Summary
-------

Allow `var` to be used when declaring the formal parameters of implicitly typed lambda expressions.


Goals
-----

- Align the syntax of a formal parameter declaration in an implicitly typed lambda expression with the syntax of a local variable declaration.

Non-goals
---------

- Align the syntax of any other kind of variable declaration, e.g., a formal parameter of a method, with the syntax of a local variable declaration.

Motivation
----------

A [lambda expression](https://docs.oracle.com/javase/specs/jls/se9/html/jls-15.html#jls-15.27.1) may be *implicitly typed*, where the types of all its formal parameters are inferred:

    (x, y) -> x.process(y)    // implicitly typed lambda expression

Java SE 10 makes implicit typing available for local variables:

    var x = new Foo();
    for (var x : xs) { ... }
    try (var x = ...) { ... } catch ...

For uniformity with local variables, we wish to allow 'var' for the formal parameters of an implicitly typed lambda expression:

    (var x, var y) -> x.process(y)   // implicit typed lambda expression

One benefit of uniformity is that modifiers, notably annotations, can be applied to local variables and lambda formals without losing brevity:

    @Nonnull var x = new Foo();
    (@Nonnull var x, @Nullable var y) -> x.process(y)

Description
-----------

For formal parameters of implicitly typed lambda expressions, allow the reserved type name `var` to be used, so that:

    (var x, var y) -> x.process(y)

is equivalent to:

    (x, y) -> x.process(y)

An implicitly typed lambda expression must use `var` for all its formal parameters or for none of them. In addition, `var` is permitted only for the formal parameters of implicitly typed lambda expressions --- explicitly typed lambda expressions continue to specify manifest types for _all_ their formal parameters, so it is not permitted for some formal parameters to have manifest types while others use `var`. The following examples are illegal:

    (var x, y) -> x.process(y)         // Cannot mix 'var' and 'no var' in implicitly typed lambda expression
    (var x, int y) -> x.process(y)     // Cannot mix 'var' and manifest types in explicitly typed lambda expression

In theory, it would be possible to have a lambda expression like the last line above, which is semi-explicitly typed (or semi-implicitly typed, depending on your point of view). However, it is outside the scope of this JEP because it deeply affects type inference and overload resolution. This is the main reason for keeping the restriction that a lambda expression must specify all manifest parameter types or none. We also want to enforce that the type inferred for a parameter of an implicitly typed lambda expression is the same whether `var` is used or not. We may return to the problem of partial inference in a future JEP. Also, we do not wish to compromise the brevity of the shorthand syntax, so we won't allow expressions like:

    var x -> x.foo()

Alternatives
------------

Continue declaring implicitly typed lambda expressions as in Java SE 8.


Risks and Assumptions
---------------------

This JEP has no risk of source incompatibility when `var` is added before a parameter name in an implicitly typed lambda expression, because the type inferred for the parameter without `var` is the same as the type inferred with `var`.