I don't completely seee the point - in more established OO languages you'd make two one-line wrapper classes that both implement Get() and be done with it. Not stuck at all.. Extra indirection, yes, but is your performance going to block on parsing mime mails?
I assumed you would do the same thing as was done in the article.
Create an interface and have a function that takes implementations of that interface as a parameter. The difference would be that in Java or C# you would need to explicitly mark Header and MIMEHeader as implementing emailHeader
I don't think much will come of discussing what is/isn't possible in a language given that they are all Turing complete. Usually posts like this are used for discussing what's easier or cleaner in a language.
For what it's worth I think having dynamic interfaces is cool, but I can only really think of one place I have actually relied on that capability.
Mostly the "dynamic"ness is nice for decoupling code. You can have a package(A) containing a function that takes an interface a parameter, a package(B) containing an implementation of the interface in A and a package (C) which passes the implementation in B to the function in A.
Given this situation C needs to import A and B but neither A nor B need to know about each-other.
