Uniform tear-off syntax

(based on discussion in #376)
Proposing new syntax for tear-offs to cover several use cases. General form: obj.(thing-to-tear-off)
Examples:
a.(+) // operator tear-off
a.(unary-)
a.(get x) // getter tear-off
a.(set x) // setter tear-off
Foo.(new) // default constructor tear-off
Foo.(new named) // named constructor tear-off

[eernstg]

Cf. this discussion on dart-misc:

It would also be useful to consider tear-offs of instance methods independently of the receiver. That is, a method with k positional arguments is torn off as a function with k + 1 positional arguments, and the first argument is the receiver.

Getters and setters could be handled as described above, we'd just change a from an expression denoting an instance to a type literal: Class.(thing-to-tear-off-with-receiver-argument).

[rrousselGit]

I don't see myself ever using a.(get x) when I can write () => a.x.

They take the same number of characters, but the closure approach is easier to refactor.

What about:

#a
#+a
#-a
#a.x
#=a.x
#Foo
#Foo.named

?

[rrousselGit]

The character doesn't really matter.
We can change it to something else, like ~ or ::

[natebosch]

It would also be useful to consider tear-offs of instance methods independently of the receiver.

Does this have the semantics of picking a concrete implementation statically? Or will it have the semantics of wrapping with a closure and the concrete implementation is still picked at runtime?

I don't think the former would work:

class Foo {
  int get _private => 1;

  int get public => 2;

  void doStuff() {
    print(_private + public);
  }
}

// Some other library.
class Bar implements Foo {
  @override
  void doStuff() {}

  @override
  int get public => 3;

  // No override of _private possible
}

void main() {
  void Function(Foo) tornOff = Foo::doStuff;
  tornOff(Foo()); // OK
  tornOff(Bar()); // throws? What would the error be?
}

I think the latter would be fine. Effectively Foo::doStuff is syntactic sugar for (Foo f, <other args>) => f.doStuff(<other args>);

[eernstg]

Does this have the semantics of picking a concrete implementation statically?

No, the idea is simply that with abstract class C { S foo(T); } a tear-off like myC.foo yields something that works like (T t) => myC.foo(t), and C.foo yields (C self, T t) => self.foo(t). So that's what you call 'the latter'.

[kasperpeulen]

What about a default first parameter name called it

=> a + it // operator tear-off
=> a - it
=> a.x // getter tear-off
=> a.x = it // setter tear-off
=> Foo(it) // default constructor tear-off
=> Foo.named(it) // named constructor tear-off

[kasperpeulen]

Right, my point is that I don`t think we really need new tear-off syntax, if we have #265.

[eernstg]

There is one thing that a tear-off is capable of doing that we can't emulate with a manual rewrite, namely the special behavior for equality tests (operator ==): Two tear-offs of the same method will test equal iff they were torn off the same receiver (o1.foo == o2.foo iff identical(o1, o2)). This property could be carried over to the new kind of tear-off (where we tear an instance method off of a class, yielding a function that adds an extra positional argument accepting the receiver): They could be equal iff they tear off the same method of the same statically known receiver type. A similar property could be defined for getters and setters and constructors.

[RedHatter]

Isn't this the same functionality of this proposal that was aproved back in 2015?

[eernstg]

It is similar, but the generalized tear-offs as described in said proposal are not supported in Dart today (not in the specification, not in the implementation).