Challenge 2: Checking a Partially Applied Function's Type in GHCi — Possible Solution
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GHCi session:

    Prelude> let add :: Int -> Int -> Int; add x y = x + y
    Prelude> :t (add 5)
    (add 5) :: Int -> Int

-- add's real type is Int -> (Int -> Int) -- a function taking one Int
-- and returning ANOTHER function of type Int -> Int. Applying add to
-- just one argument, 5, fully satisfies that first arrow. What's left
-- over -- what (add 5) actually evaluates to -- is exactly the second
-- piece of that chain: a function of type Int -> Int, still waiting
-- for one more Int before it produces a final Int. This is precisely
-- why GHCi reports (add 5) :: Int -> Int rather than some kind of
-- error or incomplete-call placeholder -- there's no such thing as an
-- "incomplete call" in Haskell, only a real function value that
-- happens to still expect more arguments.

WHY THIS WORKS AS AN ANSWER
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This uses GHCi's own :t command to inspect a partially applied
function's real type, and the explanation correctly ties the result
back to the chapter's own Int -> (Int -> Int) decomposition of the
original two-argument signature.