Created
May 9, 2022 13:55
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`checkAndEntail`
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| /** | |
| * Type guard for cases where one field of a structure determines | |
| * the nullability of a selection of other fields. | |
| * In the best case this will only check the `discriminator` field | |
| * against the given `discriminant`, but you will also get a runtime error | |
| * if any of the given `entailed` fields are `null`-ish when you don't | |
| * expect them to be. | |
| */ | |
| export function checkAndEntail<T, I extends keyof T, J extends keyof T>( | |
| t: T, | |
| discriminator: I, | |
| discriminant: T[I], | |
| entailed: [J], | |
| ): t is T & { | |
| [K in keyof T]-?: NonNullable<T[K]>; | |
| } { | |
| if (t[discriminator] !== discriminant) { | |
| return false; | |
| } | |
| return entailed.every(k => t[k] != null) || unreachable(); | |
| } | |
| // Where `unreachable` is something like | |
| const unreachable = (msg?: string): never => { | |
| throw new Error("unreachable"); | |
| }; |
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