Re: understanding backquote

[Olaf Rogalsky]

>> I have problems in understanding the semantics of backquote. Consider
>> the following function:
...
> Backquote and comma operate at read time. The comma means "Don't quote 
> this part", so it will be evaluated when the code is run. It doesn't get 
> evaluated at read time, though, because that would mess up if it has 
> variable references, e.g.
Ah, I always thought, that comma expressions are evaluated at read time.

> (defun test (a) `,(+ 1 a))
>
> If you looked at this function, it would have to be
>
> (lambda (a) (+ 1 a))
>
> It can't be folded at read time.
Yes, I thought this will produce an error.

The elisp manual does not make it totally clear, at which time comma
expressions are evaluated:

  Chapter 9.4 Backquote:

  Backquote constructs allow you to quote a list, but selectively
  evaluate elements of that list. In the simplest case, it is identical
  to the special form quote (described in the previous section; see
  Quoting). For example, these two forms yield identical results:

  `(a list of (+ 2 3) elements)
       => (a list of (+ 2 3) elements)

  '(a list of (+ 2 3) elements)
       => (a list of (+ 2 3) elements)

  The special marker ‘,’ inside of the argument to backquote indicates a
  value that isn’t constant. The Emacs Lisp evaluator evaluates the
  argument of ‘,’, and puts the value in the list structure:

Thanks for clarifying thing up for me.

Is there a way to evaluate parts of a defun during read-time or
byte-compile-time?

Olaf