A practical mode system for recursive definitions

In call-by-value languages, some mutually-recursive value definitions can be safely evaluated to build recursive functions or cyclic data structures, but (let rec x = + 1) contain vicious circles and their evaluation fails at runtime. We propose a new static analysis check the absence of such runtime failures. present set declarative inference rules, prove its soundness with respect reference source-level semantics Nordlander, Carlsson, Gill (2008), show that it (right-to-left) directed into an algorithmic in surprisingly simple way. Our implementation this replaced existing used by OCaml programming language, fragile syntactic/grammatical criterion which let several subtle bugs slip through as language kept evolving. document issues arise when advanced features real-world functional (exceptions first-class modules, GADTs, etc.) interact safety checking for definitions.