Initial stab at a flow-type layer

layers/+lang/flow-type/README.org

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+#+TITLE: flow-type layer
+
+* Description
+This layer adds some [[https://flowtype.org/][flow]] related functionality to Emacs:
+ - Show the flow-type of the object under the cursor using Eldoc
+ - Add flow checking to flycheck (based on the flycheck-flow package)
+ - company-mode completion using company-flow
+
+* Install
+To use this configuration layer, add it to your =~/.spacemacs=. You will need to
+add =flow-type= to the existing =dotspacemacs-configuration-layers= list in this
+file.
+

layers/+lang/flow-type/funcs.el

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+(defun flow-type/call-process-on-buffer-to-string (command)
+  (with-output-to-string
+    (call-process-region (point-min) (point-max) shell-file-name nil standard-output nil shell-command-switch command)))
+
+(defun flow-type/type-description (info)
+  (let ((type (alist-get 'type info)))
+    (if (string-equal type "(unknown)")
+        (let ((reasons (alist-get 'reasons info)))
+          (if (> (length reasons) 0) (alist-get 'desc (aref reasons 0))))
+      type)))
+
+(defun flow-type/type-at-cursor ()
+  (let ((output (flow-type/call-process-on-buffer-to-string
+                 (format "flow type-at-pos --retry-if-init=false --json %d %d" (line-number-at-pos) (+ (current-column) 1)))))
+    (unless (string-match "\w*flow is still initializing" output)
+      (flow-type/type-description (json-read-from-string output)))))
+
+(defun flow-type/init-mode ()
+  (set (make-local-variable 'eldoc-documentation-function) 'flow-type/type-at-cursor))

layers/+lang/flow-type/packages.el

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+;;; packages.el --- flow-type layer packages file for Spacemacs.
+;;
+;; Copyright (c) 2012-2016 Sylvain Benner & Contributors
+;;
+;; Author: Jonathan del Strother <jdelStrother@gmail.com>
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+
+(defconst flow-type-packages
+  '(company
+    (company-flow :toggle (configuration-layer/package-usedp 'company))
+    (flycheck-flow :toggle (configuration-layer/package-usedp 'flycheck))
+    js2-mode
+    web-mode))
+
+(defun flow-type/post-init-js2-mode()
+  (push 'flow-type/init-mode js2-mode-hook))
+
+(defun flow-type/post-init-web-mode()
+  (push 'flow-type/init-mode react-mode-hook))
+
+(defun flow-type/post-init-company()
+  (spacemacs|add-company-hook js2-mode)
+  (when (configuration-layer/layer-usedp 'react)
+    (spacemacs|add-company-hook react-mode)))
+
+(defun flow-type/init-company-flow ()
+  (use-package company-flow
+    :defer t
+    :init
+    (progn
+       (push 'company-flow company-backends-js2-mode)
+       (when (configuration-layer/package-usedp 'web-mode)
+         (push 'company-flow company-backends-react-mode))
+    )
+    :config
+    (when (configuration-layer/package-usedp 'web-mode)
+      (push 'react-mode company-flow-modes)))
+  )
+
+(defun flow-type/init-flycheck-flow()
+  (with-eval-after-load 'flycheck
+    (use-package flycheck-flow
+      :config
+      (progn
+        ;; Don't run flow if there's no @flow pragma
+        (custom-set-variables '(flycheck-javascript-flow-args (quote ("--respect-pragma"))))
+        ;; Run flow in react-mode files
+        (flycheck-add-mode 'javascript-flow 'react-mode)
+        ;; After running js-flow, run js-eslint
+        (flycheck-add-next-checker 'javascript-flow 'javascript-eslint)
+      ))))