Configure CoC. It doesn't work as well as LSP by itself, though.

nvim/.config/nvim/coc-settings.json

@@ -0,0 +1,19 @@
+{
+    "languageserver": {
+        "clangd": {
+            "command": "clangd",
+            "filetypes": ["c", "cpp"],
+            "rootPatterns": ["compile_flags.txt", "compile_commands.json" ]
+        },
+        "haskell": {
+            "command": "hie-wrapper",
+            "filetypes": ["haskell"],
+            "rootPatterns": [".stack.yaml", "cabal.config", "package.yaml"]
+	},
+        "scry": {
+            "command": "scry",
+            "filetypes": ["crystal"],
+            "rootPatterns": ["shard.yml"]
+        }
+    }
+}