support for 'var' in lambda parameters
forax at univ-mlv.fr
Wed Sep 27 14:06:11 UTC 2017
----- Mail original -----
> De: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> À: "Zheka Kozlov" <orionllmain at gmail.com>
> Cc: "amber-dev" <amber-dev at openjdk.java.net>
> Envoyé: Mercredi 27 Septembre 2017 14:47:43
> Objet: Re: support for 'var' in lambda parameters
> On 27/09/17 13:20, Zheka Kozlov wrote:
>> Why should we allow `var` in lambda parameters? Who will ever need
>> writing `(var x) -> x` instead of `x -> x`?
> first of all, to clarify things, this is an experimental feature - which
> is why it was left out from the JEP 286 integration into jdk10/master.
> Secondly, JEP 286 is not just about 'var' in local variable declaration
> (although I agree that's the most frequent use case). You can also use
> 'var' in other places too, such as for loops and try-with-resources.
> From an uniformity perspective, one could argue that lambda parameters
> are another place where the 'var' treatment could be applied.
> As you point out, lambdas do provide a more succint version to declare
> implicit parameters - but in a language that is evolving to use 'var' in
> more places (pattern matching will likely use 'var' for pattern
> bindings), I think that, from a pedagogical question at least, it makes
> sense to allow 'var' in all places where you want to declare a variable
> but you don't care about giving it an explicit type.
yes, it's for completeness.
I would love to see if it is possible to use var in an explicitly typed lambda, it's an interesting use case.
>> 2017-09-27 19:07 GMT+07:00 Maurizio Cimadamore
>> <maurizio.cimadamore at oracle.com <mailto:maurizio.cimadamore at oracle.com>>:
>> I've recently pushed a changeset  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
>> 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 //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
>> 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 220.127.116.11). For
>> that reason, we won't go down there now (but it's something worth
>> considering in the future).
>>  -
More information about the amber-dev