Challenge 1: A Hand-Rolled myLength — Possible Solution
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Main.hs:

myLength :: [a] -> Int
myLength [] = 0                    -- base case: an empty list has length 0
myLength (_:xs) = 1 + myLength xs  -- recursive case: 1 for the head, plus the rest's length

main :: IO ()
main = print (myLength [10, 20, 30, 40])

Output:

    4

Explanation:
myLength follows the exact base-case/recursive-case shape the chapter
introduces with sumList. The base case handles the empty list
directly, returning 0 with no further recursion needed. The recursive
case matches any non-empty list as (_:xs) -- the underscore discards
the head's actual value entirely, since myLength doesn't need to know
WHAT the head is, only that there is one -- and adds 1 for it, then
recurses on the tail xs. Each recursive call peels off exactly one
element until the list is exhausted and the base case is finally hit.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reimplements a real standard-library function (length) using
only the base-case/recursive-case pattern the chapter introduces,
with a discard pattern used correctly since the head's value itself
is never needed.