export opaque type ID: string = string;


function validateID(x: string): ID | void {
    if ( /* some validity check passes */ ) {
        return x;
    }

    return undefined;
}

import type {ID} from './exports';

function formatID(x: ID): string {
    return "ID: " + x; // Ok! IDs are strings.
}

function toID(x: string): ID {
    return x; // Error: strings are not IDs.
}