Challenge 1: UserId and ProductId, Caught at Compile Time — Possible Solution
====================================================================

Main.hs:

newtype UserId = UserId Int
newtype ProductId = ProductId Int

greetUser :: UserId -> String
greetUser (UserId n) = "Hello, user #" ++ show n

Broken attempt:

main :: IO ()
main = putStrLn (greetUser (ProductId 42))

Representative compile error:

    Main.hs:7:28: error:
        * Couldn't match type 'ProductId' with 'UserId'
          Expected: UserId
            Actual: ProductId
        * In the first argument of 'greetUser', namely '(ProductId 42)'

Fixed:

main :: IO ()
main = putStrLn (greetUser (UserId 42))

Output:

    Hello, user #42

Explanation:
UserId and ProductId both wrap the exact same underlying
representation, Int -- at runtime, there's genuinely no difference in
how the two are stored. But because they're declared as distinct
newtypes, the compiler treats them as completely different types.
greetUser's signature demands specifically a UserId, so passing a
ProductId -- even though it's "just an Int underneath" -- is rejected
at compile time, exactly the kind of ID mix-up the chapter's own tip-
box names as a real, practical newtype use case.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reproduces the exact ID-mix-up scenario the chapter's own tip-
box describes, showing the real compile error produced when two
newtypes with identical underlying representations are accidentally
swapped.