Add Variants

Author: gvergnaud

src/index.ts

@@ -3,3 +3,4 @@ import * as Pattern from './patterns';
 export { match } from './match';
 export { isMatching } from './is-matching';
 export { Pattern, Pattern as P };
+export { Variant, implementVariants } from './variants';

src/variants.ts

@@ -0,0 +1,41 @@
+import { Compute } from './types/helpers';
+import { Pattern } from './types/Pattern';
+
+export type Variant<k, d = never> = Compute<{ tag: k; value: d }>;
+
+/**
+ * VariantPatterns can be used to match a Variant in a
+ * `match` expression.
+ */
+type VariantPattern<k, p> = { tag: k; value: p };
+
+type AnyVariant = Variant<string, unknown>;
+
+type Narrow<variant extends AnyVariant, k extends variant['tag']> = Extract<
+  variant,
+  Variant<k, unknown>
+>;
+
+type Constructor<k, v> = [v] extends [never]
+  ? () => Variant<k>
+  : unknown extends v
+  ? <t>(value: t) => Variant<k, t>
+  : {
+      (value: v): Variant<k, v>;
+      <p extends Pattern<v>>(pattern: p): VariantPattern<k, p>;
+    };
+
+type Impl<variant extends AnyVariant> = {
+  [k in variant['tag']]: Constructor<k, Narrow<variant, k>['value']>;
+};
+
+export function implementVariants<variant extends AnyVariant>(): Impl<variant> {
+  return new Proxy({} as Impl<variant>, {
+    get: <k extends keyof Impl<variant>>(_: Impl<variant>, tag: k) => {
+      return (...args: [value?: Narrow<variant, k>['value']]) => ({
+        tag,
+        ...(args.length === 0 ? {} : { value: args[0] }),
+      });
+    },
+  });
+}

tests/variants.test.ts

@@ -0,0 +1,166 @@
+import { match, Variant, implementVariants, P } from '../src';
+
+// APP code
+type Shape =
+  | Variant<'Circle', { radius: number }>
+  | Variant<'Square', { sideLength: number }>
+  | Variant<'Rectangle', { x: number; y: number }>
+  | Variant<'Blob', number>;
+
+type Maybe<T> = Variant<'Just', T> | Variant<'Nothing'>;
+
+const { Just, Nothing } = implementVariants<Maybe<unknown>>();
+const { Circle, Square, Rectangle, Blob } = implementVariants<Shape>();
+
+describe('Variants', () => {
+  it('should work with exhaustive matching', () => {
+    const area = (x: Shape) =>
+      match(x)
+        .with(Circle({ radius: P.select() }), (radius) => Math.PI * radius ** 2)
+        .with(Square(P.select()), ({ sideLength }) => sideLength ** 2)
+        .with(Rectangle(P.select()), ({ x, y }) => x * y)
+        .with(Blob(P._), ({ value }) => value)
+        .exhaustive();
+
+    expect(area(Circle({ radius: 1 }))).toEqual(Math.PI);
+    expect(area(Square({ sideLength: 10 }))).toEqual(100);
+    expect(area(Blob(0))).toEqual(0);
+
+    // @ts-expect-error
+    expect(() => area({ tag: 'UUUPPs' })).toThrow();
+  });
+
+  it('should be possible to nest variants in data structures', () => {
+    const shapesAreEqual = (a: Shape, b: Shape) =>
+      match({ a, b })
+        .with(
+          {
+            a: Circle({ radius: P.select('a') }),
+            b: Circle({ radius: P.select('b') }),
+          },
+          ({ a, b }) => a === b
+        )
+        .with(
+          {
+            a: Rectangle(P.select('a')),
+            b: Rectangle(P.select('b')),
+          },
+          ({ a, b }) => a.x === b.x && a.y === b.y
+        )
+        .with(
+          {
+            a: Square({ sideLength: P.select('a') }),
+            b: Square({ sideLength: P.select('b') }),
+          },
+          ({ a, b }) => a === b
+        )
+        .with(
+          {
+            a: Blob(P.select('a')),
+            b: Blob(P.select('b')),
+          },
+          ({ a, b }) => a === b
+        )
+        .otherwise(() => false);
+
+    expect(
+      shapesAreEqual(Circle({ radius: 2 }), Circle({ radius: 2 }))
+    ).toEqual(true);
+    expect(
+      shapesAreEqual(Circle({ radius: 2 }), Circle({ radius: 5 }))
+    ).toEqual(false);
+    expect(
+      shapesAreEqual(Square({ sideLength: 2 }), Circle({ radius: 5 }))
+    ).toEqual(false);
+  });
+
+  it('Variants with type parameters should work', () => {
+    const toString = (maybeShape: Maybe<Shape>) =>
+      match(maybeShape)
+        .with(Nothing(), () => 'Nothing')
+        .with(
+          Just(Circle({ radius: P.select() })),
+          (radius) => `Just Circle { radius: ${radius} }`
+        )
+        .with(
+          Just(Square(P.select())),
+          ({ sideLength }) => `Just Square sideLength: ${sideLength}`
+        )
+        .with(
+          Just(Rectangle(P.select())),
+          ({ x, y }) => `Just Rectangle { x: ${x}, y: ${y} }`
+        )
+        .with(Just(Blob(P.select())), (area) => `Just Blob { area: ${area} }`)
+        .exhaustive();
+
+    expect(toString(Just(Circle({ radius: 20 })))).toEqual(
+      `Just Circle { radius: 20 }`
+    );
+    expect(toString(Nothing())).toEqual(`Nothing`);
+  });
+
+  it('should be possible to put a union type in a variant', () => {
+    // with a normal union
+
+    const maybeAndUnion = (
+      x: Maybe<{ type: 't'; value: string } | { type: 'u'; value: number }>
+    ) =>
+      match(x)
+        .with(Nothing(), () => 'Non')
+        .with(
+          Just({ type: 't' as const, value: P.select() }),
+          (x) => 'typeof x: string'
+        )
+        .with(
+          Just({ type: 'u' as const, value: P.select() }),
+          (x) => 'typeof x: number'
+        )
+        .exhaustive();
+
+    expect(maybeAndUnion(Nothing())).toEqual('Non');
+    expect(maybeAndUnion(Just({ type: 't', value: 'hello' }))).toEqual(
+      'typeof x: string'
+    );
+    expect(maybeAndUnion(Just({ type: 'u', value: 2 }))).toEqual(
+      'typeof x: number'
+    );
+  });
+
+  it('should be possible to create a variant with several type parameters', () => {
+    // Result
+    type Result<E, A> = Variant<'Success', A> | Variant<'Err', E>;
+
+    const { Success, Err } = implementVariants<Result<unknown, unknown>>();
+
+    type SomeRes = Result<string, { hello: string }>;
+
+    const x = true ? Success({ hello: 'coucou' }) : Err('lol');
+
+    const y: SomeRes = x;
+
+    const complexMatch = (x: Result<string, { shape: Shape }>) => {
+      return match(x)
+        .with(Err(P.select()), (msg) => `Error: ${msg}`)
+        .with(
+          Success({ shape: Circle(P.select()) }),
+          ({ radius }) => `Circle ${radius}`
+        )
+        .with(
+          Success({ shape: Square(P.select()) }),
+          ({ sideLength }) => `Square ${sideLength}`
+        )
+        .with(Success({ shape: Blob(P.select()) }), (area) => `Blob ${area}`)
+        .with(
+          Success({ shape: Rectangle(P.select()) }),
+          ({ x, y }) => `Rectangle ${x + y}`
+        )
+        .exhaustive();
+    };
+
+    expect(complexMatch(Success({ shape: Circle({ radius: 20 }) }))).toEqual(
+      'Circle 20'
+    );
+    expect(complexMatch(Success({ shape: Blob(20) }))).toEqual('Blob 20');
+    expect(complexMatch(Err('Failed'))).toEqual('Error: Failed');
+  });
+});