Rick Hanson <cryptor...@gmail.com> writes:

Hi Rick,

> Here is a problem (again!) with using a pil backquote expression,
> where the user, like you or me, is stuck on "CL-unquote thinking".
> Let's call g again, but now we switch the places of the dates and
> times.  In this case, we might expect the answer now to be "No";
> however, the answer will remain "Yes".
> : (g Is $S1$ before $S2$?
>    The answer is
>    `(if (and
>            (<= $D2$ $D1$)
>            (<= $T2$ $T1$) )
>        "Yes"
>        "No" ) )
> -> "Is \"1999-09-09 20:12:05\" before \"2000-03-04 18:03:03\"? The
> answer is \"Yes\""
> Why?
> Because when the backquote expression gets resolved by the reader, the
> symbols $D1$, $D2$, $T1$, and $T2$ are not bound to any values
> (i.e. will evaluate to NIL).  And since
> (<= NIL NIL)
> evals to T, g will always see "Yes" as its last argument.
> Sorry.  That means you have to come up with another way to eval "code"
> in your DSL (i.e the stuff under g).
> I hope this make sense.  This is what Alex has been trying to tell us
> all along.  (I only now just realized this; so you can see how far
> I've come along: not much.)

thanks for your improvements of my rather crude function and for the
final hint that using (date) in the examples was a rather bad idea since
(date) works with NIL args too, it just gives different results.

This backquote/comma mechanism in Emacs Lisp seemed so natural to me I
never really thought about the way it works. So I should rather think
about function calls then read syntax. But maybe not in public, I don't
want to drive more people off the mailing list ;-)

However, I will use your factorized code as starting point for new
investigations, thanks again! 


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