fix: Don't forget branding in value type of Record.

[untio11]

Before this fix, branding information was lost design time on the key and value types of a Record type:

type BrandedString = The<typeof BrandedString>;
const BrandedString = string.withBrand('BrandedString');

type BrandedKVRecord = The<typeof BrandedKVRecord>;
const BrandedKVRecord = record(BrandedString, BrandedString);

// Old behaviour:
expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<string, string>>();

// New behaviour:
expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<string, BrandedString>>();

This same helper type (Unwidened<T>) can be used on the key type too, however, this breaks compatibility with .literal() because DeepUnbranded<T> can't really deal with branded key types yet.

[pavadeli]

I suggest we add some more tests. See my suggestion below. Unfortunately, not all of those tests pass right now.

Suggested change

	test('Branded types', () => {
	// Branded values have a particular interaction with the Record type.
	type BrandedString = The<typeof BrandedString>;
	const BrandedString = string.withBrand('BrandedString');
	type BrandedKVRecord = The<typeof BrandedKVRecord>;
	const BrandedKVRecord = record('BrandedKVRecord', BrandedString, BrandedString);
	// Currently, branded types are not supported as Record key types. They are instead widened to the unbranded base type:
	expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<string, BrandedString>>();
	// The problem with branded keytypes arises when trying to create a literal of the record type.
	expectTypeOf(
	// This `.literal()` would give a TS error because the `DeepUnbranding` can't deal with branded key types.
	BrandedKVRecord.literal({
	a: 'b',
	}),
	).toEqualTypeOf<Record<string, BrandedString>>();
	});
	// Branded values have a particular interaction with the Record type.
	describe('branded types', () => {
	test('Branded string', () => {
	type BrandedString = The<typeof BrandedString>;
	const BrandedString = string.withBrand('BrandedString');
	type BrandedKVRecord = The<typeof BrandedKVRecord>;
	const BrandedKVRecord = record('BrandedKVRecord', BrandedString, BrandedString);
	// Currently, branded types are not supported as Record key types. They are instead widened to the unbranded base type:
	expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<string, BrandedString>>();
	const branded = BrandedString('branded');
	const regular = String('abc');
	expectTypeOf({ [regular]: regular }).not.toMatchTypeOf<BrandedKVRecord>();
	expectTypeOf({ [regular]: branded }).toEqualTypeOf<BrandedKVRecord>();
	expectTypeOf({ [branded]: branded }).toEqualTypeOf<BrandedKVRecord>();
	// The problem with branded keytypes arises when trying to create a literal of the record type. We don't apply the `Narrow` utility to
	// the key type. This `.literal()` would give a TS error because the `DeepUnbranding` can't deal with branded key types.
	expectTypeOf(BrandedKVRecord.literal({ a: 'b' })).toEqualTypeOf<Record<string, BrandedString>>();
	});
	test('Branded union', () => {
	type IntOrUndefined = The<typeof IntOrUndefined>;
	const IntOrUndefined = int.or(undefinedType);
	expectTypeOf<IntOrUndefined>().toEqualTypeOf<int | undefined>();
	type BrandedKVRecord = The<typeof BrandedKVRecord>;
	const BrandedKVRecord = record('BrandedKVRecord', int, IntOrUndefined);
	// Currently, branded types are not supported as Record key types. They are instead widened to the unbranded base type:
	expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<number, int | undefined>>();
	});
	test('Branded object value', () => {
	type BrandedObject = The<typeof BrandedObject>;
	const BrandedObject = object({ a: string }).withBrand('BrandedObject');
	type BrandedVRecord = The<typeof BrandedVRecord>;
	const BrandedVRecord = record('BrandedVRecord', string, BrandedObject.or(literal('whatever')));
	expectTypeOf<BrandedVRecord>().toEqualTypeOf<Record<string, BrandedObject | 'whatever'>>();
	const unbranded = { a: 'abc' };
	const branded = BrandedObject.literal(unbranded);
	const someString = String('abc');
	expectTypeOf({ [someString]: unbranded }).not.toMatchTypeOf<BrandedVRecord>();
	expectTypeOf({ [someString]: branded }).toMatchTypeOf<BrandedVRecord>();
	});
	});

[pavadeli]

I believe something like Narrow would be a better name than Unwidened, do you agree? And secondly, please move utility types like this to the interfaces.ts file

[untio11]

It's a subtle difference.

• Narrowing: string -> Branded<string, '...'>
• Widening: Branded<string, '...'> -> string

I called the type Unwidened because it stops the widening case from happening. The problem is that we receive a branded string that accidentally gets widened, not that we received a normal string that we needed to narrow down.

Let me know if you still want me to change the name. I'll move the type to interfaces.ts in any case.