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Explore an invited talk by Richard A. Eisenberg from Jane Street that delves into the concept of type levels in type inference and generalization algorithms used in OCaml and GHC compilers. Learn how levels, originally pioneered by Didier Rémy in 1992 (then called ranks), serve as a powerful organizing principle in type inference implementation. Discover how levels increase as the algorithm descends into an abstract syntax tree, helping determine which type variables should be generalized. The presentation covers what levels are, their application in generalization, and additional uses found for them in both OCaml and GHC compilers beyond their original purpose. This hour-long talk was presented at the WITS 2025 workshop on January 20, 2025, sponsored by ACM SIGPLAN.
