Interface Foldable<A>

Foldable type class represents data structures that can be folded to a summary value.

This interface provides the universal fold operations (foldLeft, foldRight) that work consistently across all data structures. The fold method is intentionally excluded because it has different semantics for sum types vs collections:

• Sum types (Option, Either, Try): fold(onEmpty, onValue) — pattern match
• Collections (List, Set, Map): fold(initial, fn) — left-reduce accumulator

Each type category defines its own fold with the appropriate signature.