Re: defmacro with built-in gensym declaration and initialization

[akater <nuclearspace@gmail.com>]

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"Basil L. Contovounesios" <contovob@tcd.ie> writes:

> Let's just say I would sooner see native arglists gain support for
> keyword arguments ;).
>
> For non-native arglists, we could always extend cl-defmacro or some
> other definition definer.

Sorry, I suspect I misunderstood.  What's a native arglist?  Doesn't
defmacro already have native support for &optional and &rest keywords?

> I was referring to the arity of the macro being defined, not that of
> defmacro.

I guess I'm confused, again.  The arity of macro being defined remains
exactly the same.  See the --partition-by example in the original
message.  defmacro/&gensym is a drop-in replacement; it doesn't change
any arities.

> It and its variant macroexp-let2* are relevant wherever the macro author
> wants to avoid evaluating an argument more than once, but improvements
> are always welcome.

That's usually called “once-only” but unlike once-only, macroexp-let2
also requires a test function which is exactly why I called it “an
overcomplicated once-only”.  &gensym is also relevant wherever the macro
author wants to avoid evaluating an argument more than once, and where
the argument needs to be evaluated unconditionally, but it's more
concise, both in terms of token count and nesting depth.  One variation
of defmacro/&gensym accessible in the linked repository does perform the
same elimination of bindings as macroexp-let2, only it uses a hardcoded
test function, namely macroexp-const-p.

>> Since we're moving to
>> natively compiled Elisp, I was thinking it's going to become less
>> relevant in near future, and &gensym covers most use cases of once-only.
>
> I don't see how native compilation changes how existing and new Elisp
> macros ought to be written.

What's the purpose of TEST in macroexp-let2, then, other than to
minimise bindings in the expansion?  (Which I presume is only relevant
when Elisp's byte compiler is used to compile the expanded form.)

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