Challenge 2: summarizeList Using an As-Pattern — Possible Solution
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Main.hs:

summarizeList :: [Int] -> String
summarizeList [] = "The list is empty."
summarizeList full@(x:_) = "The list " ++ show full ++ " starts with " ++ show x

main :: IO ()
main = do
  putStrLn (summarizeList [])
  putStrLn (summarizeList [10, 20, 30])

Output:

    The list is empty.
    The list [10,20,30] starts with 10

Explanation:
full@(x:_) binds TWO things at once from a single successful match:
full captures the entire matched list (used later via show full to
print [10,20,30]), while x simultaneously captures just the head
(10). Neither binding requires re-matching the list a second time or
reconstructing it from x -- both are available directly from the one
pattern match, exactly the convenience the chapter's own firstTwo
example demonstrates. The empty-list case is handled separately as
its own equation, since [] never matches the (x:_) pattern at all.

WHY THIS WORKS AS AN ANSWER
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This uses an as-pattern to bind both the whole list and its head in
one match, matching the chapter's own firstTwo shape, with a separate
equation correctly handling the empty-list case the as-pattern alone
can't cover.