When I was experimenting with Haskell kinds, and trying to get the kind of ->, and this showed up:

$ ghci
...
Prelude> :k (->)
(->) :: ?? -> ? -> *
Prelude>

Instead of the expected * -> * -> *. What are the ?? and ? things? Do they mean concrete types or "kind variables"? Or something else?

haskell types type-systems
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