refactor: replace instanceof with symbol-based brands on class objects

src/boolean/__test__/type.spec.js

@@ -19,7 +19,7 @@ describe('boolean type functions', () => {
    })
    it('converts any type to boolean', () => {
       for (const T in math.types) {
-         if (T instanceof Type) assert(boolean.resolve([T]))
+         if (Type.holds(T)) assert(boolean.resolve([T]))
       }
    })       
 })

src/core/Implementations.js

@@ -1,13 +1,23 @@
+const implementationsBrand = Symbol()
 export class Implementations {
    constructor(impOrImps) {
       if (Array.isArray(impOrImps)) {
          this.matchers = impOrImps
       } else this.matchers = [impOrImps]
+      this[implementationsBrand] = true
    }
+
+   // Returns true if entity is an Implementations
+   static holds(entity) {return entity[implementationsBrand]}
 }
 
+const igBrand = Symbol()
 export class ImplementationsGenerator {
    constructor(f) {
       this.generate = f
+      this[igBrand] = true
    }
+
+   // Returns true if entity is an ImplementationsGenerator
+   static holds(entity) {return entity[igBrand]}
 }

src/core/Type.js

@@ -1,5 +1,7 @@
 import ArrayKeyedMap from 'array-keyed-map'
 
+const typeBrand = Symbol() // invisible outside this file
+
 // Generic types are callable, so we have no choice but to extend Function
 export class Type extends Function {
    constructor(f, options = {}) {
@@ -19,6 +21,7 @@ export class Type extends Function {
          }
       })
 
+      this[typeBrand] = true
       this.test = f
       // we want property `from` to end up as an array of Matchers:
       this.from = options.from
@@ -57,15 +60,16 @@ export class Type extends Function {
       return rewired
    }
 
-   toString() {
-      return this.typeName || `[Type ${this.test}]`
-   }
+   toString() {return this.typeName || `[Type ${this.test}]`}
+
+   // Returns true if entity is a Type
+   static holds(entity) {return entity[typeBrand]}
 }
 
 export const Undefined = new Type(
    t => typeof t === 'undefined',
    {zero: undefined, one: undefined, nan: undefined})
-export const TypeOfTypes = new Type(t => t instanceof Type)
+export const TypeOfTypes = new Type(t => Type.holds(t))
 export const Unknown = new Type(() => true) // Danger, do not merge!
 Unknown._doNotMerge = true
 
@@ -74,7 +78,7 @@ export const OneOf = (...types) => {
    if (!types.length) {
       throw new RangeError('cannot choose OneOf no types at all')
    }
-   const nonType = types.findIndex(T => !(T instanceof Type))
+   const nonType = types.findIndex(T => !Type.holds(T))
    if (nonType >= 0) {
       throw new RangeError(
          `OneOf can only take type arguments, not ${types[nonType]}`)
@@ -106,7 +110,7 @@ export const ReturnType = f => f.returns ?? Unknown
 
 export const whichType = typs => Returns(TypeOfTypes, item => {
    for (const type of Object.values(typs)) {
-      if (!(type instanceof Type)) continue
+      if (!Type.holds(type)) continue
       if (type.test(item)) return type.refine(item, whichType(typs))
    }
    let errorMsg = ''

src/core/TypeDispatcher.js

@@ -81,10 +81,10 @@ export class TypeDispatcher {
          // For special cases like types, config, etc, we can wrap
          // a function in ImplementationsGenerator to produce the thing
          // we should really merge:
-         if (val instanceof ImplementationsGenerator) val = val.generate()
+         if (ImplementationsGenerator.holds(val)) val = val.generate()
 
          // Now dispatch on what sort of thing we are supposed to merge:
-         if (val instanceof Type) {
+         if (Type.holds(val)) {
             if (val._doNotMerge) {
                throw new TypeError(`attempt to merge unusable type '${val}'`)
             }
@@ -105,13 +105,13 @@ export class TypeDispatcher {
 
          // Everything else we coerce into Implementations and deal with
          // right here:
-         if (val instanceof Matcher) val = new Implementations(val)
+         if (Matcher.holds(val)) val = new Implementations(val)
          if (Array.isArray(val)) val = new Implementations(val)
          if (isPlainFunction(val)) {
             throw new RangeError(
                `function value for ${key} must be merged within a 'match' call`)
          }
-         if (!(val instanceof Implementations)) {
+         if (!Implementations.holds(val)) {
             val = new Implementations(match(Passthru, val))
          }
 
@@ -307,7 +307,7 @@ export class TypeDispatcher {
          }
          if (generatingDeps
              && typeof result === 'object'
-             && !(result instanceof Type)
+             && !Type.holds(result)
          ) {
             return DependencyRecorder(result, key, this, bhvix)
          }
@@ -354,7 +354,7 @@ export class TypeDispatcher {
          behave.set(bhvix, item)
          if (generatingDeps
              && typeof item === 'object'
-             && !(item instanceof Type)
+             && !Type.holds(item)
          ) {
             return DependencyRecorder(item, key, this, bhvix)
          }
@@ -469,7 +469,8 @@ const DependencyRecorder = (object, path, repo, bhvix) => new Proxy(object, {
       const result = Reflect.get(target, prop, receiver)
       // pass internal methods through, as well as resolve calls,
       // since we record dependencies within the latter:
-      if (prop.startsWith('_')
+      if (typeof prop === 'symbol'
+          || prop.startsWith('_')
           || prop === 'resolve'
           || (typeof result === 'function' && 'isDispatcher' in result)
       ) {
@@ -484,7 +485,7 @@ const DependencyRecorder = (object, path, repo, bhvix) => new Proxy(object, {
       // dependencies on its properties (e.g. math.config.predictable)
       // So proxy the return value, except for types, which must maintain
       // strict referential identity:
-      if (typeof result === 'object' && !(result instanceof Type)) {
+      if (typeof result === 'object' && !Type.holds(result)) {
          return DependencyRecorder(result, newPath, repo, bhvix)
       } else return result
    }
@@ -508,7 +509,7 @@ const DependencyWatcher = (object, path, ixList, repo) => new Proxy(object, {
    get(target, prop, receiver) {
       // Only thing we need to do is push the watching down
       const result = Reflect.get(target, prop, receiver)
-      if (typeof result === 'object' && !(result instanceof Type)) {
+      if (typeof result === 'object' && !Type.holds(result)) {
          const newPath = [path, prop].join('.')
          return DependencyWatcher(result, newPath, ixList, repo)
       }

src/core/TypePatterns.js

@@ -1,7 +1,12 @@
 import {ReturnType, Type, Undefined, Unknown} from './Type.js'
 import {isPlainFunction} from './helpers.js'
 
+const tpBrand = Symbol()
+
 export class TypePattern {
+   constructor() {
+      this[tpBrand] = true
+   }
    match(_typeSequence, _options={}) {
       throw new Error('Specific TypePatterns must implement match')
    }
@@ -10,6 +15,9 @@ export class TypePattern {
    }
    equal(other) {return other.constructor === this.constructor}
    toString() {return 'Abstract Pattern (?!)'}
+
+   // Returns true if entity is a TypePattern
+   static holds(entity) {return entity[tpBrand]}
 }
 
 class MatchTypePattern extends TypePattern {
@@ -91,8 +99,8 @@ class PredicatePattern extends TypePattern {
 }
 
 export const pattern = patternOrSpec => {
-   if (patternOrSpec instanceof TypePattern) return patternOrSpec
-   if (patternOrSpec instanceof Type) {
+   if (TypePattern.holds(patternOrSpec)) return patternOrSpec
+   if (Type.holds(patternOrSpec)) {
       return new MatchTypePattern(patternOrSpec)
    }
    if (Array.isArray(patternOrSpec)) return new SequencePattern(patternOrSpec)
@@ -206,11 +214,16 @@ export const needsCollection = (template) => {
    return 'actual' in template
 }
 
+const matcherBrand = Symbol()
 export class Matcher {
    constructor(spec, facOrBehave) {
       this.pattern = pattern(spec)
       this.does = facOrBehave
+      this[matcherBrand] = true
    }
+
+   // Returns true if entity is a matcher
+   static holds(entity) {return entity[matcherBrand]}
 }
 
 export const match = (spec, facOrBehave) => new Matcher(spec, facOrBehave)

src/core/__test__/helpers.spec.js

@@ -7,8 +7,8 @@ import {match, Matcher, TypePattern} from '../TypePatterns.js'
 describe('Core helpers', () => {
    it('defines what Matchers are', () => {
       const matcher = match([TypeOfTypes, Undefined], -3)
-      assert(matcher instanceof Matcher)
-      assert(matcher.pattern instanceof TypePattern)
+      assert(Matcher.holds(matcher))
+      assert(TypePattern.holds(matcher.pattern))
    })
 
    it('detects plain objects', () => {