Lambdas

[CJ-Johnson]

This document proposes a path forward to add lambdas to Carbon. It further proposes augmenting function declarations to create a more continuous syntax between the two categories of functions. In short, both lambdas and function declarations will be introduced with the fn keyword. The presence of a name distinguishes a function declaration from a lambda expression, and the rest of the syntax applies to both kinds. By providing a valid lambda syntax in Carbon, migration from from C++ to Carbon will be made easier and more idiomatic. In C++, lambdas are defined at their point of use and are often anonymous, meaning replacing them solely with function declarations would create an ergonomic burden compounded by the need for the migration tool to select a name.

Associated discussion docs:

• Lambdas Discussion 1
• Lambdas Discussion 2
• Lambdas Discussion 3
• Lambdas Discussion 4

[zygoloid]

Looks good to me, thanks, and sorry for the slow review cycle!

[chandlerc]

Very happy with the direction here. A bunch of comments, but they're all focused on the proposal write up, not the specific proposed feature.

[chandlerc]

Another suggestion for the proposal -- I think it'd be nice to add some comparison for how the Carbon lambdas would look in C++.

This doesn't need to get into features that C++ lambdas have and Carbon's don't, just showing how analogous C++ lambdas would look so that folks can compare.

[KateGregory]

I'm really liking this. A few small comments. More calling examples, ideally right after the first syntax example. "This would be invoked like this:" kind of thing. I see just one (though I may have missed others) example of a lambda not being put into a variable but being used right away, the comparator in the sort example. This is terrific but may be missed by someone less interested in positional parameters, so some other examples of unnamed lambdas just being used would be great. I would hate for a reader to think that the #1 use of lambdas is to put them into a named variable or value.

[CJ-Johnson]

I'm really liking this. A few small comments. More calling examples, ideally right after the first syntax example. "This would be invoked like this:" kind of thing. I see just one (though I may have missed others) example of a lambda not being put into a variable but being used right away, the comparator in the sort example. This is terrific but may be missed by someone less interested in positional parameters, so some other examples of unnamed lambdas just being used would be great. I would hate for a reader to think that the #1 use of lambdas is to put them into a named variable or value.

Thanks for the feedback, Kate! I tried to address it by adding both // In a variable: and // In a function call: variants to the top level examples. Hopefully this will clarify things :)

[chandlerc]

I think this LGTM, checking with leads before approving.

[zygoloid]

LGTM. Some extremely minor nit-picking with equivalent versions of some of the examples in C++ here, but I'm happy with this either way; I don't think these affect the understandability of the proposal and certainly don't affect the proposed approach.

[chandlerc]

Just confirming that @KateGregory gave a thumbs up offline here -- this is good to go.

@CJ-Johnson - wanted to give you a chance to merge the suggestions from @zygoloid, but also happy to merge them for you and merge the PR whenever.

[chandlerc]

Accepting the suggestions as requested and merging!