Re: eval-when-compile

[PJ Weisberg <pj@irregularexpressions.net>]

On Fri, Jul 27, 2012 at 10:50 AM, Achim Gratz <Stromeko@nexgo.de> wrote:
> Pascal J. Bourguignon writes:
>> C-h f eval-when-compile RET
>>
>>     Like `progn', but evaluates the body at compile time if you're compiling.
>>     Thus, the result of the body appears to the compiler as a quoted constant.
>>     In interpreted code, this is entirely equivalent to `progn'.
>>
>> What more can I add?
>
> I read that.  It doesn't seem to work that way or has some strings
> attached that aren't obvious to me from the documentation.
>
>> The above source will evaluate, when you compile the file:
>>
>>     (defvar unquoted-t "true")
>>     '(defvar quoted-nil "false")
>
> No.  They will evaluate to
>
> unquoted-t
> (defvar quoted-nil "false")
>
> The unquoted variant was only there to check if maybe there's another
> `(quote …)´ snuck in.  I expected the defvar form to compile, but it
> doesn't.

You missed this part:

     Thus, the result of the body appears to the compiler as a quoted constant.

So they really evaluate to:

'unquoted-t
'(defvar quoted-nil "false")

Which of course does nothing, so the compiler optimizes it away.

You know what you could do?

---
(eval
 (eval-when-compile
   (if nil
       '(defvar quoted-t "true")
     '(defvar quoted-nil "false")))
---

That'll compile to:

---
(eval
 '(defvar quoted-nil "false"))
---

-PJ

Gehm's Corollary to Clark's Law: Any technology distinguishable from
magic is insufficiently advanced.