nominal-types
v0.2.0
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Nominal types for better typesafety
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Nominal
The right way to do types in typescript.
Installation
npm install nominal-types
yarn install nominal-types
pnpm install nominal-types
Usage
Basic
The most immediate benefit of nominal types is preventing confusion between two types. In regular Typescript you run into this problem:
type Minutes = number
type Seconds = number
const minutesToSeconds = (minutes: Minutes) => minutes * 60
const seconds: Seconds = 420
// uh-oh, we can use Minutes and Seconds interchangeably
minutesToSeconds(seconds)
Nominal types solve this problem
import { Nominal, nominal } from 'nominal-types';
type Minutes = Nominal<'Minutes', number>;
type Seconds = Nominal<'Seconds', number>;
const minutesToSeconds = (minutes: Minutes) => minutes * 60
// You can directly type cast or use nominal.make
const seconds = nominal.make<Seconds>(420)
const minutes = 1337 as Minutes
// doesn't work, yay type safety
minutesToSeconds(seconds)
// does work!
minutesToSeconds(minutes)
Another example
You can use nominal types to give your code even better type-safety guarantees.
This goes beyond just type-safety, it's a performance optimization: once you know the array is sorted, you never have to sort it again. This is enforcing that at a type level.
type SortedArray<T> = Nominal<'sortedArray', Array<T>>
const sort = <T>(arr: Array<T>): SortedArray<T> => arr.sort()
const binarySearch = <T>(
sorted: SortedArray<T>,
search: T
): number | undefined => {
/* ... */
}
const regularArray = [1, 7, 2, 3, 6, 9, 10, 4, 5]
// won't work
binarySearch(regularArray, 2)
// will work
binarySearch(sort(regularArray), 3)
This is also known as Refinement types
Composing types
We can actually make this a bit crazier, we can compose nominal types
type SortedArray<T> = Nominal<'sortedArray', Array<T>>
const sort = <T>(arr: Array<T>): SortedArray<T> => arr.sort() as SortedArray<T>
const nonEmpty = <T>(arr:Array<T>):NonEmptyArray<T> => arr.filter(Boolean) as NonEmptyArray<T>
type NonEmptyArray<K, T extends Array<K>> = Nominal<'nonEmptyArray', T>;
type NonEmptySorted<T> = NonEmptyArray<T, SortedArray<T>>;
const binarySearch = <T>(sorted: NonEmptySorted<T>): T => {
let foo = sorted[0]
return foo
}
// won't work
binarySearch(regularArray)
// still won't work
binarySearch(sort(regularArray))
binarySearch(nonEmpty(sort(regularArray)))
Examples
More examples in examples folder, you can also see them typed on replit.
| Example | Link | |-------------|-----------------------------------------------------------| | basic | https://replit.com/@CryogenicPlanet/Nominal#basic.ts | | sorting | https://replit.com/@CryogenicPlanet/Nominal#sort.ts | | composing | https://replit.com/@CryogenicPlanet/Nominal#composing.ts | | safeRecords | https://replit.com/@CryogenicPlanet/Nominal#safeRecord.ts |
Credits
You can read more about this https://zackoverflow.dev/writing/nominal-and-refinement-types-typescript
Inspiration from Ghosts of Departed Proofs (Functional Pearl)