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Re: Double unquote/unquote-splicing
From: |
Nathan Trapuzzano |
Subject: |
Re: Double unquote/unquote-splicing |
Date: |
Mon, 04 Nov 2013 12:59:21 -0500 |
User-agent: |
Gnus/5.130007 (Ma Gnus v0.7) Emacs/24.3.50 (gnu/linux) |
Stefan Monnier <address@hidden> 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.
- Double unquote/unquote-splicing, Nathan Trapuzzano, 2013/11/04
- Re: Double unquote/unquote-splicing, Stefan Monnier, 2013/11/04
- Re: Double unquote/unquote-splicing,
Nathan Trapuzzano <=
- Re: Double unquote/unquote-splicing, Nathan Trapuzzano, 2013/11/04
- Re: Double unquote/unquote-splicing, Stefan Monnier, 2013/11/04
- RE: Double unquote/unquote-splicing, Drew Adams, 2013/11/04
- Re: Double unquote/unquote-splicing, Nathan Trapuzzano, 2013/11/04
- Re: Double unquote/unquote-splicing, Stefan Monnier, 2013/11/04
- Re: Double unquote/unquote-splicing, Nathan Trapuzzano, 2013/11/05
- Re: Double unquote/unquote-splicing, Stefan Monnier, 2013/11/05
- Re: Double unquote/unquote-splicing, Stephen J. Turnbull, 2013/11/04
- Re: Double unquote/unquote-splicing, Nathan Trapuzzano, 2013/11/05
- Re: Double unquote/unquote-splicing, Stephen J. Turnbull, 2013/11/05