ReScript-aware `memo`

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The current definition of React.memo is:

@module("react")
external memo: component<'props> => component<'props> = "memo"

which is incorrect. React.memo takes an equality function as an optional second param

https://react.dev/reference/react/memo

memo(SomeComponent, arePropsEqual?)

Fortunately, this can be easily fixed without breaking

Customizing the second argument is rare in regular JS/TS projects, but not in ReScript.

ReScript's powerful type system represents boxed objects at runtime. Ironically, this makes memo almost useless in ReScript projects, since the default for memo compares shallow equality.

E.g. Playground

Users can easily make deep-equality function

let equal = \"="
// this is confusing btw...

generates

import * as Caml_obj from "./stdlib/caml_obj.js";

var equal = Caml_obj.equal;

However the deep-equal implementation is usually not what React users want. This will be a huge overhead when dealing with data that is not a persistent structure.

Assuming the user still wants the shallow-equal, we can try a much more optimized solution. The idea is simple, making recursive shallow-equal, using well-known structures.

builtin shallowEqual function in React

// https://github.com/facebook/react/blob/857ee8c/packages/shared/shallowEqual.js#L18

// `is` and `hasOwnProperty` are polyfill of `Object` static methods
function shallowEqual(objA: mixed, objB: mixed): boolean {
  if (is(objA, objB)) {
    return true;
  }

  if (
    typeof objA !== 'object' ||
    objA === null ||
    typeof objB !== 'object' ||
    objB === null
  ) {
    return false;
  }

  const keysA = Object.keys(objA);
  const keysB = Object.keys(objB);

  if (keysA.length !== keysB.length) {
    return false;
  }

  // Test for A's keys different from B.
  for (let i = 0; i < keysA.length; i++) {
    const currentKey = keysA[i];
    if (
      !hasOwnProperty.call(objB, currentKey) ||
      // $FlowFixMe[incompatible-use] lost refinement of `objB`
      !is(objA[currentKey], objB[currentKey])
    ) {
      return false;
    }
  }

  return true;
}

modified for ReScript outputs

function shallowEqualPolyvar(objA: polyvar, objB: polyvar) {
  if (objA.NAME !== objB.NAME) {
    return false;
  }
  return shallowEqual(objA, objB);
}

function shallowEqualVariant(objA: variant, objB: variant) {
  // Ok... this should be done by the compiler
  if (objA.TAG !== objB.TAG) {
    return false;
  }
  return shallowEqual(objA._0, objB._0)
}

function shallowEqual(objA: mixed, objB: mixed): boolean {
  if (is(objA, objB)) {
    return true;
  }

  if (
    typeof objA !== 'object' ||
    objA === null ||
    typeof objB !== 'object' ||
    objB === null
  ) {
    return false;
  }
  
  // We cannot skip check because there is no guarantee objs are record.
  // Or maybe we can make separate function for it.

  // isPolyvar should be provided by the compiler
  const isObjAPolyvar = isPolyvar(objA)
  const isObjBPolyvar = isPolyvar(objB)
  if (isObjAPolyvar !== isObjBPolyvar) {
    return false;
  } else if (isObjAPolyvar) {
    return shallowEqualPolyvar(objA, objB);
  }

  // isVariant should be provided by the compiler
  const isObjAVariant = isVariant(objA)
  const isObjBVariant = isVariant(objB)
  if (isObjAVariant !== isObjBVariant) {
    return false;
  } else if (isObjAVariant) {
    return shallowEqualVariant(objA, objB)
  }

  const keysA = Object.keys(objA);
  const keysB = Object.keys(objB);

  if (keysA.length !== keysB.length) {
    return false;
  }

  // Test for A's keys different from B.
  for (let i = 0; i < keysA.length; i++) {
    const currentKey = keysA[i];
    if (
      !hasOwnProperty.call(objB, currentKey) ||
      !is(objA[currentKey], objB[currentKey])
    ) {
      return false;
    }
  }

  return true;
}

it seems like compiler support is needed first 😅

Inizia individuando il binding ReScript che definisce React.memo e consulta il riferimento a React.memo collegato nell’issue. Determina quale supporto del compilatore è necessario per il comportamento proposto di uguaglianza superficiale consapevole di ReScript; il lavoro è considerato completato quando esistono un design concordato e le modifiche corrispondenti al binding e al compilatore, ma l’issue non indica file o test.

Scritto dal modello di indicizzazione a partire dal testo della issue.

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