The essence of monotonic state

Alexandre Pilkiewicz (1) , François Pottier (1)

1 GALLIUM - Programming languages, types, compilation and proofs

We extend a static type-and-capability system with new mechanisms for expressing the promise that a certain abstract value evolves monotonically with time; for enforcing this promise; and for taking advantage of this promise to establish non-trivial properties of programs. These mechanisms are independent of the treatment of mutable state, but combine with it to offer a flexible account of "monotonic state". We apply these mechanisms to solve two reasoning challenges that involve mutable state. First, we show how an implementation of thunks in terms of references can be assigned types that reflect time complexity properties, in the style of Danielsson (2008). Second, we show how an implementation of hash-consing can be assigned a specification that conceals the existence of an internal state yet guarantees that two pieces of input data receive the same code if and only if they are equal.

Mots clés: hash-consing; thunks; type-based complexity-checking; amortized complexity; mono-tonic state; hidden state; specification; capabilities; types

Domaines: Langage de programmation [cs.PL]

Alexandre Pilkiewicz, François Pottier. The essence of monotonic state. TLDI 2011: The Sixth ACM SIGPLAN Workshop on Types in Language Design and Implementation, Jan 2011, Austin, United States. ⟨10.1145/1929553.1929565⟩. ⟨hal-01081193⟩