A syntax for type signatures with mixins

[makoConstruct]

It is sometimes desirable to require a parameter to implement a class and a mixin, for instance:

mixin Indented {
   int get indentLevel;
}

class IndentedLine{
   Widget with Indented child;

But the expression Widget with Indented is not legal. There is no way to refer to that type.

I have tried

abstract class IndentedWidget extends Widget with Indented;

class IndentedLine{
   IndentedWidget child;

But this wont do here. When you actually create an IndentedWidget, you generally can't do it by extending IndentedWidget. You do it by extending StatelessWidget with Indented, which despite implementing both Widget and Indented, is not castable to IndentedWidget.

[eernstg]

You are asking for intersection types: If you could use a term like Widget with Indented as a type then it would have to mean "a maximal type T such that T is a subtype of Widget and T is a subtype of Indented". The word "maximal" just means that we can't choose a different value for T which is a supertype of the chosen one, so it gets "as close as possible to Widget and Indented". That's exactly how we'd define an intersection type. A typical syntax would use &, so you would then write this intersection type as Widget & Indented:

class IndentedLine {
   Widget & Indented child;
}

Intersection types could be added to Dart, but it is likely to be a major project. In particular, intersection types are directly related to union types (for instance, using | to specify union types, Function(S | T) <: Function(S) & Function(T) and Function(S) | Function(T) <: Function(S & T)), and this means that the addition of intersection types basically won't work unless we also add union types, and vice versa.

See #83 and #145 for further discussions about intersection and union types. Also note #1152 where the topic of intersection types is approached from a similar angle as in this issue.

I'll close this issue as a duplicate of those other discussions.

However, in the particular case where you wish to specify a subtype of Widget and Indented, it might be possible to simply eliminate the problem by making abstract class Indented ... implements Widget. The assumption is that you aren't actually going to work with any Indented objects that aren't also Widgets, and if this is true then you might as well just include Widget among the supertypes of Indented in the first place, and then you can simply do this:

class IndentedLine {
  Indented child;
}

[makoConstruct]

Oh cool, I didn't know about implements, so many reasonable subtleties.

I ended up using runtime type checking (and if anything that doesn't implement Indent is passed in, it just defaults to indent:0), but it's nice to know that there's a way to do that.

[eernstg]

Ah, makes sense! Thanks.