Symmetric Diffs for Belt Standard Library

ReasonML implementation of fast symmetric diffs for Sets and Maps. Library is designed to be used together with Belt standard library that is shipped with BuckleScript.

Algorithm was inspired by Jane Street's Base library (that unfortunately cannot be used in BuckleScript directly).

Installation

Install the package using npm:

npm install belt-diff

Add dependency to your bsconfig.json:

"bs-dependencies": [
  "belt-diff"
],

Usage

For more examples see examples folder.

Note: Diffing function is combined together with folding/reducing functionality to allow convenient direct accumulation of changes.

Set

Following example for string set accumulates all added values into list:

let oldSet = Belt.Set.String.fromArray([|"A", "B", "C"|]);
let newSet = Belt.Set.String.(oldSet->remove("A")->remove("C")->add("D")->add("C"));
let addedItems = BeltDiff.Set.String.symmetricDiff(oldSet, newSet, ~acc=[], ~f=(either, acc) =>
  switch (either) {
  | Left(a) =>
    Js.log("Element " ++ a ++ " is only in the old set.");
    acc;
  | Right(a) =>
    Js.log("Element " ++ a ++ " is only in the new set.");
    [a, ...acc];
  }
);
// > Element A is only in the old set.
//   Element D is only in the new set.

Js.log(Array.of_list(addedItems));
// > [ 'D' ]

Map

Argument ~veq specifies function for detecting equal values with signature ('value, 'value) => bool.

Following example for string map accumulates all added values:

let oldMap = Belt.Map.String.fromArray([|("A", 1), ("B", 2), ("C", 3)|]);
let newMap =
  Belt.Map.String.(oldMap->remove("A")->set("D", 4)->set("C", 5));

let addedItems =
  BeltDiff.Map.String.symmetricDiff(
    oldMap, newMap, ~acc=[], ~veq=(==), ~f=(diffRes, acc) =>
    switch (diffRes) {
    | (key, Left(_a)) =>
      Js.log("Item with key " ++ key ++ " is only in the old map.");
      acc;
    | (key, Right(a)) =>
      Js.log("Item with key " ++ key ++ " is only in the new map.");
      [a, ...acc];
    | (key, Unequal(a, b)) =>
      Js.log(
        "Item with key "
        ++ key
        ++ " is in both, but its value was changed from "
        ++ string_of_int(a)
        ++ " to "
        ++ string_of_int(b)
        ++ ".",
      );
      acc;
    }
  );
// > Item with key A is only in the old map.
//   Item with key C is in both, but its value was changed from 3 to 5.
//   Item with key D is only in the new map.

Js.log(Array.of_list(addedItems));
// > [ 4 ]

Development

Build

npm run build

Build + Watch

npm run start

Run example

E.g.:

node lib/js/examples/MapDiffExample.js