Update: You can read even more about type erasure with lots of example code
A friend of mine @purpleyay gave a talk at try! Swift in Japan about type erasure
this week last month. For those of us who didn't attend, what's the deal with type erasure anyway?
More specifically, what is type erasure in the context of Swift and Protocols with associated types? If I could boil it down to one sentence:
Passing a generic type parameter to a protocol's associated type member.
We pass value parameters to functions. A function is usually meaningless without them. So how do we pass a type parameter?
All computing is meaningless in some sense. A computer is just a dumb machine that follows meaningless rules to perform meaningless operations on meaningless values, coming to some meaningless conclusion (or not as the halting case may be).
A protocol like
SequenceType is just floating out there in space. The compiler has no idea what kind of
Element it might contain. That's why you can't declare
var x: SequenceType. How would the compiler know the shape of
Only when a specific concrete type like
SequenceType does the compiler have enough information. That locks you in to
Array though. You're using a specific type.
What if you want a way to abstract over all
SequenceTypes? That's exactly what
AnySequence<T> does. We passed an explicit type parameter to
AnySequence<T>, which becomes the type member (associated type) to fulfill
SequenceType.Element. That gives the compiler enough concrete information to work with.
You can take that one step further by inheriting from
AnySequence<T> and giving a specific
AnyIntSequence: AnySequence<Int>. At that point you've fully erased both the generic type parameter and the generic type members of the protocol, plus erased the specific implementation details of exactly what kind of sequence you stuffed in at the beginning. Hence calling it type erasure.
You can find more information on associated types in an older post; specifically why Swift doesn't have
Protocol<T> and what the point of associated types is anyway.
This blog represents my own personal opinion and is not endorsed by my employer.