'type', 'tag', and 'kind'

Some users may prefer a discriminant other than type. The keys tag and kind are popular in various circles, and __typename is used in GraphQL communities. The key typeis just the default. You can make a new variant() function to make variants with another discriminant by calling variantFactory.

typescript
const myVariant = variantFactory('__typename');
type KEY = '__typename';
export const Animal = variantList([
    myVariant('Dog', fields<{name: string, favoriteBall?: string}>()),
    myVariant('Cat', fields<{name: string, furnitureDamaged: number}>()),
    myVariant('Snake', (name: string, pattern = 'striped') => ({name, pattern})),
]);
export type Animal<T extends TypeNames<typeof Animal, KEY> = undefined> = VariantOf<typeof Animal, T, KEY>;

typescript
const myVariant = variantFactory('__typename');
type KEY = '__typename';
export const Animal = variantList([
    myVariant('Dog', fields<{name: string, favoriteBall?: string}>()),
    myVariant('Cat', fields<{name: string, furnitureDamaged: number}>()),
    myVariant('Snake', (name: string, pattern = 'striped') => ({name, pattern})),
]);
export type Animal<T extends TypeNames<typeof Animal, KEY> = undefined> = VariantOf<typeof Animal, T, KEY>;

type Animal<'Dog'> is rendered as:

typescript
{
    __typename: "Dog";
    name: string;
    favoriteBall?: string | undefined;
}

typescript
{
    __typename: "Dog";
    name: string;
    favoriteBall?: string | undefined;
}

Most of variant's functions will accept an optional key as their final parameter, allowing the types to function with a different discriminant.

typescript
const result = match(animal, {
    Cat: ...
    Dog: ...
    Snake: ...
}, '__typename')

typescript
const result = match(animal, {
    Cat: ...
    Dog: ...
    Snake: ...
}, '__typename')