RE: message in minibuffer

["Drew Adams" <drew.adams@oracle.com>]

    Here is a sample piece of code
    (defun foo () (let ((arg "abc")) (message "Here is arg: %s" arg)))

    I evaluate this in the minibuffer
          Eval: (foo)
    and get "Here is arg: abc"
    in the [echo area].  However if I evaluate it with
          M-x foo
    I get the output Here is arg: abc
    It seems that the quotation marks show up depending on how the function
    is invoked.  Why?

Like many "functions" in Lisp, `message' does two things:

1. It displays a message in the minibuffer.

2. It returns a value: the message that it displayed - a string.

#1 is a side effect. #2 is the normal behavior of a true function.

But this is more than a question just of `message'...

When you evaluate `(foo)' using, for example, `M-:' (command
`eval-expression'), you see only the result of the evaluation - that's the
behavior of `eval-expression' (it's own side effect): print the result of
evaluation in the echo area. When the string value of `message' is printed,
it is enclosed in double-quotes, so you can see that the value is a string.

In fact, the process of evaluating `M-: (foo)' first calls (message...),
which prints your message (without quotes), but then the result of `M-:
(foo)' is printed (as a string - showing the quotes), so you never see the
first message. If you check buffer *Messages*, you will see your message,
without the quotes.

When you evaluate `(message...)' using `M-x foo', the result of the
evaluation (of `(foo)') is not echoed in the echo area, because function
`execute-extended-command' (which is bound to `M-x') does not have such a
side effect.

IOW, this is all about 1) keeping straight the difference between side
effect and resulting value, and 2) knowing the behavior of the given
functions (`eval-expression' vs `execute-extended-command').

"Are we having fun yet?", asks Zippy, tossing his "Elementary Haskell"
manual into the dryer and turning the dial to "Delicates". "We'll see if
it's really Purely Functional and 100% Fully Lazy..."

HTH.