Why don't we have two nulls?

Question

I\'ve often wondered why languages with a null representing \"no value\" don\'t differentiate between the passive \"I don\'t know what the value is\"

Answer 1

Existence of value:

• Python: vars().has_key('variableName')
• PHP: isset(variable)
• JavaScript: typeof(variable) != 'undefined'
• Perl: (variable != undef) or if you wish: (defined variable)

Of course, when variable is undefined, it's not NULL