How to report that an interactive command couldn't run?

Question

When writing my own interactive function, whats the best way to report that nothing happened.

(if line-is-empty
    (message "Found empty line, nothing to do!")
  (do-stuff))

Or is it better to use an error (even though it's not exactly an error, just that the command can't do anything useful).

(when line-is-empty
  (error "Found empty line, nothing to do!"))
(do-stuff)

Answer 1

The only thing to be considered here is the semantics.

One should use a programming language as good as possible in a sense of a natural language. I.e., the code you write should as directly express what you intent to do as possible. Only in very time-critical sections of the program one may strife from that path under the condition that one documents these deviations (including the cause of their necessity).

1. If the thing you have is an error caused by miss-use of the command use user-error.
2. If it is an abnormal state in the program use error or cl-assert.
3. If it is just worth a warning use warn.
4. If you want to notify the user about some trivial outcome of the command which does not show any visible effect otherwise use message.

All other aspects of the special forms and functions can be tailored to your needs with, e.g.,

• (signal...)
• (catch ... (throw...)...)
• (cl-block...(throw...)...(cl-return)...)
• (condition-case...)
• (unwind-protect...)
• and so on...

Answer 2

It depends on your usecase.

---

error signals an error, this means it rewinds the call stack until the previous condition-case which catches the error.

Consider following code:

(defun foo ()
  (interactive)
  (blah)
  (message "end1"))

(defun blah ()
  (error "Moep"))

"end1" will never be printed.

---

while message just logs the message and your code keeps running.

---

Also you should consider to message only if things really go wrong. This helps to reduce the noise at users interface.

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Remark on error usage from documentation:

In Emacs, the convention is that error messages start with a capital letter but do not end with a period. Please follow this convention for the sake of consistency.