A type of elements (in a hash set) or keys (in a hash map).

An equivalence test on keys. The function equal x y returns true if and only if the keys x and y are equivalent. It is up to the user to define an equivalence relation on keys. In the simplest and most common case, equivalence is just equality.

A hash function on keys. This function must be compatible with equivalence: that is, it must be the case that equiv x y implies hash x = hash y.