Challenge 3: An Incomplete Pattern, Caught by -Wincomplete-patterns — Possible Solution
====================================================================

Broken attempt — Main.hs:

data Shape = Circle Double | Rectangle Double Double

area :: Shape -> Double
area (Circle r) = pi * r * r
-- Rectangle case deliberately omitted

Terminal:

    $ ghc -Wincomplete-patterns Main.hs

Representative warning:

    Main.hs:3:1: warning: [-Wincomplete-patterns]
        Pattern match(es) are non-exhaustive
        In an equation for 'area':
            Patterns not matched: Rectangle _ _

Fixed — Main.hs:

data Shape = Circle Double | Rectangle Double Double

area :: Shape -> Double
area (Circle r) = pi * r * r
area (Rectangle w h) = w * h

Recompiling with -Wincomplete-patterns now produces no warning at all.

Explanation:
GHC's exhaustiveness checker inspects every constructor Shape actually
has (Circle and Rectangle) and compares that against the patterns
area actually covers. Since only Circle is handled, GHC identifies
the exact missing pattern -- Rectangle _ _ -- and reports it by name,
rather than a vague "might be incomplete" message. Adding the missing
equation resolves the warning entirely, since every constructor now
has a matching pattern.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reproduces the chapter's own missing-Rectangle-case example
exactly, compiles it with the recommended -Wincomplete-patterns flag
to get the real warning text, and then fixes it by adding the missing
equation, confirming the warning disappears.