The ! marker indicates injectivity. See https://github.com/ocaml/ocaml/pull/9500 and more details in Jacques Garrigue’s upcoming ML Workshop 2020 presentation https://icfp20.sigplan.org/details/mlfamilyworkshop-2020-papers/8/Tracking-injectivity-and-nominality-beyond-abstraction
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I just realized that since I’m using Async already, I can use Core_kernel.Unique_id (which uses the counter method).