This project contains:

• mhash, to implement hashing for types in a way that is compatible with many concrete hashing functions
• ppx_deriving_mhash to automatically derive such hashers

The mhash library comes with an implementation of the FNV algorithm.

After adding (preprocess (pps ppx_deriving_hhash)) in dune, this should work:

# type foo = {
  x : int;
  y : string;
} [@@deriving mhash] ;;
type foo = { x : int; y : string; }
val hasher_foo : foo Mhash.hasher =
  {Mhash.hash_into = <fun>}
val hash_foo : algo:('a, 'b) Mhash.hash_algo -> foo -> 'b =
  <fun>

# #require "mhash.fnv";;
# hash_foo ~algo:Mhash_fnv.int {x=1; y="foo"};;
- : int = 3535931368763302282

• [@nohash] on a type will skip hashing for this type. Useful when a type declaration contains some function fields, or other fields that shouldn't affect the hash.
• [@hasher <expr>] forces the ppx to use <expr> as the hasher for the given type. The attribute for a type ty must be an expression of type ty Mhash.hasher