Re: Double unquote/unquote-splicing

[Nathan Trapuzzano <nbtrap@nbtrap.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> There are two reasons why I'm resisting it:
> 1- the above code looks clean, but I can't understand it at all.
>    Your alternative code is less clean and not easy to understand, but
>    I *can* understand it.

We'll just have to disagree there.  But even granting that the latter is
easier to understand, I can't imagine anyone figuring out how to write
it from scratch.

> 2- Since we don't allow (\` (a (\, 1 2 3))), it's weird to allow
>    ``(a ,,@x) since one possible expansion for it when x=(1 2 3) is
>    (\` (a (\, 1 2 3))).
>    Another way to say it is that we should allow (\, 1 2 3) and (\,@
>    1 2 3), but that can't be used with the ,e and ,@e syntax, so it'll
>    stay as a second-rate citizen.

This is incorrect.  ``(a ,,@x) where x=(1 2 3) would evaluate to
(\` (a (\, 1) (\, 2) (\, 3))).  The first comma in ,,@ has the effect of
being applied member-wise to each element spliced out of ,@.  Cf. CLHS
`Backquote' (http://www.lispworks.com/documentation/HyperSpec/Body/02_df.htm).

> Isn't the above the same as
>
>    (defmacro once-only (names &rest body)
>      (let ((gensyms (loop for n in names collect (gensym))))
>        `(let (,@(loop for g in gensyms collect `(,g (gensym))))
>          `(let (,@(list ,@(loop for g in gensyms for n in names collect ``(,,g ,,n))))
>            ,(let (,@(loop for n in names for g in gensyms collect `(,n ,g)))
>               ,@body)))))

I believe so.  (FWIW, I don't think it's that easy to tell, though I'm
not great at this.  I derived the second form I presented from the first
form step-by-step using the rules in CLHS.)

The bottom line to me is that the behavior of Elisp differs from both CL
and Scheme, and there doesn't seem to be a good reason for it.

In further support of my argument :)

"The backquote syntax was particularly powerful when nested. This
occurred primarily within macro-defining macros; because such were coded
primarily by wizards, the ability to write and interpret nested
backquote expressions was soon surrounded by a certain mystique. Alan
Bawden of MIT acquired a particular reputation as backquote-meister in
the early days of the Lisp Machine." - "The Evolution of Lisp", Gabriel,
Steele.

I lifted that quote from a StackOverflow thread and have no idea whether
it's authentic.  But I think it's true.