alessioprestileo

import {Feet, Meters, Measurement, toMeasurement} from './units-of-measurement';

function area<T extends Measurement>(width: T, height: T): T {
    return toMeasurement<T>(width * height);
}

const widthFeet = toMeasurement<Feet>(4);
const heightFeet = toMeasurement<Feet>(5);
const areaFeet = area<Feet>(widthFeet, heightFeet);

alessioprestileo

export type Meters = number & {__tagMeters: never};
export type Feet = number & {__tagFeet: never};
export type Measurement = Meters | Feet;

export function toMeasurement<T extends Measurement>(val: number): T {return val as T}

alessioprestileo

type FString = string & { __compileTimeOnly: any };

const failure1: FString = 'hello world'; // Error: Type 'string' is not assignable to type '{ __compileTimeOnly: any; }'.
const failure2: FString = { __compileTimeOnly: 'whatever' }; // Error: Type '{ __compileTimeOnly: string; }' is not assignable to type 'string'.

const success1: FString = 'hello world' as FString;
const success2: FString = { __compileTimeOnly: 'whatever' } as FString;

alessioprestileo

type FString = string & { __compileTimeOnly: any };