following conditions are met:

- The major mode is one of js-mode, js2-mode, or flow-js2-mode.
- The file resides in a Flow project (there is a .flowconfig file).
- The file has a `// @flow` or `/* @flow */` tag.

* Update README

* Remove redundant lambda

Author: danielmartin

README.org

@@ -106,8 +106,9 @@
    | HTML                  | [[https://github.com/vscode-langservers/vscode-html-languageserver][html]]                                      | Yes           | npm install -g vscode-html-languageserver-bin |                              |
    | Haskell               | [[https://github.com/haskell/haskell-ide-engine][IDE engine]]                                | [[https://github.com/emacs-lsp/lsp-haskell][lsp-haskell]]   | [[https://github.com/haskell/haskell-ide-engine][IDE engine]]                                    |                              |
    | Java                  | [[https://github.com/eclipse/eclipse.jdt.ls][Eclipse JDT LS]]                            | [[https://github.com/emacs-lsp/lsp-java][lsp-java]]      | Automatic by [[https://github.com/emacs-lsp/lsp-java][lsp-java]]                         | Yes                          |
-   | Javascript/Typescript | [[https://github.com/theia-ide/typescript-language-server][typescript-language-server]]  (recommended) | Yes           | npm i -g typescript-language-server           |                              |
-   | Javascript/Typescript | [[https://github.com/sourcegraph/javascript-typescript-langserver][javascript-typescript-stdio]]               | Yes           | npm i -g javascript-typescript-langserver     |                              |
+   | JavaScript/TypeScript | [[https://github.com/theia-ide/typescript-language-server][typescript-language-server]]  (recommended) | Yes           | npm i -g typescript-language-server           |                              |
+   | JavaScript/TypeScript | [[https://github.com/sourcegraph/javascript-typescript-langserver][javascript-typescript-stdio]]               | Yes           | npm i -g javascript-typescript-langserver     |                              |
+   | JavaScript Flow       | [[https://flow.org][flow]] (add-on if working on a Flow file)   | Yes           | [[https://flow.org][flow]]                                          |                              |
    | Ocaml                 | [[https://github.com/freebroccolo/ocaml-language-server][ocaml-language-server]]                     | Yes           | [[https://github.com/freebroccolo/ocaml-language-server][ocaml-language-server]]                         |                              |
    | PHP                   | [[https://github.com/felixfbecker/php-language-server][php-language-server]]                       | Yes           | [[https://github.com/felixfbecker/php-language-server][php-language-server]]                           |                              |
    | Python                | [[https://github.com/palantir/python-language-server][pyls]]                                      | Yes           | pip install 'python-language-server[all]'     | Yes                          |

lsp-clients.el

@@ -143,6 +143,68 @@ finding the executable with variable `exec-path'."
                   :server-id 'ts-ls))
 
 
+
+;;; JavaScript Flow
+(defcustom lsp-clients-flow-server "flow"
+  "The Flow executable to use.
+Leave as just the executable name to use the default behavior of
+finding the executable with variable `exec-path'."
+  :group 'lsp-flow
+  :risky t
+  :type 'file)
+
+(defcustom lsp-clients-flow-server-args '("lsp")
+  "Extra arguments for starting the Flow language server."
+  :group 'lsp-flow
+  :risky t
+  :type '(repeat string))
+
+(defun lsp-clients-flow-tag-present-p (file-name)
+  "Checks if the '// @flow' or `/* @flow */' tag is present in
+the contents of FILE-NAME."
+  (with-temp-buffer
+    (insert-file-contents file-name)
+    (save-excursion
+      (goto-char (point-min))
+      (let (stop found)
+        (while (not stop)
+          (when (not (re-search-forward "[^\n[:space:]]" nil t))
+            (setq stop t))
+          (if (equal (point) (point-min))
+              (setq stop t)
+            (backward-char))
+          (cond ((or (looking-at "//+[ ]*@flow")
+                     (looking-at "/\\**[ ]*@flow"))
+                 (setq found t)
+                 (setq stop t))
+                ((looking-at "//")
+                 (forward-line))
+                ((looking-at "/\\*")
+                 (when (not (re-search-forward "*/" nil t))
+                   (setq stop t)))
+                (t (setq stop t))))
+        found))))
+
+(defun lsp-clients-flow-project-p (file-name)
+  "Checks if FILE-NAME is part of a Flow project, that is, if
+there is a .flowconfig file in the folder hierarchy."
+  (locate-dominating-file file-name ".flowconfig"))
+
+(defun lsp-clients-flow-activate-p (file-name major-mode)
+  "Checks if the Flow language server should be enabled for a
+particular FILE-NAME and MAJOR-MODE."
+  (and (member major-mode '(js-mode js2-mode flow-js2-mode))
+       (lsp-clients-flow-project-p file-name)
+       (lsp-clients-flow-tag-present-p file-name)))
+
+(lsp-register-client
+ (make-lsp-client :new-connection (lsp-stdio-connection
+                                   (-const `(,lsp-clients-flow-server
+                                             ,@lsp-clients-flow-server-args)))
+                  :activation-fn 'lsp-clients-flow-activate-p
+                  :add-on? t
+                  :server-id 'flow-ls))
+
 ;;; Vue
 (defcustom lsp-clients-vue-server "vls"
   "The vue-language-server executable to use.

lsp-mode.el

@@ -654,6 +654,12 @@ INHERIT-INPUT-METHOD will be proxied to `completing-read' without changes."
 
   ;; major modes supported by the client.
   (major-modes)
+  ;; Function that will be called to decide if this language client
+  ;; should manage a particular buffer. The function will be passed
+  ;; the file name and major mode to inform the decision. Setting
+  ;; `activation-fn' will override `major-modes' and `remote?', if
+  ;; present.
+  (activation-fn)
   ;; Break the tie when major-mode is supported by multiple clients.
   (priority 0)
   ;; Unique identifier for
@@ -3453,9 +3459,11 @@ remote machine and vice versa."
     (--when-let (->> lsp-clients
                      hash-table-values
                      (-filter (-lambda (client)
-                                (and (-contains? (lsp--client-major-modes client) buffer-major-mode)
-                                     (-some-> client lsp--client-new-connection (plist-get :test?) funcall)
-                                     (eq (---truthy? remote?) (---truthy? (lsp--client-remote? client)))))))
+                                (and (or
+                                      (-some-> client lsp--client-activation-fn (funcall buffer-file-name buffer-major-mode))
+                                      (and (-contains? (lsp--client-major-modes client) buffer-major-mode)
+                                           (eq (---truthy? remote?) (---truthy? (lsp--client-remote? client)))))
+                                     (-some-> client lsp--client-new-connection (plist-get :test?) funcall)))))
       (-let (((add-on-clients main-clients) (-separate 'lsp--client-add-on? it)))
         ;; Pick only one client (with the highest priority) that is not declared as add-on? t.
         (cons (and main-clients (--max-by (> (lsp--client-priority it) (lsp--client-priority other)) main-clients))