Provide options to select hie, hls or ghcide as the exe

Author: alanz

lsp-haskell.el

@@ -43,7 +43,10 @@
   "The path for starting the haskell-ide-engine
 server. hie-wrapper exists on HIE master from 2018-06-10"
   :group 'lsp-haskell
-  :type '(choice string))
+  :type '(choice (const "hie-wrapper")
+                 (const "haskell-language-server-wrapper")
+                 (const "ghcide")
+                 string))
 
 ;;;###autoload
 (defcustom lsp-haskell-process-args-hie