In GHCi, why does the kind of the function arrow `:kind (->)` include question marks `(->) :: ?? -> ? -> *`? [duplicate]
https://stackoverflow.com/questions/5359707
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Possible Duplicate:
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 (,)
(,) :: * -> * -> *
Solution
The ? and ?? kinds refer to GHC extensions, specifically unboxed types. http://hackage.haskell.org/trac/ghc/wiki/IntermediateTypes has a diagram showing relationships between the extended kinds ? (all possible types), # (unboxed types), ?? (boxed or normal unboxed types — "least upper bound of # and *"), (#) (unboxed tuples, which can only be used in a small number of contexts). (The standard kind * refers to normal boxed types.)
