support for 'var' in lambda parameters
maurizio.cimadamore at oracle.com
Wed Sep 27 12:47:43 UTC 2017
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.
> 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 188.8.131.52). For
> that reason, we won't go down there now (but it's something worth
> considering in the future).
>  -
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