Lambda the Ultimate

Proving that particular methods within a code base are functionally pure - deterministic and side-effect free - would aid verification of security properties including function invertibility, reproducibility of computation, and safety of untrusted code execution. Until now it has not been possible to automatically prove a method is functionally pure within a high-level imperative language in wide use such as Java. We discuss a technique to prove that methods are functionally pure by writing programs in a subset of Java called Joe-E; a static verifier ensures that programs fall within the subset. In Joe-E, pure methods can be trivially recognized from their method signature.

The paper includes a nice discussion of security benefits that can stem from being able to identify pure functions (of course, it is not obvious that guarantees at the programming language level are maintained at the run time level). I am sure many here have opinions about whether it makes more sense to try to graft purity on an imperative language, exposing it as an added feature, or to move programmers to functional languages..