Smalltalk type system

Question

I'm very new to Smalltalk and would like to understand a few things and confirm others (in order to see if I'm getting the idea or not):

1) In Smalltalk variables are untyped?

2) The only "type check" in Smalltalk occurs when a message is sent and the inheritance hierarchy is climbed up in order to bind the message to a method? And in case the class Object is reached it throws a run time error because the method doesn't exist?

3) There are no coercions because there are no types...?

4) Is it possible to overload methods or operators?

5) Is there some kind of Genericity? I mean, parametric polymorphism?

6) Is there some kind of compatibility/equivalence check for arguments when a message is sent? or when a variable is assigned?

Most questions probably have very short answers (If I'm in the right direction).

Answer 1

1) Variables have no declared types. They are all implicitly references to objects. The objects know what kind they are.

2) There is no implicit type check but you can do explicit checks if you like. Check out the methods isMemberOf: and isKindOf:.

3) Correct. There is no concept of coercion.

4) Operators are just messages. Any object can implement any method so, yes it has overloading.

5) Smalltalk is the ultimate in generic. Variables and collections can contain any object. Languages that have "generics" make the variables and collections more specific. Go figure. Polymorphism is based on the class of the receiver. To do multiple polymorphism use double dispatching.

6) There are no implicit checks. You can add your own explicit checks as needed.

Answer 2

Answer 3) you can change the type of an object using messages like #changeClassTo:, #changeClassToThatOf:, and #adoptInstance:. There are, of course caveats on what can be converted to what. See the method comments.