[AnimalMuppet]

Imagine that you have a binary-only third-party library, one function of which returns an opaque type. Imagine that you want to unpack that and observe the state of some data inside it (perhaps guided by online hints), and the vendor won't give you the ability to do so because 1) they're busy, or 2) they want a lot of money, or 3) they're out of business. You don't want to have to subvert the type system, but you still kind of need to.

And if you're going to tell me that the only reason you need to in this scenario is because the third-party vendor didn't do their job (including creating the types) right, I'd agree. That doesn't actually change what you need to do, though.

[munk-a]

I agree, but in that case you're fighting against the typing system more than typing itself, whatever you are doing with that thing and however you would abstractly define that type (maybe as "We expect this thing to have the type of the thing that terrible third party library usually gives us back") you will make assumptions based on the fact that that value is actually one of those and you never want those assumptions violated. In this case the typing system is too restrictive and it might be a case where duck-typing would come in handy (since you can usually post-apply types in that sort of a setting).