FoldLaws

Law equations for a Fold[S, A] — a read-only optic whose carrier F has ForgetfulFold[F] and whose write-side is pinned to Unit.

These laws encode the two universal properties of any monoidal fold — foldMap(const(mempty)) collapses to mempty, and foldMap is a monoid homomorphism in the target monoid. The homomorphism property is stated here (additive Int monoid, sufficient to witness the law); see also dev.constructive.eo.laws.eo.FoldMapHomomorphismLaws which states it for any optic with ForgetfulFold[F] and is reused by Traversal / Lens. FoldLaws re-states the empty-constant variant here because it is a genuine stand-alone property of Fold that does not need the homomorphism law's two-function phrasing.

Note: FoldLaws does not include a foldMap consistency check against an externally-supplied Foldable[F]. For Fold.apply[F, A] that consistency is a corollary of how the carrier Forget[F] wires its ForgetfulFold instance through Foldable[F].foldMap. Users who want that check should exercise it as a spec-level property test against the concrete constructor.