# Module Diffing_with_keys

    module Diffing_with_keys: sig .. end

When diffing lists where each element has a distinct key, we can refine the diffing patch by introducing two composite edit moves: swaps and moves.

`Swap`s exchange the position of two elements. `Swap` cost is set to `2 * change - epsilon`. `Move`s change the position of one element. `Move` cost is set to `delete + addition - epsilon`.

When the cost `delete + addition` is greater than `change` and with those specific weights, the optimal patch with `Swap`s and `Move`s can be computed directly and cheaply from the original optimal patch.

    type 'a with_pos = {

`` | `pos : `int`;`
`` | `data : `'a`;`

`}`

    val with_pos : 'a list -> 'a with_pos list

    type ('l, 'r, 'diff) mismatch =

`|` | `Name of `{`
`` | `pos : `int`;`
`` | `got : `string`;`
`` | `expected : `string`;`
`` | `types_match : `bool`;`
`}``
`|` | `Type of `{`
`` | `pos : `int`;`
`` | `got : `'l`;`
`` | `expected : `'r`;`
`` | `reason : `'diff`;`
`}``

    type ('l, 'r, 'diff) change =

`|` | `Change of `('l, 'r, 'diff) mismatch``
`|` | `Swap of `{`
`` | `pos : `int * int`;`
`` | `first : `string`;`
`` | `last : `string`;`
`}``
`|` | `Move of `{`
`` | `name : `string`;`
`` | `got : `int`;`
`` | `expected : `int`;`
`}``
`|` | `Insert of `{`
`` | `pos : `int`;`
`` | `insert : `'r`;`
`}``
`|` | `Delete of `{`
`` | `pos : `int`;`
`` | `delete : `'l`;`
`}``

This specialized version of changes introduces two composite changes: `Move` and `Swap`

    val prefix : ('l, 'r, 'diff) change Format_doc.printer

    module Define: functor (D : Diffing.Defs  with type eq := unit) -> sig .. end