Why does let binding sometimes seem to fail to override a value?

Question

In this example, the message isn't shown, but it's still logged (message-log-max is ignored).

(let ((inhibit-message t)
      (message-log-max nil))
  (hs-minor-mode))

The text Showing all blocks ... done is shown in the *Messages* buffer.

---

However, this overrides message-log-max as expected.

(let ((inhibit-message t)
      (message-log-max-orig message-log-max))
  (setq message-log-max nil)
  (hs-minor-mode)
  (setq message-log-max message-log-max-orig))

Why isn't the first example working? I've found let-binding message-log-max works in other contexts.

Answer 1

(You can just replace your (hs-minor-mode) to show the same effect, BTW.)

You're getting fooled by the echoing of the result of evaluation provided by a command such as C-x C-e or M-:.

Calls to message etc. within the let are NOT logged. But if you use something like C-x C-e to evaluate your code then that command echoes the result of evaluating, and that echoing happens after the let.

In your second example, what gets echoed by C-x C-e is the result of evaluating the last sexp in the let, which is the second setq, which produces a numeric result (probably 1000). That is, for the second example, you see 1000 logged, but not your message -- because the call to message (or hs-minor-mode) is not the last thing evaluated, so the message isn't returned as a result and isn't echoed by C-x C-e.