12.2.1. Promises · Functional Programming in OCaml

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In Lwt, a promise is a write-once reference: a value that is permitted to mutate at most once. When created, it is like an empty box that contains nothing. We say that the promise is pending. Eventually the promise can be resolved, which is like putting something inside the box. Instead of being resolved, the promise can instead be rejected, in which case the box is filled with an exception. Regardless of whether the promise is resolved or rejected, once the box is filled, its contents may never change.For now, we will mostly forget about concurrency. Later we'll come back and add incorporate it. But there is one part of the design for concurrency that we need to address now. When we later start using functions for OS-provided concurrency, such as concurrent reads and writes from files, there will need to be a division of responsibilities:The client code that wants to make use of concurrency will need to access promises: query whether they are resolved or pending, and make use of the …