Haskell layer: add support for lsp backend

CHANGELOG.develop

@@ -1357,6 +1357,7 @@ Other:
     (thanks to Magnus Therning)
 - Fixed jump handlers conflict for haskell and intero (thanks to CthulhuDen)
 - Fix haskell jump handler for dante (thanks to cjay)
+- Enable LSP mode using HIE.
 **** Helm
 - Limited =ripgrep= results to 512 columns by default, and added
   =spacemacs-helm-rg-max-column-number= layer variable to configure the above

layers/+lang/haskell/README.org

@@ -14,6 +14,7 @@
     - [[#intero][=intero=]]
     - [[#dante][=dante=]]
     - [[#ghc-mod][=ghc-mod=]]
+    - [[#lsp][=lsp=]]
   - [[#optional-extras][Optional extras]]
     - [[#structured-haskell-mode][structured-haskell-mode]]
     - [[#hindent][hindent]]
@@ -77,6 +78,7 @@ This layer requires some [[https://www.haskell.org/cabal/][cabal]] packages:
 - =hoogle= (optional for =haskell-mode= and =helm-hoogle=)
 - =ghc-mod= (optional for completion)
 - =intero= (optional for completion)
+- =hie= (optional for completion)
 
 To install them, use the following command (or the =stack= equivalent):
 
@@ -100,8 +102,8 @@ For information about setting up =$PATH=, check out the corresponding section in
 the FAQ (~SPC h SPC $PATH RET~).
 
 ** Completion support
-This layer provides several completion backends - =intero=, =dante=, =ghci= and
-=ghc-mod=. =ghci= (=company-ghci=) is used by default, because it doesn’t
+This layer provides several completion backends - =intero=, =dante=, =ghci=,
+=ghc-mod=, and =lsp=. =ghci= (=company-ghci=) is used by default, because it doesn’t
 require any dependencies, and it works with both =stack= and pure =cabal=
 projects. The completion backend can be changed manually, by setting the value
 of the =haskell-completion-backend= variable. Note that in order to enable
@@ -152,6 +154,14 @@ Also note that =ghc-mod= only works with the =GHC= version that was used to
 build =ghc-mod=. You can check which version was used by calling
 =ghc-mod --version=.
 
+*** =lsp=
+=lsp= requires an appropriate installation of =hie= to provide the Haskell language server. 
+=hie= is built on =ghc-mod=, so many of the same considerations apply.
+
+Enabling the =lsp= backend requires the =lsp= layer to be enabled, and provides access to
+all the additional =lsp-mode= keybindings. As such it is more of a full backend than just
+a completion backend.
+
 ** Optional extras
 The Haskell layer supports some extra features, which can be enabled through the
 layer variables.

layers/+lang/haskell/config.el

@@ -17,7 +17,7 @@
 
 (defvar haskell-completion-backend 'ghci
   "Completion backend used by company.
-Available options are `ghci', `intero', `dante', and `ghc-mod'. Default is
+Available options are `ghci', `intero', `dante', `lsp' and `ghc-mod'. Default is
 `ghci'.")
 
 (defvar haskell-enable-hindent nil

layers/+lang/haskell/funcs.el

@@ -28,6 +28,7 @@
   "Conditionally setup haskell backend."
   (pcase haskell-completion-backend
     (`ghci (spacemacs-haskell//setup-ghci))
+    (`lsp (spacemacs-haskell//setup-lsp))
     (`intero (spacemacs-haskell//setup-intero))
     (`dante (spacemacs-haskell//setup-dante))
     (`ghc-mod (spacemacs-haskell//setup-ghc-mod))))
@@ -36,6 +37,7 @@
   "Conditionally setup haskell backend."
   (pcase haskell-completion-backend
     (`ghci (spacemacs-haskell//setup-ghci-company))
+    (`lsp (spacemacs-haskell//setup-lsp-company))
     (`intero (spacemacs-haskell//setup-intero-company))
     (`dante (spacemacs-haskell//setup-dante-company))
     (`ghc-mod (spacemacs-haskell//setup-ghc-mod-company))))
@@ -50,6 +52,31 @@
     :backends (company-ghci company-dabbrev-code company-yasnippet)
     :modes haskell-mode))
 
+;; LSP functions
+
+(defun spacemacs-haskell//setup-lsp ()
+  "Setup lsp backend"
+  (if (configuration-layer/layer-used-p 'lsp)
+      (progn
+        ;; The functionality we require from this is not an autoload, but rather some
+        ;; top-level code that registers a LSP server type. So we need to load it
+        ;; directly and can't rely on it being autoloaded.
+        (require 'lsp-haskell)
+        (lsp))
+    (message "`lsp' layer is not installed, please add `lsp' layer to your dotfile.")))
+
+(defun spacemacs-haskell//setup-lsp-company ()
+  "Setup lsp auto-completion"
+  (if (configuration-layer/layer-used-p 'lsp)
+      (progn
+        (spacemacs|add-company-backends
+          :backends company-lsp
+          :modes haskell-mode
+          :append-hooks nil
+          :call-hooks t)
+        (company-mode))
+    (message "`lsp' layer is not installed, please add `lsp' layer to your dotfile.")))
+
 ;; ghc-mod functions
 
 (defun spacemacs-haskell//setup-ghc-mod ()

layers/+lang/haskell/packages.el

@@ -28,6 +28,8 @@
         ;; dante completion backend
         (dante :requires company)
 
+        lsp-haskell
+
         flycheck
         (flycheck-haskell :requires flycheck)
         ggtags
@@ -40,6 +42,10 @@
         hlint-refactor
         ))
 
+(defun haskell/init-lsp-haskell()
+  (use-package lsp-haskell
+    :defer t))
+
 (defun haskell/init-cmm-mode ()
   (use-package cmm-mode
     :defer t))