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Comment by na85 | Hacker News Reader

Why on earth doesn't LinkedList already have a size() method? I'm thinking of typed languages, I guess, but surely any sane collection type knows how to perform addition/concatenation with other instances of its own type, right?

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There is a built in function for it in Prolog. But when it's presented as an example of how to implement it in the same way that list classes are often used as examples in OO languages even if they language implementation includes one.

This is describing how you would implement LinkedList. Obviously, the size method has to be defined somewhere, and this is showing a potential implementation of it

Prolog is a logic programming language. It's not object-oriented, so it doesn't have objects or methods.

If you want to know the length of a list you can use the predicate length/2:

  ?- length([a,b,c,d], N).
  N = 4.

The example above just shows you how length/2 works (albeit with some CLP operators that are not typical).

Edit: also, lists are not a "collection type" or any type at all. They're predicates, like everything else in the language. See my comment below - the easiest way to think of them is as patterns (just like in regexes, only Turing complete).

Those collections have to be implemented by someone. You seem to be assuming that these collections spring fully formed into the world and are black boxes without an implementation.

I think you are willingly missing my point, because I believe it is obvious that I am not making that assumption.

Looking at the other responses to your comment, clearly it is not obvious that you're not making that assumption.

If that's not what you were trying to say, then I still don't understand what you were trying to say.

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