A ppx writer that generates hash functions from type expressions and definitions.

Type definitions: [@@deriving hash] Expressions: [%hash_fold: TYPE] and [%hash: TYPE] Types, record fields: [@hash.ignore]

    type t = {
      s : string;
      x : (int * bool) list;
    }
    [@@deriving hash]

This will generate a function hash_fold_t : Hash.state -> t -> Hash.state.

Where Hash is Ppx_hash_lib.Std.Hash.

The generated function follows the structure of the type; allowing user overrides at every level. This is in contrast to ocaml's builtin polymorphic hashing Hashtbl.hash which ignores user overrides.

Also generated is a direct hash-function hash : t -> Hash.hash_value. This function will be named hash_<T> when != "t".

The direct hash function is the one suitable for Hashable.Make.

In a module signature, type t [@@deriving hash] will add both hash_fold_t and hash (or hash_<T>) to the signature.

A folding hash function is accessed/created as [%hash_fold: TYPE]. A direct hash function is accessed/created as [%hash: TYPE].

Types can be annotated with [@hash.ignore] so that they are not incorporated into the computed hash value.

  type second_only = (string [@hash.ignore]) * int [@@deriving hash]

Mutable records fields must have such an annotation.

    type t = {
      mutable s : string [@hash.ignore];
      x : (int * bool) list;
    }
    [@@deriving hash]

The annotation [@compare.ignore] (and [@ignore]) implies [@hash.ignore], in order to preserve the invariant that compare x y = 0 implies hash x = hash y.

So long as all types in support hashing, the following common pattern:

    module T = struct
      type t = <TYPE-EXP> [@@deriving compare, sexp]
      let hash = Hashtbl.hash
    end
    include T
    include Hashable.Make (T)

Can this be written as:

    module T = struct
      type t = <TYPE-EXP> [@@deriving compare, hash, sexp]
    end
    include T
    include Hashable.Make (T)

ppx/ppx_hash/doc/design.notes
ppx/ppx_hash/runtime-lib/make_std.ml