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

[James Noble]

On 29/07/2016, at 8:21AM, Vitek, Jan <j.vitek at northeastern.edu> wrote:
> 
>  Like types will never cause runtime failures, while concrete types may.

OK, so now I'm concerned that I'm missing something.

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...?

James