Filtering & Analyzing

The last section covered how to create and group variants. Here, we'll cover how to narrow variant types into more specific subsets and automatically extract variants from a module based on various types.

Filtering the discriminated union

The Animal type allows some filtering out of the box. We have access to the type of a specific form through the Animal<'snake'> syntax, and can create a type union Animal<'dog'> | Animal<'cat'> as expected. Remember that it's possible to compute Animal<'dog'> yourself with the built in Extract<T, U> type.

ts
type Dog = Extract<Animal, {type: 'dog'}>;

ts
type Dog = Extract<Animal, {type: 'dog'}>;

It's also possible to leverage thatExtract<T, U> type to filter the union to only the possibilities matching a given interface. For example, our variant may be structured such that every winged animal has a wingCount: number property.

typescript
// assume every winged animal has a wingCount property
// and other animals don't.
type WingedAnimal = Extract<Animal, {wingCount: number}>;

typescript
// assume every winged animal has a wingCount property
// and other animals don't.
type WingedAnimal = Extract<Animal, {wingCount: number}>;

isOfVariant

It's possible to use isOfVariant to narrow a variable to a variant type. isOfVariant() is a user-defined type guard that expects two parameters

1. The variable to be evaluated
2. The variant module to compared against

typescript
const flap = (animal: WingedAnimal) => {...} 
declare var a: Animal;
if (isOfVariant(a, WingedAnimal)) {
    // a is now known to be an WingedAnimal
    // so this is safe.
    flap(a);
}

typescript
const flap = (animal: WingedAnimal) => {...} 
declare var a: Animal;
if (isOfVariant(a, WingedAnimal)) {
    // a is now known to be an WingedAnimal
    // so this is safe.
    flap(a);
}

Note it's possible to construct the variant module isOfVariant is expecting on the fly.

typescript
if (isOfVariant(a, variantList([Animal.bird, Animal.pegasus]))) {
    flap(a);
}

typescript
if (isOfVariant(a, variantList([Animal.bird, Animal.pegasus]))) {
    flap(a);
}