package ppx_hash
Install
Dune Dependency
Authors
Maintainers
Sources
sha256=878f2ef0312d27cd14ed491e56f0a7e696710dc43bbf16e0f35cd6446f5a676d
md5=44b6a5acdd7eabe11348499f4ef2a054
README.md.html
title: ppx_hash parent: ../README.md
A ppx writer that generates hash functions from type expressions and definitions.
Syntax
Type definitions: [@@deriving hash]
Expressions: [%hash_fold: TYPE]
and [%hash: TYPE]
Record fields: [@no_hashing]
Basic usage
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
, as a wrapper around the hash-fold function.. (named hash_<T>
when
The direct hash function is the one suitable for Hashable.Make
.
Signature
In a module signature, type t [@@deriving hash]
will add both hash_fold_t
and hash
(or hash_<T>
) to the signature.
Hashing without a type definition
A folding hash function is accessed/created as [%hash_fold: TYPE]
. A direct hash function is accessed/created as [%hash: TYPE]
.
Special support for record fields
Record fields can be annotated with [@no_hashing]
so that they are not incorporated into the computed hash value. In the case of mutable fields, there must be such an annotation.
type t = {
mutable s : string; [@no_hashing]
x : (int * bool) list;
} [@@deriving hash]
Adapting code to ppx_hash
So long as all types in
module T = struct
type t = <TYPE-EXP> [@@deriving sexp, compare]
let hash = Hashtbl.hash
end
include T
include Hashable.Make(T)
Can this be written as:
module T = struct
type t = <TYPE-EXP> [@@deriving sexp, compare, hash]
end
include T
include Hashable.Make(T)
More information
ppx/ppx_hash/doc/design.notes
ppx/ppx_hash/runtime-lib/make_std.ml