The read surface the path functions need: exactly the four UGM
methods they call, nothing more. A host with its own index (an
adjacency table over 100,000 typed edges, say) can hand in an
object implementing just these four instead of building a UGM for
a read-only search, and the compiler, not a cast, keeps the
contract honest: the day a search reads a fifth method, this type
widens and every such host fails to typecheck rather than at
runtime. UGM satisfies it by construction (it is a Pick of UGM).
The read surface the path functions need: exactly the four UGM methods they call, nothing more. A host with its own index (an adjacency table over 100,000 typed edges, say) can hand in an object implementing just these four instead of building a UGM for a read-only search, and the compiler, not a cast, keeps the contract honest: the day a search reads a fifth method, this type widens and every such host fails to typecheck rather than at runtime.
UGMsatisfies it by construction (it is a Pick of UGM).