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emacselispstring-interning

(read-command ...) returns strange object upon blank input

I need to prompt the user for a command, using read-command, then take an action based on whether or not the user entered the empty string on the minibuffer.

Most of the time, read-command returns a valid command. But when the user enters a blank string (by just hitting enter in the minibuffer), a strange object is returned, which is neither nil, nor a string, nor a command (according to stringp and commandp).

ELISP> (setq strange-object (read-command "Enter some command" nil) )
ELISP> (equal nil strange-object)
nil
ELISP> (if strange-object "YES" "NO")
"YES"
ELISP> (mapcar (lambda (fun) (funcall fun strange-object)) (list 'stringp 'commandp 'char-or-string-p 'functionp 'integerp 'listp) )
(nil nil nil nil nil nil)
ELISP> (prin1-to-string strange-object)
""
ELISP> (equal "" strange-object)
nil

How can I:

  1. Access the read-syntax of this object?
  2. Determine the type of the object without matching against every type predicate defined here
  3. Test for equality against this object?
  4. Why is read-command misbehaving? Specification of read-command:

read-command is a built-in function in `minibuf.c'.

(read-command PROMPT &optional DEFAULT-VALUE)

Read the name of a command and return as a symbol. Prompt with PROMPT. By default, return DEFAULT-VALUE or its first element if it is a list.

[back]

I expected nil, since that is what I passed as DEFAULT-VALUE, but apparently read-command doesn't care.

I've tried loading the source of minibuf.c to see what is going on, but I haven't been able to get that to work either. I've downloaded emacs-23.3b.tar.gz from here and had emacs look for the source there, but it wasn't able to find it. Neither does a minibuf.c seem to exist there. Very frustrating stuff, I will appreciate any pointers.

Solution

  • The Elisp manual is your friend. C-h i, choose Elisp. Then i and enter read-command. As part of the description you see this:

     The argument DEFAULT specifies what to return if the user enters
 null input.  It can be a symbol, a string or a list of strings.
 If it is a string, `read-command' interns it before returning it.
 If it is a list, `read-command' interns the first element of this
 list.  If DEFAULT is `nil', that means no default has been
 specified; then if the user enters null input, the return value is
 `(intern "")', that is, a symbol whose name is an empty string.

    Read the full description. But this alone should help you understand. The value returned is a symbol that has an empty name.