A total ordering function over the keys. This is a two-argument function f such that f e1 e2 is zero if the keys e1 and e2 are equal, f e1 e2 is strictly negative if e1 is smaller than e2, and f e1 e2 is strictly positive if e1 is greater than e2. Example: a suitable ordering function is the generic structural comparison function Stdlib.compare.

The equality predicate used to compare keys.

A hashing function on keys. It must be such that if two keys are equal according to equal, then they have identical hash values as computed by hash. Examples: suitable (equal, hash) pairs for arbitrary key types include

• ((=), hash) for comparing objects by structure (provided objects do not contain floats)
• ((fun x y -> compare x y = 0), hash) for comparing objects by structure and handling Stdlib.nan correctly
• ((==), hash) for comparing objects by physical equality (e.g. for mutable or cyclic objects).