Programmers talk a lot about types, but what is a “type”, anyway? It is, in essence, the set of all possible values for some variable. Defining such a set gives us some information about what we can do with the value of that variable, in general. For example, when speaking about numbers, we might say, “let x be any integer” or “let y be any real number not equal to zero”. These statements tell us what we can do with these values; we know that we can add, subtract, or multiply x an...
