Just a note that we could use some docs on:
- reference types
- value types (see also User-defined value types aka structs and I assume a struct keyword will be added too)
- = vs. ==
- == vs. is
- is vs. isa
== vs is vs isa
The "==" operation is deceptively simple. When a data structuer has overriden the "Equals" method, .NET/Boo checks the values of one data structure against another and returns true if they are identical or false if they are not. By default, .NET provides "Equals" overloads for most of the builtin data-types like int, float, decimal, string, and etc. Using "==", two seperate objects can be evaluated and it can be determined if, by value, they are equal or not.
The "isa" keyword determines if a variable is of a particular type. "isa" is useful in determining if a variable implements an interface, or derives from a common base class.
The "is" keyword serves a different, and somewhat more complex function. Sometimes you might want to go beyond the usual check of "are these two variables of equal value?" and see, "are these two variables actually pointing to the same object?" This is where "is" shines; it looks at the reference of two variables and determines if they both point to the same object!
This technique is normally used when you receive an object by poking a dictionary or invoking a method; if the identity of var1 and var2 is the same, then they are obviously equal--because they are the same object.
/* True, False, True! */ a = 0 b = 0 c = 4 print a == b print c == b print a == 0
/* Prints out... True, True, False! */ class Food: pass class Sandwich(Food): pass hamAndSwiss = Sandwich() print hamAndSwiss isa Food print hamAndSwiss isa Sandwich print hamAndSwiss isa int
/* prints, True, False */ var1 = "hello, world!" var2 = var1 var3 = "hey, carl!" print var1 is var2 print var1 is var3 //2 different objects pointed at!