Rewriting Code from Interpreted to Compiled Language – Terminology

Real world? Try carpentry. Most people have played around in a workshop a little.

You measure a piece of lumber and cut it. You need 20 of these. But to save time you take the previous one you cut, lay it flush with the next piece of lumber, and mark where you need to cut.

After 20 times you'll notice that your saw has a non-zero width and/or your pencil doesn't mark exactly where you want to cut. Errors build up and the first piece of lumber won't match the last piece of lumber.

OO, according to Alan Kay is all about message passing, and that's it. You will see that qualities such as polymorphism and encapsulation are actually derived from message passing.

Now that stance is in fact very extreme, because it basically means that only Smalltalk and languages alike would qualify.

I think you can define OO as building your system on entities, that fully encapsulate their state, and that are rendered exchangeable due to their inherent polymorphous qualities. One could thus argue a purely OO language ensures these two core qualities are always met. What renders OO languages "impure" would be mechanisms that allow the creation of constructs that do not meet these criteria, such as the possibilities to:

• declare public fields
• declare variables that can only hold instances of a specific class and its subclasses (i.e. variables should be typed against interfaces, which is what the biological objects communicate through in Kay's anology), which is rendered even narrower if the class in question is final, as that leaves even less room for polymorphism
• declare primitives

Then again, IMHO language purity is more a weakness than a strength. OO is not a silver bullet. No single paradigm is.