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Subject: [std-proposals] Terse lambda quick outline of an alternative
From: Михаил Найденов (mihailnajdenov_at_[hidden])
Date: 2020-02-27 04:19:30


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



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