Hi Guido, On 16/09/2020 5:54 am, Guido van Rossum wrote:
On Tue, Sep 15, 2020 at 7:31 PM Benjamin Peterson <benjamin@python.org <mailto:benjamin@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-c...>
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