One of the things that puzzles me about this library is its intended purpose. "Types don't know #" was clearly about hash types for use in hash tables, which explained why keying the hashes was mandatory. But it is unclear to me why MD5, SHA-1, and other cryptographic hashes are present here; [...] I believe the HashAlgorithm concept is too overloaded. It _requires_ that hashes be both keyable and extendable-output, [...] A user that wants to use this library with a hash that is not included here is going to be pushed to include functionality the hash does not provide and that they might not know how to achieve.
That is actually a good point: Why are those requirements on the HashAlgorithm concept at all? The library is (mostly) about allowing to hash types, i.e. the framework surrounding `hash_append`. I understand that a specific way (currently being discussed here on the list) of construction is required for implementing `hash_append` for unordered containers. But besides that, why does the library require specific constructors at all? The HashAlgorithm is provided by the user as an instance. How the users constructed that isn't relevant for the purpose of the library, is it? To my understanding having support for seeds could be highlighted as best practice for specific use cases, but not as a general requirement. I understand that the library provides ways to extract a hash out of an Algorithm. So including `result_type` and `result` makes sense, although given that this is C++11 the former seems a bit outdated as it can be determined by the latter. But besides that there are no requirements on HashAlgorithm relied upon by the library, are there? For user convenience I can see `SeedableHashAlgorithm` as a subset of `HashAlgorithm` as a useful, additional concept to include in the library. The library could provide traits to check a given type for a specific concept (when lacking C++20 concepts) This would give users and hash authors more freedom in which algorithm is used. Of course it could be stated, that all included algorithms conform to the `SeedableHashAlgorithm` concept, if they (meaningfully) are.