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Haskell Weird Kinds: Kind of (->) is ?? -> ? -> *

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 (,)
(,) :: * -> * -> *
function haskell types ghci
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