Maybe

在haskell裡，我們有：

Maybe a = Just a | Nothing

Typescript中實現

class Optional<T> {
    private _isValid: boolean = null;
    private _value: T = null;
    
    constructor (v: T = null) {
        if (v) {
            this._isValid = true;
            this._value = v;
        } else {
            this
 
._isValid = false;
        }
    }

    getValue () {
        return this._value;
    }

    isValid () {
        return this._isValid;
    }
}

// 使用
// 保證返回的是一個Optional物件，而不是Null
// 可以走通整個後續流程
function getPlayerById(id: number): Optional<Player> {
	let player = world.players[i];
	return new Optional
 
(player);
}