如何在Scala中实现`type`关键字

Question

Is type in scala just a syntactic sugar that acts like a macro during compile time, or is there some other magic involved? In a situation like this:

type Bar1 = Foo
type Bar2 = Foo
val bar1 = injector.getInstance(classOf[Bar1])
val bar2 = injector.getInstance(classOf[Bar2])

Would the DI container be able to distinguish between the two 'type aliases' ?

Answer 1

It's just syntactic sugar that assigns a different name to the same type. The compiler does not generate a new type so is not able to distinguish type aliases of the same type from each other, or from the original type. So for example:

type Foo = String
def foo(f:Foo)
val s : String = "bar"
foo(s)

would compile just fine.

In the same way, a DI container would be unable to distinguish the two cases you've shown at runtime because there were never two distinct types at compile time.

Answer 2

I think that a DI container using Scala reflection and macros could distinguish them (though classOf doesn't make sense in this case), but probably shouldn't: it would violate principle of least surprise rather strongly. Rather more usefully, it can distinguish List[Apple] and List[Orange] despite their classOf being the same.