A pattern for almost homomorphic functions

Modern type systems present the programmer with a trade-off between correctness and code complexity--more precise, or exact, types that allow only legal values prevent runtime errors while less precise types enable more reuse. Unfortunately, the software engineering benefits of reuse and avoiding duplicate code currently outweigh assurance gains of exact types. We factor out a pattern common in conversions that result from using exact types as a reusable function, extending existing generic programming techniques to avoid code duplication and enable reuse.

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