Exercise 7.4 Confusion about fp-oo HOT 3 CLOSED

Changed the wording to "Fix this problem by adding a new link in the inheritance hierarchy."

I'm pretty sure this is the way Ruby does it. Older Smalltalk too (though I think the later versions split Object up into several pieces).

Here's the way to understand MetaAnything.

Its superclass:

• any object must be a subclass of Anything (either direct or indirect).
• MetaAnything is an object
• Therefore: it must be a subclass of Anything.
• It's a direct subclass because there's nothing to put between it and Anything.

Its class link:

• Any object in the system must have a class.
• Any object's class must be descended from Anything so that it has access to methods like :class and :method-missing.
• MetaAnything is an object.
• Therefore, its class link must point to Anything or something that's descended from it.
• There are two choices: Anything or MetaAnything.
  • If MetaAnything were its own class, you'd be able to send MetaAnything :new. But there's no point to having the ability to make new MetaAnythings. It seems like trying to do that should be an error -- a :method-missing error.
  • That leaves Anything.

I'm going to improve the hints for these problems.

Also note that in the next release, this whole chapter moves to the optional Part V. It adds nothing to the reader's knowledge of Clojure or functional programming techniques.

from fp-oo.

Thanks Brian, that helped a lot.

Looking back, am I wrong or is MetaAnything's class arrow in the diagram pointing the wrong way (should have the arrowhead pointing at Anything)?

from fp-oo.

I don't think so. It has a left-pointing :class arrow that points to Anything. The arrow pointing to MetaAnything is Anything's :class arrow. So there's a weird circularity: Anything's class is MetaAnything and MetaAnything's class is Anything. (!)

Don't worry: none of this is obvious. Every time I work on such questions, I have to work it out from the beginning. It never gets intuitive.

from fp-oo.