Can I pass an extension method as a parameter?

[Dale King]

So been playing with the new extension methods and I have to ask isn't there any way to pass an extension method method as a parameter? I have read the section on tearoffs, but the example there is stupid as it declares an extension on a type, but never actually references that type

At the very list isn't there a way to take an extension like:

extension Foo on int { int doubled() => this * 2 }

and turn Foo.doubled into an instance of int Function(int)?

[Nate Bosch]

Extension methods are syntactically closer to an instance method than a static function. You can tear off an extension method where it is applied on an instance, not statically. That is `1.doubled` will tear off an `int Function()`. To get an `int Function(int)` you'll need to wrap it as `(int i) = i.doubled()`.

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[Dale King]

Yeah, I think that this is not specific to extension methods and rather a missing feature of Dart with regard to instance methods.

For comparison here is the documentation for method references in Java which is the equivalent of Dart tear-offs: https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html. Java supports 4 types of method references:

Reference to a static method	ContainingClass::staticMethodName
Reference to an instance method of a particular object	containingObject::instanceMethodName
Reference to an instance method of an arbitrary object of a particular type	ContainingType::methodName
Reference to a constructor	ClassName::new

So Dart only supports the first 2 cases, a reference to a top-level or static function or a reference to an instance method of a particular object. It does not support references to an instance method of an arbitrary object.

For dart it would look like Foo.doubled would an expression of type `int Function(int)` that would be the equivalent of a lambda of the form `(int x) => x.doubled()`.

So basically Dart does not treat instance methods as first class citizens and they cannot be passed to higher level functions.

Really wish we had Flutter built on a real language like Kotlin.

On Thursday, December 5, 2019 at 1:42:02 PM UTC-5, Nate Bosch wrote:

Extension methods are syntactically closer to an instance method than a static function. You can tear off an extension method where it is applied on an instance, not statically. That is `1.doubled` will tear off an `int Function()`. To get an `int Function(int)` you'll need to wrap it as `(int i) = i.doubled()`.

On Thu, Dec 5, 2019 at 10:17 AM Dale King <dale...@gmail.com> wrote:

So been playing with the new extension methods and I have to ask isn't there any way to pass an extension method method as a parameter? I have read the section on tearoffs, but the example there is stupid as it declares an extension on a type, but never actually references that type

At the very list isn't there a way to take an extension like:

extension Foo on int { int doubled() => this * 2 }

and turn Foo.doubled into an instance of int Function(int)?

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[Erik Ernst]

There's a request for constructor tear-offs in https://github.com/dart-lang/language/issues/216, and that request has substantial support and might well be added to Dart. Similarly, https://github.com/dart-lang/language/issues/691 proposes some generalizations covering setters and getters, and the case where `Class.member` is used to tear off a function that accepts the receiver as an extra argument would fit nicely into that approach.

So we may not cover all the cases yet, but these ideas seem useful! ;-)

[Dale King]

Good to know. Those other cases are vital. In the end I refactored the code in a different way where I did not need to pass them, but it would be nice to have that ability.