[Grace-core] Should Grace abandon "gradual typing" for "like typing"?

[Vitek, Jan]

> if I've got a like type such as 
> 
> type Fooable = like { foo }     // something Fooable has a Foo method
> 
> what happens if I assign an object that doesn't have a Foo method to that variable (and, say, never actually call the foo method).  Presumably that object is flowing in e.g. from a variable or parameter of type Unknown (aka Dynamic)
> so we don''t know at compile-time (or type-check type) whether or not
> the object is Fooable... 
> 
> method assignToFooable( other : Unknown  -> like Fooable  {   
>      // now I'm being extra explicit other is dynamically typed
>      def x : like Fooable = other
>      return x 
> }
> 
> Does the assignment to a Fooable cause an error or not?  (I guess I should read the formalsims, but...) 
> 
> I thought Dart in checked mode *would* raise a dynamic error there. 
> I thought Thorn would too, and if we can be sure that other does 
> not have a foo method we get an eager error…?


No error in Thorn, Dart (unchecked), TypeScript, Hack.

But if you remove the “like” annotation, there will be an error.

-j