In GHCi, why does the kind of the function arrow `:kind (->)` include question marks `(->) :: ?? -> ? -> *`?
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In GHCi (version 7.0.2), if I ask for the kind of the function type, the result has question marks:

    Prelude> :kind (->)
    (->) :: ?? -> ? -> *

Why does the kind include question marks instead of just asterisks `* -> * -> *`?  What do the question marks mean?  Why do other types just use asterisks?

    Prelude> :kind (,)
    (,) :: * -> * -> *

Thanks.