Re: understanding backquote

[Barry Margolin <barmar@alum.mit.edu>]

In article <mailman.4175.1433272668.904.help-gnu-emacs@gnu.org>,
 Olaf Rogalsky <olaf.rogalsky@aol.de> wrote:

> Hi,
> 
> I have problems in understanding the semantics of backquote. Consider
> the following function:
> 
> (defun test () `,(* (+ 1 2) (+ 3 4)))
> 
> Now let's have a look at the defintion of the symbol:
> 
> (symbol-function 'test)
> => (lambda nil (* (+ 1 2) (+ 3 4))) 
> 
> Huhh??? I was thinking, that the above definition is equivalent to the
> following:
> 
> (defun test () 21)
> 
> Ok, this was unexpected, but perhaps the compiler does it "right":
> (disassemble (byte-compile 'test))
> =>  byte code:
>       args: nil
>     0       constant  21
>     1       return    
> 
> Yes, it does. It is only, that I suspect, that this has nothing to do
> with backquotes, but rather with constant folding optimization.
> 
> Can anybody explain this to me?

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.

(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.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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