support for 'var' in lambda parameters

[Zheka Kozlov]

Why should we allow `var` in lambda parameters? Who will ever need writing
`(var x) -> x` instead of `x -> x`?

2017-09-27 19:07 GMT+07:00 Maurizio Cimadamore <
maurizio.cimadamore at oracle.com>:

> Hi,
> I've recently pushed a changeset [1] to enable use of 'var' in lambda
> parameters. Below is some rationale on the implemented support.
>
> In the current (e.g. JDK 9) grammar, we have two versions of lambdas:
> *implicitly*- and *explicitly*-typed. Furthermore, an implicitly typed
> lambda can come in two form (there's no terminology for it, so I'll make up
> one): *compact* and *parenthesized*.
>
> Let's see some examples:
>
> x->x //implicit, compact
> (x)->x //implicit, parenthesized
> (String x)->x //explicit
>
> So, the first question is - when is it ok to use `var`? There are two
> choices here:
>
> 1. allow `var` on all implicit lambdas
> 2. allow `var` only on implicit parenthesized lambdas
>
>
> We concluded that (2) is the option that makes more sense - this allows
> for a smooth verbosity curve (see below from most compact to least compact):
>
> x->x
> (x)->x //added parens
> (var x)->x //added 'var'
> (String x)->x //ok, fully typed
>
> Second question: we basically have now two kind of implicitly typed
> parameters - those that omit type info in full (e.g. the formal decl is
> just an identifier) and those using `var`. Can we mix and match between
> these e.g.
>
> (var x, y)->x+y
>
> We have concluded that there's no need to mix and match. In fact, it is
> more likely that one form of implicit parameter declaration will supplant
> the other in the long run.
>
> Third question: what about parameter modifiers such as `final` and/or
> annotations? Historically, such modifiers were not allowed in implicit
> lambda parameters (as those were just identifiers, and it poses several
> parser challenges to e.g. allow annotations there); on the other hand,
> local variables using `var` support both modifiers and annotations. We
> concluded that, for consistency, we have to do the same when `var` occurs
> in a lambda parameter.
>
> Last question: can I mix implicit an explicit parameters?
>
> (var x, String y)-> x + y
>
> While that would be nice, it must be noted that this is a much deeper
> change, with ramifications in overload selection, since implicit and
> explicit lambdas are currently two disjoint sets, with very different
> behavior w.r.t. overload resolution (see definitions of 'pertinent to
> applicability' in JLS 15.12.2.2). For that reason, we won't go down there
> now (but it's something worth considering in the future).
>
> Maurizio
>
> [1] - http://mail.openjdk.java.net/pipermail/amber-dev/2017-Septem
> ber/002138.html
>
>
>