Hi Guido,
On 16/09/2020 5:54 am, Guido van Rossum wrote:
On Tue, Sep 15, 2020 at 7:31 PM Benjamin Peterson <benja...@python.org
<mailto:benja...@python.org>> wrote:
On Tue, Sep 15, 2020, at 20:10, Greg Ewing wrote:
> Maybe the PEP should propose an AST of its own, which would initially
> be a third thing separate from either of the existing ones, with
> the possibility of adopting it as the ast module representation
> some time in the future.
The rust approach to this problem to pass only a token stream to the
macro. Then, the macro writer brings their own parser.
And (I looked this up [1]) it returns another token stream which is then
re-parsed by the standard parser. I'm not quite sure who is responsible
for determining where the macro ends -- the standard parser or the macro
parser.
A nice side effect of this would be that (hopefully) the macro syntax
doesn't need to have the ugly special cases needed to allow `import! foo
as bar` and `from! mod import foo as bar` (though these still need to be
predefined macros).
I'm not entirely sure how this would preserve the hygiene of the macros
though, since the macro's parser could just do a token-by-token
substitution without checking the context, effectively what a macro in C
does. Forcing an AST on the macros restricts macros somewhat (e.g. it
requires the input arguments to look like well-formed expressions) but
that's probably a good thing.
(Hm, or not. How would you implement something like PEP 505 using
macros? It introduces new tokens like `?.` and `??`.
You can't implement any new operators with the PEP.
Having said that, a None-aware attribute operator could be implemented
something like this:
maybe!(a.b)
which would translate to:
$tmp = a; None if $tmp is None else ($tmp.b)
Cheers,
Mark.
[1] https://doc.rust-lang.org/book/ch19-06-macros.html
--
--Guido van Rossum (python.org/~guido <http://python.org/~guido>)
/Pronouns: he/him //(why is my pronoun here?)/
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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