Documentation

A type class that is inhabited by types that can compute a hash of their content.

The base instances of this class should only live in dedicated "hashing packages" such as unison-hashing-v2, whose types and implementations should never change.

Trivial wrapper instances can be written around these, but note these pipelines from MyType ==> SomeHashingType ==> Hash must take care not to change the MyType ==> SomeHashingType conversion, once written.

For example, we might have a representation of some namespace in memory

data Namespace = Namespace Terms Types OtherStuff CachesAndWhatnot

with a somewhat equivalent "hashing" type in some "hashing package", with a ContentAddressable instance

data HashingNamespace = Namespace Terms Types

We can of course make our own convenience instance

instance ContentAddressable Namespace where
  contentHash = contentHash . namespaceToHashingNamespace

But we must make sure that the implementation of namespaceToHashingNamespace never changes the fields in the corresponding HashingNamespace, even as features are added to or removed from Namespace.