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Subject: Re: [std-proposals] Terse lambda quick outline of an alternative
From: Anton Zhilin (antonyzhilin_at_[hidden])
Date: 2020-02-27 06:41:32


An abbreviated lambda proposal can't be considered complete without the
ability to omit the "return" keyword.

Please see this review of Rust lambda syntax for C++:
https://gist.github.com/Anton3/3926dc16b9815d022475cb4b680a9744

Compare:
1. Today
[](const auto& a, const auto& b) { return a < b; }
2. Yours
[] a, b { return a < b; }
3. Mine
|a, b| a < b

27.02.2020, 13:19 Михаил Найденов via Std-Proposals <
std-proposals_at_[hidden]:

> Hello, I am aware of the multiple failed attempts at "abbreviated lambda",
> summarized here <https://brevzin.github.io/c++/2020/01/15/abbrev-lambdas/>recently.
> Nevertheless, this feature seems to be too important to abandon and I
> wanted to share some thoughts I have on the subject.
>
> Ultimately, as far as "terseness" (alone) goes, it is the function
> argument that are the biggest culprit
> [](const auto& a, const auto& b) { return a < b; }
>
> As you can see, if we aim to improve verbosity alone, we should look no
> further then the params
>
> With that in mind, I believe we can split the "abbreviated lambda" goal in
> two tasks - one for the params and one for the body, assuming the one,
> regarding the body will be harder because more issues must be solved.
>
> The only issue, outlined in the blog regarding params is the double
> parsing - the fact the compiler has to parse until => (if any) to know, if
> it deals with abbr. params .
> [](a) => ; //< arg
> [](a){}; //< no arg, just void(a) signature
>
> *Proposal*
>
> Allow lambda arguments to be declared without parentheses.
> A single item introduces an argument with that name.
>
> []a,b { return a < b; }
>
> This way the parser will immediately know with type of param list it deals
> with.
>
> It will be possible to add a type as well
>
> []a, auto b { return a < b; }
>
> The argument list is bound b/w [] and { or mutable or ->
>
> []{ ... }
> []a, b { ... }
> []a, b -> R { ... }
> [=]a, b mutable { ... }
>
>
> *My question is*
> Does anyone see any fatal implementation issues, before I go ahead and
> write a formal proposal.
>
> Thank You
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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