Eq

Compare-by-key equality. Non-structural, so not available in on-chain code: on-chain equality is always structural (see Eq).

Derives an Eq instance for type A (case class, enum, or sealed trait)

Asserts that f implements structural equality for A and wraps it as an Eq[A].

This is the only way (besides derived) to introduce an Eq instance usable in on-chain code: the compiler plugin rejects bare lambdas because on-chain === and Eq-based operations are always lowered to structural comparison of the runtime representation — a non-structural f would be silently ignored there. By wrapping f you assert that f(a, b) iff a and b are structurally equal (a.toData == b.toData); the compiler is free to replace it with an equivalent representation-optimal comparison.