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August 18, 2022 08:59
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| /** | |
| * Exhaustive visitation for discriminated unions. | |
| * Let's say you have something like | |
| * | |
| * type T = { type: "number", value: number } | { type: "string", value: string }; | |
| * const t: T = ...; | |
| * | |
| * Then you can say | |
| * | |
| * discriminate(t, "type", { | |
| * number: ({ value }) => ..., // `value` is a `number` here! | |
| * string: ({ value }) => ..., // and a `string` here! | |
| * }) | |
| */ | |
| export function discriminate< | |
| Union extends Discriminated<Discriminator>, | |
| Discriminator extends Discriminators<Union>, | |
| Arms extends DiscriminateArms<Union, Discriminator>, | |
| >( | |
| value: Union, | |
| discriminator: Discriminator, | |
| arms: Restrict<Arms, DiscriminateArms<Union, Discriminator>>, | |
| ): DiscriminateResult<Union, Discriminator, Arms>; | |
| export function discriminate< | |
| Union extends Discriminated<Discriminator>, | |
| Discriminator extends Discriminators<Union>, | |
| Arms extends Partial<DiscriminateArms<Union, Discriminator>>, | |
| Fallback, | |
| >( | |
| value: Union, | |
| discriminator: Discriminator, | |
| arms: Restrict<Arms, Partial<DiscriminateArms<Union, Discriminator>>>, | |
| fallback: (value: Union) => Fallback, | |
| ): DiscriminateResult<Union, Discriminator, Arms> | Fallback; | |
| export function discriminate< | |
| Union extends Discriminated<Discriminator>, | |
| Discriminator extends Discriminators<Union>, | |
| Arms extends Partial<DiscriminateArms<Union, Discriminator>>, | |
| Fallback, | |
| >( | |
| value: Union, | |
| discriminator: Discriminator, | |
| arms: Restrict<Arms, Partial<DiscriminateArms<Union, Discriminator>>>, | |
| fallback?: (value: Union) => Fallback, | |
| ): DiscriminateResult<Union, Discriminator, Arms> | Fallback { | |
| // Implementing this so that TS accepts it is hard to impossible. | |
| // The code is simple enough, though. | |
| // See also `match` above. | |
| type Result = DiscriminateResult<Union, Discriminator, Arms>; | |
| const arm = arms[value[discriminator]] as ( | |
| <Variant extends Union>(value: Variant) => Result | |
| ) | undefined; | |
| // eslint-disable-next-line @typescript-eslint/no-non-null-assertion | |
| return (arm ?? fallback)!(value); | |
| } | |
| type Discriminated<Discriminator extends string | number> = { | |
| [Discriminant in Discriminator]: string | number; | |
| }; | |
| type Discriminators<Union> = keyof Union & (string | number); | |
| type DiscriminateArms< | |
| Union extends Discriminated<Discriminator>, | |
| Discriminator extends Discriminators<Union>, | |
| Out = unknown, | |
| > = { | |
| [Discriminant in Union[Discriminator]]: ( | |
| variant: Extract<Union, Record<Discriminator, Discriminant>>, | |
| ) => Out; | |
| }; | |
| type Restrict<T, U> = Exclude<keyof T, keyof U> extends never ? T : U; | |
| type DiscriminateResult< | |
| Union extends Discriminated<Discriminator>, | |
| Discriminator extends Discriminators<Union>, | |
| Arms extends Partial<DiscriminateArms<Union, Discriminator>>, | |
| > = Arms extends Partial<DiscriminateArms<Union, Discriminator, infer Out>> | |
| ? Out | |
| : never; |
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