Generation of comparison functions from types
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README.md

ppx_compare

Generation of fast comparison functions from type expressions and definitions.

Ppx_compare is a ppx rewriter that derives comparison functions from type representations. The scaffolded functions are usually much faster than ocaml's Pervasives.compare. Scaffolding functions also gives you more flexibility by allowing you to override them for a specific type and more safety by making sure that you only compare comparable values.

Syntax

Type definitions: [@@deriving compare] Expressions: [%compare: TYPE] and [%compare.equal: TYPE] Record fields: [@compare.ignore]

Basic usage

We use ppx_deriving/ppx_type_conv, so type definitions are annotated this way:

type s = v * w [@@deriving compare]

This will generate compare_s : s -> s -> int function that relies on compare_v : v -> v -> int and compare_w : w -> w -> int.

Compare is not DWIM (do what I mean): it will scaffold a fast well behaved comparison (reflexive, transitive, symmetric...) function however it does not try to follow any "natural ordering". For instance arrays of characters are not sorted lexicographically.

Base types (options,int,array,lists,char,floats...) have the same comparison order as Pervasives.compare (provided their type parameters also do for the polymorphic ones).

Records fields are compared in the order they are defined (left to right); tuples fields are compared left to right. When we compare two branches of a sum whichever ones comes first in the definition is considered lowest. Variants compare in the order they are listed (increasing top-to-bottom). Polymorphic variants use the same ordering as the ocaml runtime.

Calling compare for type ts

In compliance (or conformance) with Janestreet's coding standard we assume that type named t are the main types in a module and

type t = S.t * T.t [@@deriving compare]

will call the functions S.compare and T.compare instead of calling S.compare_t and T.compare_t. This will also generate a compare : t -> t -> int function.

Signature

type t [@@deriving compare] in a module signature will add val compare : t -> t -> int in the signature.

Comparison without a type definition

Sometimes you just want a comparison without having to create a new type. You can create such a comparison function using the [%compare: ..] extension point:

let gt x y = [%compare: float * int * [`A | `B | `C] ] x y

You can use the type _, in which case the corresponding values will be ignored (i.e. compared using fun _ _ -> 0). For instance:

assert ([%compare: _ list] [ true ] [ false ] = 0);
assert ([%compare: _ list] [] [ false ] <> 0);

You can also check for equality using [%compare.equal: ..], which produces a function that returns true precisely when [%compare: ..] returns 0.

Special support for record fields

The comparison ignores record fields which are annotated with [@compare.ignore].

    type t =
      { a : float  [@compare.ignore]
      ; b : string
      }
    [@@deriving compare]