Re: Double unquote/unquote-splicing

[Stefan Monnier]

>> 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).

This is even worse since it means that the expansion of ,@foo depends on
the context.

> I believe so.  (FWIW, I don't think it's that easy to tell, though I'm
> not great at this.

Back to problem nb 1, eh?

> "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.

That sounds about right: it's only for wizards.

Nested backquotes were largely broken in Elisp and it took many years
for someone to notice.  I do use them occasionally, but only in fairly
simple ways.  The resulting code is largely impenetrable, so I don't
want to encourage it.


        Stefan