block11.ts

declare class MeterTag {
   private __kind: 'meter'
}
type Meter = number & MeterTag;
declare class FootTag {
   private __kind: 'foot'
}
type Foot = number & FootTag

function createMeter(value: number): Meter {
   //This *must* be the only way to create a Meter
   return value as Meter;
}
function createFoot(value: number): Foot {
   //This *must* be the only way to create a Foot
   return value as Foot;
}

function fromFootToMeter(feet: Foot): Meter {
   return createMeter(0.3048 * feet);
}

function move(m:Meter) {
  console.log('move', m, 'meter');
}

const distance1 = createMeter(10);
const distance2 = createFoot(10);

move(distance1) // compile
move(distance2) // Argument of type Foot is not assignable to Meter
move(fromFootToMeter(distance2)) // compile

move(distance1 + distance2)
// Argument of type number is not assignable to Meter, the tag is lost