What's a good use of an Expando?

[mythz]

The API docs for it are pretty bare with the core API revolved around overloading the indexer '[]' operator:

http://api.dartlang.org/docs/releases/latest/dart_core/Expando.html

I can't find any examples of when it's a good idea to use an Expando even in the core libraries?

What is Expando for? when should we use it? and what benefits does it have over a simple Map?

[John Messerly]

It lets you associate data with any object. This can be pretty convenient.

On the VM, it has weak reference behavior. In JS, it stores data on the object itself, in a hidden way.

Unlike associating data via a Map, using an Expando won't keep the key object alive. So it's essentially a weak map.

As to why it isn't used much: I can't speak for everyone, but I avoid it because of:

https://code.google.com/p/dart/issues/detail?id=7558

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[Alex Tatumizer]

Is it a weak identity hashmap then?
Named "expando" to sound more romantic?
Where O(N^2) comes from in a hashmap?
I'm confused :(
I was wondering about expando, but was afraid to ask - thought it was something just sitting there until someone notices it and removes. No?

[Joseph Moniz]

It's use still seems some what confusing. Is it a class you extend, kind of like a mixin for [] and []=, or is it used in paramaterized type manner as it's constructor seems to suggest that somehow forwards method invocations to an instance of it's paramaterized type?

- Joseph Moniz

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"Wake up early, Stay up late, Change the world"

[John Messerly]

On Tue, Apr 16, 2013 at 12:09 PM, Alex Tatumizer <tatu...@gmail.com> wrote:

Is it a weak identity hashmap then?

Yup.

 

Where O(N^2) comes from in a hashmap?

Yeah, it's a bug :)

 

I was wondering about expando, but was afraid to ask - thought it was something just sitting there until someone notices it and removes. No?

Weak maps are a useful feature. They're being added to ES6 too: http://wiki.ecmascript.org/doku.php?id=harmony:weak_maps

[John Messerly]

On Tue, Apr 16, 2013 at 12:31 PM, Joseph Moniz <joseph...@gmail.com> wrote:

It's use still seems some what confusing. Is it a class you extend, kind of like a mixin for [] and []=, or is it used in paramaterized type manner as it's constructor seems to suggest that somehow forwards method invocations to an instance of it's paramaterized type?

Think of it as a map, where you use objects as keys. Use like:

final myProp = new Expando<int>('optional human friendly name for debugging');

someFunction(Node node) {

  // Associate data with some arbitrary object.

  // Maybe it came from a library I don't control, like dart:html, so I can't just add a field there  

  myProp[node] = 42;

}

someOtherFunction(node) {

  // Get the data we associated earlier.

  int theAnswer = myProp[node];

  print(theAnswer);

}

See also:

http://api.dartlang.org/docs/releases/latest/dart_core/Expando.html

[Alex Tatumizer]

> Weak maps are a useful feature

Absolutely. I use them very often in java. I just couldn't recognize them in expandos. Not documented well.

Good to know, thanks!

[Sean Eagan]

ES6's WeakMap doesn't expose iteration functionality so as to keep the GC semantics unobservable.  I assume Expando doesn't implement Map (or include Map in it's name) for the same reason.

Cheers,
Sean Eagan