Option and Null in Dynamic Languages(rntz.net)

    1> X = #{"a" => 1}.
    #{"a" => 1}
    2> maps:find("a", X).
    {ok,1}
    3> maps:find("x", X).
    error

    {ok, Val} = maps:find("a", X).

  from itertools import chain
  
  def getOpt(d, k):
      return [d[key]] if key in d else []

  def values_in(keys, d):
      return list(chain(*[ getOpt(d,key) 
                           for key in keys ]))

  def values_in(keys, d)
    keys.flat_map {|k| d.has_key?(k) ? [d[k]] : []}
  end

  def values_in(dict, keys):
      marker = object()
      return [x
              for x in (dict.get(k, marker)
                        for k in keys)
              if x is not marker]

        //Let's define some object
        var obj = {a: true, b: true, c: true, d: true, hasOwnProperty: true}

        //Now let's make them falsy in different ways
        obj.a = undefined; //set to undefined
        obj.b = null; //set to null
        obj.c = false; //set to false
        delete obj.d; //delete key
        delete obj.hasOwnProperty //delete key, but present in prototype

        console.log(obj.a) //--> undefined
        console.log(obj.b) //--> null
        console.log(obj.c) //--> false
        console.log(obj.d) //--> undefined
        console.log(obj.hasOwnProperty) //--> [Function]

        console.log('a' in obj) //--> true
        console.log('b' in obj) //--> true
        console.log('c' in obj) //--> true
        console.log('d' in obj) //--> false
        console.log('hasOwnProperty' in obj) //--> true

        console.log(obj.a == null) //--> true
        console.log(obj.b == null) //--> true
        console.log(obj.c == null) //--> false
        console.log(obj.d == null) //--> true
        console.log(obj.hasOwnProperty == null) //--> false 

        for(var key in obj) {
          console.log(key) //--> prints a, b, c
        }

        local tab = {a = true, b = true, c = true}
        
        -- by default there are none, but we can defined a fallback metatable similar to a prototype method like js's obj.prototype.hasOwnProperty

        setmetatable(tab, {__index={c='meta c', d='meta d'}})

        -- if it is important that a key is explicitly set to false, do so, otherwise simply remove it
        tab.a = false --set to false
        tab.b = nil --set to nil
        tab.c = nil --set to nil, present in fallback table
        tab.d = false --set to false, present in fallback table

        print(tab.a) --false
        print(tab.b) --nil
        print(tab.c) --meta c
        print(tab.d) --false