GHCi has no way to query arity of type constructor
In non-generative type constructors (type synonyms and type families), we have a property called arity, which determines how many arguments must be supplied to saturate it.
I propose to add this information into output of :info
in ghci. For example:
ghci> type T :: Proxy a -> Maybe a; type T x = Nothing
ghci> :i T
type T :: forall a. Proxy a -> Maybe a
type T x = Nothing
-- Arity: 2 (Visible: 1, Invisible: 1)
-- Defined at <interactive>:14:31