[haskell] support dynamically set backend

layers/+lang/haskell/README.org

@@ -113,6 +113,19 @@ completion, you’ll have to enable the =auto-completion= layer as well.
                   (haskell :variables haskell-completion-backend 'intero)))
 #+END_SRC
 
+Backend can be chosen on a per project basis using directory local variables
+(files named =.dir-locals.el= at the root of a project), an example to use the
+=intero= backend:
+
+#+BEGIN_SRC elisp
+  ;;; Directory Local Variables
+  ;;; For more information see (info "(emacs) Directory Variables")
+
+  ((haskell-mode (haskell-completion-backend . intero)))
+#+END_SRC
+
+*Note:* you can easily add a directory local variable with ~SPC f v d~.
+
 *** =company-ghci=
 [[https://github.com/juiko/company-ghci][company-ghci]] communicates directly with =ghci=, in order to provide completion.
 To use it, you have to call =haskell-process-load-or-reload= (=SPC s b=).

layers/+lang/haskell/funcs.el

@@ -22,7 +22,58 @@
     (haskell-navigate-imports)
     (haskell-mode-format-imports)))
 
-;; Dante Functions
+
+;; Completion setup functions
+
+(defun spacemacs-haskell//setup-backend ()
+  "Conditionally setup haskell backend."
+  (pcase haskell-completion-backend
+    (`ghci (spacemacs-haskell//setup-ghci))
+    (`intero (spacemacs-haskell//setup-intero))
+    (`dante (spacemacs-haskell//setup-dante))
+    (`ghc-mod (spacemacs-haskell//setup-ghc-mod))))
+
+(defun spacemacs-haskell//setup-company ()
+  "Conditionally setup haskell backend."
+  (pcase haskell-completion-backend
+    (`ghci (spacemacs-haskell//setup-ghci-company))
+    (`intero (spacemacs-haskell//setup-intero-company))
+    (`dante (spacemacs-haskell//setup-dante-company))
+    (`ghc-mod (spacemacs-haskell//setup-ghc-mod-company))))
+
+
+;; ghci functions
+
+(defun spacemacs-haskell//setup-ghci ()
+  (interactive-haskell-mode))
+
+(defun spacemacs-haskell//setup-ghci-company ()
+  (spacemacs|add-company-backends
+    :backends (company-ghci company-dabbrev-code company-yasnippet)
+    :modes haskell-mode))
+
+
+;; ghc-mod functions
+
+(defun spacemacs-haskell//setup-ghc-mod ()
+  (ghc-init))
+
+(defun spacemacs-haskell//setup-ghc-mod-company ()
+  (spacemacs|add-company-backends
+    :backends (company-ghc company-dabbrev-code company-yasnippet)
+    :modes haskell-mode))
+
+
+;; Dante functions
+
+(defun spacemacs-haskell//setup-dante ()
+  (dante-mode)
+  (add-to-list 'spacemacs-jump-handlers-haskell-mode 'xref-find-definitions))
+
+(defun spacemacs-haskell//setup-dante-company ()
+  (spacemacs|add-company-backends
+    :backends (dante-company company-dabbrev-code company-yasnippet)
+    :modes haskell-mode))
 
 (defun spacemacs-haskell//dante-insert-type ()
   (interactive)
@@ -31,6 +82,16 @@
 
 ;; Intero functions
 
+(defun spacemacs-haskell//setup-intero ()
+  (interactive-haskell-mode)
+  (intero-mode)
+  (add-to-list 'spacemacs-jump-handlers-haskell-mode 'intero-goto-definition))
+
+(defun spacemacs-haskell//setup-intero-company ()
+  (spacemacs|add-company-backends
+    :backends (company-intero company-dabbrev-code company-yasnippet)
+    :modes haskell-mode))
+
 (defun haskell-intero/insert-type ()
   (interactive)
   (intero-type-at :insert))

layers/+lang/haskell/packages.el

@@ -53,7 +53,8 @@
   (use-package cmm-mode
     :defer t))
 
-(defun haskell/post-init-company ())
+(defun haskell/post-init-company ()
+  (add-hook 'haskell-mode-local-vars-hook #'spacemacs-haskell//setup-company))
 
 (defun haskell/init-company-cabal ()
   (use-package company-cabal
@@ -65,12 +66,7 @@
 
 (defun haskell/init-company-ghci ()
   (use-package company-ghci
-    :defer t
-    :init
-    (spacemacs|add-company-backends
-      :backends (company-ghci company-dabbrev-code company-yasnippet)
-      :modes haskell-mode)
-    (add-hook 'haskell-mode-hook 'interactive-haskell-mode)))
+    :defer t))
 
 (defun haskell/init-company-ghc ()
   (use-package company-ghc
@@ -79,11 +75,6 @@
 (defun haskell/init-ghc ()
   (use-package ghc
     :defer t
-    :init
-    (spacemacs|add-company-backends
-      :backends (company-ghc company-dabbrev-code company-yasnippet)
-      :modes haskell-mode)
-    (add-hook 'haskell-mode-hook 'ghc-init)
     :config
     (progn
       (dolist (mode haskell-modes)
@@ -108,13 +99,6 @@
 (defun haskell/init-intero ()
   (use-package intero
     :defer t
-    :init
-    (spacemacs|add-company-backends
-      :backends (company-intero company-dabbrev-code company-yasnippet)
-      :modes haskell-mode)
-    (add-hook 'haskell-mode-hook 'interactive-haskell-mode)
-    (add-hook 'haskell-mode-hook 'intero-mode)
-    (add-to-list 'spacemacs-jump-handlers-haskell-mode 'intero-goto-definition)
     :config
     (progn
       (spacemacs|diminish intero-mode " λ" " \\")
@@ -152,12 +136,6 @@
 (defun haskell/init-dante ()
   (use-package dante
     :defer t
-    :init
-    (spacemacs|add-company-backends
-      :backends (dante-company company-dabbrev-code company-yasnippet)
-      :modes haskell-mode)
-    (add-hook 'haskell-mode-hook 'dante-mode)
-    (add-to-list 'spacemacs-jump-handlers-haskell-mode 'xref-find-definitions)
     :config
     (progn
       (dolist (mode haskell-modes)
@@ -192,6 +170,8 @@
     :defer t
     :init
     (progn
+      (add-hook 'haskell-mode-local-vars-hook #'spacemacs-haskell//setup-backend)
+
       (defun spacemacs//force-haskell-mode-loading ()
         "Force `haskell-mode' loading when visiting cabal file."
         (require 'haskell-mode))