Add @siteimprove/alfa-selective package

packages/alfa-selective/package.json

@@ -0,0 +1,37 @@
+{
+  "$schema": "http://json.schemastore.org/package",
+  "name": "@siteimprove/alfa-selective",
+  "homepage": "https://siteimprove.com",
+  "version": "0.10.0",
+  "license": "MIT",
+  "description": "An implementation of a selective functor for modelling conditional function application",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/siteimprove/alfa.git",
+    "directory": "packages/alfa-selective"
+  },
+  "bugs": "https://github.com/siteimprove/alfa/issues",
+  "main": "src/index.js",
+  "types": "src/index.d.ts",
+  "files": [
+    "src/**/*.js",
+    "src/**/*.d.ts"
+  ],
+  "dependencies": {
+    "@siteimprove/alfa-either": "^0.10.0",
+    "@siteimprove/alfa-equatable": "^0.10.0",
+    "@siteimprove/alfa-functor": "^0.10.0",
+    "@siteimprove/alfa-hash": "^0.10.0",
+    "@siteimprove/alfa-json": "^0.10.0",
+    "@siteimprove/alfa-mapper": "^0.10.0",
+    "@siteimprove/alfa-predicate": "^0.10.0",
+    "@siteimprove/alfa-refinement": "^0.10.0"
+  },
+  "devDependencies": {
+    "@siteimprove/alfa-test": "^0.10.0"
+  },
+  "publishConfig": {
+    "access": "public",
+    "registry": "https://npm.pkg.github.com/"
+  }
+}

packages/alfa-selective/src/index.ts

@@ -0,0 +1 @@
+export * from "./selective";

packages/alfa-selective/src/selective.ts

@@ -0,0 +1,103 @@
+import { Either, Left, Right } from "@siteimprove/alfa-either";
+import { Equatable } from "@siteimprove/alfa-equatable";
+import { Functor } from "@siteimprove/alfa-functor";
+import { Hash, Hashable } from "@siteimprove/alfa-hash";
+import { Serializable } from "@siteimprove/alfa-json";
+import { Mapper } from "@siteimprove/alfa-mapper";
+import { Predicate } from "@siteimprove/alfa-predicate";
+import { Refinement } from "@siteimprove/alfa-refinement";
+
+export class Selective<S, T = never>
+  implements
+    Functor<T>,
+    Iterable<S | T>,
+    Equatable,
+    Hashable,
+    Serializable<Selective.JSON<S, T>> {
+  public static of<T>(value: T): Selective<T> {
+    return new Selective(Left.of(value));
+  }
+
+  private readonly _value: Either<S, T>;
+
+  private constructor(value: Either<S, T>) {
+    this._value = value;
+  }
+
+  public map<U>(mapper: Mapper<T, U>): Selective<S, U> {
+    return new Selective(
+      this._value.either(
+        (value) => Left.of(value) as Either<S, U>,
+        (value) => Right.of(mapper(value))
+      )
+    );
+  }
+
+  public if<P extends S, U>(
+    refinement: Refinement<S, P>,
+    mapper: Mapper<P, U>
+  ): Selective<Exclude<S, P>, T | U>;
+
+  public if<U>(
+    predicate: Predicate<S>,
+    mapper: Mapper<S, U>
+  ): Selective<S, T | U>;
+
+  public if<U>(
+    predicate: Predicate<S>,
+    mapper: Mapper<S, U>
+  ): Selective<S, T | U> {
+    return this._value.either(
+      (value) =>
+        predicate(value) ? new Selective(Right.of(mapper(value))) : this,
+      () => this
+    );
+  }
+
+  public else<U>(mapper: Mapper<S, U>): Selective<never, T | U> {
+    return new Selective<never, T | U>(
+      Right.of(
+        this._value.either<T | U>(
+          (value) => mapper(value),
+          (value) => value
+        )
+      )
+    );
+  }
+
+  public get(): S | T {
+    return this._value.get();
+  }
+
+  public equals<S, T>(value: Selective<S, T>): boolean;
+
+  public equals(value: unknown): value is this;
+
+  public equals(value: unknown): boolean {
+    return value instanceof Selective && value._value.equals(this._value);
+  }
+
+  public hash(hash: Hash): void {
+    this._value.hash(hash);
+  }
+
+  public *iterator(): Iterator<S | T> {
+    yield this._value.get();
+  }
+
+  public [Symbol.iterator](): Iterator<S | T> {
+    return this.iterator();
+  }
+
+  public toJSON(): Selective.JSON<S, T> {
+    return this._value.toJSON();
+  }
+
+  public toString(): string {
+    return `Selective { ${this._value} }`;
+  }
+}
+
+export namespace Selective {
+  export type JSON<S, T = never> = Either.JSON<S, T>;
+}

packages/alfa-selective/test/selective.spec.ts

@@ -0,0 +1,58 @@
+import { test } from "@siteimprove/alfa-test";
+
+import { Refinement } from "@siteimprove/alfa-refinement";
+
+import { Selective } from "../src/selective";
+
+const isFoo: Refinement<string, "foo"> = (string): string is "foo" =>
+  string === "foo";
+
+const isBar: Refinement<string, "bar"> = (string): string is "bar" =>
+  string === "bar";
+
+test("#if() conditionally applies a function to a selective value", (t) => {
+  Selective.of("foo")
+    .if(isFoo, (value) => {
+      t.equal(value, "foo");
+    })
+    .if(isBar, () => {
+      t.fail();
+    });
+});
+
+test(`#else() applies a function to a selective value that matched no other
+      conditions`, (t) => {
+  Selective.of("bar")
+    .if(isFoo, () => {
+      t.fail();
+    })
+    .else((value) => {
+      t.equal(value, "bar");
+    });
+});
+
+test("#get() returns the value of a selective", (t) => {
+  t.equal(
+    Selective.of("foo")
+      .if(isFoo, () => "was foo")
+      .get(),
+    "was foo"
+  );
+
+  t.equal(
+    Selective.of("bar")
+      .if(isFoo, () => "was foo")
+      .get(),
+    "bar"
+  );
+});
+
+test(`#map() applies a function to a matched selective value`, (t) => {
+  t.equal(
+    Selective.of("foo")
+      .if(isFoo, () => "was foo")
+      .map((string) => string.toUpperCase())
+      .get(),
+    "WAS FOO"
+  );
+});

packages/alfa-selective/tsconfig.json

@@ -0,0 +1,34 @@
+{
+  "$schema": "http://json.schemastore.org/tsconfig",
+  "extends": "../tsconfig.json",
+  "files": ["src/index.ts", "src/selective.ts", "test/selective.spec.ts"],
+  "references": [
+    {
+      "path": "../alfa-either"
+    },
+    {
+      "path": "../alfa-equatable"
+    },
+    {
+      "path": "../alfa-functor"
+    },
+    {
+      "path": "../alfa-hash"
+    },
+    {
+      "path": "../alfa-json"
+    },
+    {
+      "path": "../alfa-mapper"
+    },
+    {
+      "path": "../alfa-predicate"
+    },
+    {
+      "path": "../alfa-refinement"
+    },
+    {
+      "path": "../alfa-test"
+    }
+  ]
+}

packages/tsconfig.json

@@ -78,6 +78,7 @@
     { "path": "alfa-rules" },
     { "path": "alfa-scraper" },
     { "path": "alfa-sarif" },
+    { "path": "alfa-selective" },
     { "path": "alfa-selector" },
     { "path": "alfa-sequence" },
     { "path": "alfa-set" },