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@RikkiGibson RikkiGibson/adt.ts
Created Mar 11, 2016

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Fake ADTs in TypeScript
type Meters = { meters: number };
type Feet = { feet: number };
type Distance = Meters | Feet;
function isMeters(distance: Distance): distance is Meters {
return typeof (distance as Meters).meters === 'number';
function isFeet(distance: Distance): distance is Feet {
return typeof (distance as Feet).feet === 'number';
function checkDistance(x: Distance): string {
if (isMeters(x)) {
return "meters";
} else if (isFeet(x)) {
return "feet";
return "neither";
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