Challenge 3: A Missing Eq Constraint, Caught at Compile Time — Possible Solution
====================================================================

Broken attempt — Main.hs:

bothSame :: a -> a -> Bool   -- missing the Eq a => constraint
bothSame x y = x == y

Representative compile error:

    Main.hs:2:15: error:
        No instance for (Eq a) arising from a use of '=='
        Possible fix:
          add (Eq a) to the context of the type signature for:
            bothSame :: forall a. a -> a -> Bool

Fixed — Main.hs:

bothSame :: Eq a => a -> a -> Bool
bothSame x y = x == y

main :: IO ()
main = print (bothSame 5 5)

Output:

    True

Explanation:
The original signature `a -> a -> Bool` promises bothSame works for
EVERY possible type a, with no restriction at all -- but the function
body uses ==, which only exists for types that have an Eq instance.
GHC catches this contradiction directly: it can't prove == is legal
for an unconstrained, fully generic a, since some type without an Eq
instance could theoretically be substituted in. Adding `Eq a =>` to
the signature narrows the promise to only types that DO support ==,
which resolves the contradiction and lets the body type-check
correctly.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reproduces the exact "No instance for (Eq a)" compile error the
chapter's warn-box describes for a missing constraint, and the fix --
adding Eq a => -- resolves it precisely because it narrows the
function's promise to match what its own body actually requires.