`checkAndEntail`

checkAndEntail.ts

/**
 * 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");
};