[知识点] Existential types

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Let's explore existential types--these are types that are known to the component that produces them, but unknowable to the component that consumes them. Think about a hypothetical virtual machine that produces some compiled representation of some source code and it can then run the compiled code. In short, our VM compiles into type a that is unknown and unknowable to anything outside the VM; the only thing that the outside world knows is that it can take the value of type a and run it.

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