#### ppx_hash

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

### Syntax

Type definitions: `[@@deriving hash]`

Expressions: `[%hash_fold: TYPE]`

and `[%hash: TYPE]`

Types, record fields: `[@hash.ignore]`

### 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`

. This function will be 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]`

.

### Ignoring part of types

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]
```

### Special support for `ppx_compare`

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`

.

### Adapting code to `ppx_hash`

So long as all types in

```
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)
```

### More information

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

>= "0.23.0"

>= "2.0.0"

>= "v0.15" & < "v0.16"

>= "v0.15" & < "v0.16"

>= "v0.15" & < "v0.16"

>= "4.08.0"

>= "v0.15.0"

>= "0.6"

>= "0.5.0" & < "0.5.2"

< "v0.14.0"

>= "v0.15.0"

>= "v0.15.0"