lang/haskell: fail gracefully if stack isn't installed

modules/lang/haskell/+intero.el

@@ -2,8 +2,10 @@
 ;;;###if (featurep! +intero)
 
 (def-package! intero
-  :hook (haskell-mode . intero-mode)
+  :after haskell-mode
   :config
+  (add-hook 'haskell-mode-hook #'+haskell|init-intero)
+
   (unless (executable-find "stack")
     (warn! "Couldn't find stack. Intero has been disabled."))
 

modules/lang/haskell/autoload.el

@@ -0,0 +1,10 @@
+;;; lang/haskell/autoload.el -*- lexical-binding: t; -*-
+
+;;;###autoload
+(defun +haskell|init-intero ()
+  "Initializes `intero-mode' in haskell-mode, unless stack isn't installed.
+This is necessary because `intero-mode' doesn't do its own error checks."
+  (when (derived-mode-p 'haskell-mode)
+    (if (executable-find "stack")
+        (intero-mode +1)
+      (message "Couldn't find stack. Refusing to enable intero-mode."))))