fix: Apply review suggestions by @monnier

CHANGES.md

@@ -17,6 +17,8 @@ unreleased
       behavior can be enabled permanently by customizing
       `merlin-construct-with-local-values` (#1644)
     - emacs: add support for opam-switch-mode (#1654, fixes #1591)
+      To fully benefit from `M-x opam-switch-mode`, the Merlin command
+      must be a filename, e.g., `(setq merlin-command "ocamlmerlin")`.
 
 merlin 4.9
 ==========

emacs/merlin.el

@@ -612,41 +612,32 @@ argument (lookup appropriate binary, setup logging, pass global settings)"
         (kill-local-variable 'merlin-erroneous-buffer)))))
 
 (defcustom merlin-stop-server-on-opam-switch t
-  "If t, stop ocamlmerlin server before the opam switch changes.
-If `(opam-switch-mode)' is loaded and the user changes the opam
-switch using `opam-switch-set-switch' or an `\"OPSW\"' menu, this
-option asks to stop the merlin server process, so that the next
-merlin command starts a new server, typically with a different
-merlin version from a different opam switch.
-
-Based on the minor mode `opam-switch-mode', see:
-https://github.com/ProofGeneral/opam-switch-mode
-
-Note: `(opam-switch-mode)' expects `merlin-command' to be `\"ocamlmerlin\"'."
-  :group 'merlin :type 'boolean)
-
-(defun merlin-stop-server-on-opam-switch ()
-  "Stop ocamlmerlin before the opam switch changes.
+  "If t, stop the Merlin server before the opam switch changes.
+If the user changes the opam switch using `opam-switch-set-switch'
+or an `\"OPSW\"' menu from `opam-switch-mode', this option asks to
+stop the Merlin server process, so that the next Merlin command
+starts a new server, typically with a different Merlin version
+from a different opam switch.
+
+See https://github.com/ProofGeneral/opam-switch-mode
+
+Note: `opam-switch-mode' triggers automatic changes of `exec-path',
+so to fully benefit from this feature, you should set `merlin-command'
+to a basename, typically: `(setq merlin-command \"ocamlmerlin\")'."
+  :type 'boolean)
+
+(defun merlin--stop-server-on-opam-switch ()
+  "Stop the Merlin server before the opam switch changes.
 This function is for the `opam-switch-mode' hook
 `opam-switch-before-change-opam-switch-hook', which runs just
-before the user changes the opam switch through
-`opam-switch-mode'.
-
-If `merlin-stop-server-on-opam-switch' is t, then the merlin
-server process is stopped, so that the next merlin command starts
-a new server, typically with a different merlin version from a
-different opam switch.
-
-Based on the minor mode `opam-switch-mode', see:
-https://github.com/ProofGeneral/opam-switch-mode
-
-Note: `(opam-switch-mode)' expects `merlin-command' to be `\"ocamlmerlin\"'."
+before the user changes the opam switch through `opam-switch-mode'."
   (when (and merlin-mode merlin-stop-server-on-opam-switch)
-    (condition-case sig
+    (condition-case _sig
         (merlin-stop-server)
       (t (message "Info: (merlin-stop-server) failed in the previous opam switch")))))
 
-(add-hook 'opam-switch-before-change-opam-switch-hook #'merlin-stop-server-on-opam-switch t)
+(add-hook 'opam-switch-before-change-opam-switch-hook
+          #'merlin--stop-server-on-opam-switch t)
 
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