Arc Forumnew | comments | leaders | submitlogin
1 point by sacado 5878 days ago | link | parent

I guess these issues will eventually be fixed. These are only bugs, not design issues.

As for is-a vs. has-a, I was wondering, isn't the biggest mistake considering that a piece of data only has one type ? That is, isa & type are broken ?

Considering values have a list of types (instead of a single type) would fix a few problems. For example, what is the type of nil ? Is it a symbol or a list ? Yes, it is both, but to do so, pg had to define the alist function ! (type nil) returns sym and cannot let you know nil is a list. What is '(1 2 3) ? Is it a cons or a list ? It is a list, but its type is only cons. type should return a list (it is already possible through annotate) and the definition of isa should be :

  (def isa (x typ)
    (mem typ (type x))
That way, we would have

  (type nil) -> '(list sym)
  (type '(1 2 3)) -> '(list cons)
  (type '(1 . 2)) -> '(cons)
And the system would remain very generic (even more than now). Thus, almkglor's scanners would work with each (provided the definitions of car and cdr were adapted) :

  (type s) -> '(cons scanner)
  (isa cons s) -> t
Other representations would be possible, including the ones we already discussed here. The name isa does not necessarily mean we have an is-a semantic (and not has-a). Once again, each item in type is an annotation and can mean many things that are left to the programmer (it can be a class, a class type, a bunch of functions, whatever).

But the "objects-have-one-type" model seems broken. At least for lists, which is quite annoying for a Lisp !



2 points by almkglor 5878 days ago | link

"objects-have-one-type" model helps with nex3's defm:

  (defm something ((t f scanner))
    (do-something-on-scanner f))
  (defm something ((t f cons))
    (do-something-on-real-cons-cell f))
In this case, if we pass an object that is-a scanner and is-a cons, which method gets called?

This is the main reason I'm advocating is-a and has-a separation. We can say that an object is-a 'cons cell and has-a scanner. If something requires that an object is-a real, element-pointer-and-next-pointer 'cons cell, as opposed to somethingthing that requires that an object has-a 'car and 'cdr, we can make the distinction.

So we can say that an object is-a 'cons cell - it's what it really is, what it's implemented with. However, a 'cons cell has-a scanner interface, and if it's proper it has-a list interface, etc.

Separating is-a and has-a could also be useful for optimization of basic parts.

For example, a basic non-optimized diff algorithm might operate on has-a 'scanner, and use 'car and 'cdr operations. However a string-scanner is really just a wrapper around a string and an index into the string, as well as the string's length. Each 'string-scanner object contains three slots: one for the string, one for the index, and one for the length. This applies to each 'cdr on a string-scanner.

Now suppose we have a version of the diff algo which specifically detects if an object is-a string-scanner. It destructures the string-scanner into the string, index, and end, and instead of carrying around a triple of (string, index, length) it only carries the index, leaving string and length into local variables. This reduces memory consumption to only one-third.

(Note that the diff algo I posted a while back actually keeps entire sections of the list, in order to properly scan through their differences; that is, it keeps several scanners)

-----

5 points by sacado 5877 days ago | link

Hmm, I think there are 2 really different concepts here :

- type declaration of real-implementation (what you call "is-a"),

- type declaration in the sense of "capabilities" an object has (what you call has-a).

I think they should really be distinguished. The former is about optimized compilation, the latter about which functions can be applied to a given object.

But optimization is linked to variables (e.g. "in this block n always holds an integer, s always holds a string and l is always a cons) and does not need to be declared until you want to compile something.

On the opposite, capabilities are linked to values (e.g., "n, s and l are all scanners, they all have scanner capabilities, you can apply car and cdr to all of them. This is currently true, but could change if values referenced by n, s or l change). These are mandatory, and have to be known dynamically (this is not a declaration in the static meaning, they can even change later). When you apply car to a variable, you must know if its attached value can answer it (and eventually how).

  (= str (string-scanner "foo bar baz"))
  (type str)
  -> (scanner string)

  (def scan (s)
    (if (no s)
      ""
      (cons (foo (car s)) (cdr s))))
There, the values held by s are considered as a scanner and a string, that is, car and cdr can be applied to them. A dispatch algorithm is applied to them on the moment we need it. If, at any moment, an object held by s cannot be applied the method car or cdr, we have an error. Until we want more speed, that's enough.

Now suppose we want more. All we have to do is :

  (def scan (s)
    (istype string-scanner s
      (if (no s)
        (cons (foo (car s) (cdr s)))))
That way, for optimization purpose, we state that s only holds string-scanner objects. It does not even have to care with the annotations you added to the value (or values) held by s. If an object held by s is not really a string-scanner, well, anything could happen.

I might be wrong, but I think super-optimizing CL compilers work that way. You say them "optimize that function, and btw, this var always holds strings, don't even bother checking and dispatching the right function".

-----

4 points by almkglor 5877 days ago | link

Quite accurate. The main thing is that I think people should use has-a for everyday programming, and only use is-a if absolutely necessary, e.g. optimization.

My second proposal, probably lost somewhere in the confusion, is that has-a information would be connected to an object's is-a type.

-----

3 points by Jesin 5877 days ago | link

This could be part of a temporary solution:

  (def my-isa (obj typ)
    (let a type.obj
      (if atom.a
          (is a typ)
          (some [is _ typ] a)))

-----

3 points by sacado 5877 days ago | link

Yes, but then it does not work with the core functions (notably each & friends), as they are relying on isa. Renaming your function isa does not work either (that would be too easy) : atom and some call isa themselves, so you get in an infinite loop. Btw, obj is a macro (at least in Anarki, I don't know if it's present in the official Arc2), so your code has a red flag on it (although it does seem to work).

I tried this, it does work :

  (redef isa (x typ)
    (isa-rec type.x typ))

  (def isa-rec (types typ)
    (if
      (no types) nil
      (is types typ) t
      (isnt type.types 'cons) nil
      (is (car types) typ) t
      (isa-rec (cdr types) typ)))

  (isa nil 'sym) -> t
  (isa (annotate '(sym list) nil) 'sym) -> t
  (isa (annotate '(sym list) nil) 'list) -> t
  (isa (annotate '(sym list) nil) 'cons) -> nil
And now the funny part :

  (redef car (x)
    (if (isa x 'int)
      (if (> rep.x 0)
        'a
        nil)
      (old x)))

  (redef cdr (x)
    (if (isa x 'int)
      (if (> rep.x 0)
        (annotate type.x (- rep.x 1))
        nil)
      (old x)))
Both car and cdr now work on objects of type 'int (and objects of type 'cons, as before). If an object is both an int and a cons, its int being is taken into consideration. Every operation requiring 'cons cells or calling car and cdr can now be overridden to use ints instead (an int being a list whose car is the symbol 'a and whose cdr is that num - 1).

  (car 1)
  -> a
  (cdr 1)
  -> 0
  (cddr 2)
  -> 0
  (car (annotate '(int cons)) 3)
  -> a
  (len (annotate '(int cons)) 3)
  -> 3
Very funny things to do from there, but watch your fingers.

-----

2 points by sacado 5877 days ago | link

Hmm, Playing with this stuff, I ran into that :

  (each c (annotate '(int cons) 3) (prn c))
  -> Error: "Function call on inappropriate object #3(tagged (int cons . nil) 3) (0)"
The problem is that each calls acons and alist, but they are defined in terms of (is (type x) 'cons) instead of (isa x 'cons). Once you redefine them, it works fine.

pg, do you still accept suggestions about the core functions ? Shouldn't acons and alist be defined with isa instead of is ? That would let us redefine them more easily. Btw, what do you think of all these discussions about types ?

-----