Diffing_with_keys(3)

💅 NAME

okay so bestie, welcome to the Diffing_with_keys(3) page from the OCaml library ✨. It's literally about when you're diffing lists where each element has a distinct key, and we can refine the diffing patch by introducing two composite edit moves: swaps and moves. it's giving refined-patch energy, fr fr.

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🧚 Module

okay the module itself is Module Diffing_with_keys and it's just:

Module Diffing_with_keys : sig end

no cap, that's the whole signature vibe check 💅.

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✨ Documentation

okay so Module Diffing_with_keys is right here:

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.

Swaps exchange the position of two elements. Swap cost is set to 2 * change - epsilon.

Moves 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 Swaps and Moves can be computed directly and cheaply from the original optimal patch.

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here's the with_pos situation, bestie 🎀:

type 'a with_pos = {
 pos : int ;
 data : 'a ;
 }

and the val:

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

then we got the mismatch type, which is giving "whoops that's not it" energy 😭:

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 ;
 }

and the change type, the main character of the whole page 👑:

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 ✨.

also prefix is right here:

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

and Define is like:

module Define : (D : sig end) -> sig end

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bestie reminder: every type, field, constructor, val, module signature, and cost formula above is exactly the technical tea — don't rename or merge anything, okay? 🫶

mwah 💖 that's the whole page! it came from OCamldoc, version/date 2026-06-22, and it was Diffing_with_keys(3) the entire time. periodt. 🌈