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2 points by rocketnia 3518 days ago | link | parent

"What do people think of this? Basically I want all list operations to treat atoms as degenerate dotted lists. Does this have any adverse implications for other aggregates (tables)?"

I don't see a problem, but I may not be the right person to ask. If it came down to it, I'd be willing to use 'table-pushnew, 'alist-pushnew, 'maxheap-pushnew, and so on. :-p

What other list operations do you have in mind? While 'pushnew makes sense to use with degenerate dotted lists, it's a very special case: It only clobbers the list at a finite depth, and that depth is actually zero. (Or zero in the unmodified list, anyway.)



1 point by akkartik 3518 days ago | link

"What other list operations do you have in mind?"

Well, I've already changed reclist and some so things like this work:

  > (find 3 3)
  3
  > (find 3 '(2 . 3))
  3
  > (find 4 '(2 . 3))
  nil
More: https://github.com/nex3/arc/blob/76d078bcd0/arc.arc.t. I expect I've still missed some cases, and I'll keep tweaking them as I run into them until someone objects.

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3 points by rocketnia 3518 days ago | link

Oh, whoops. I forgot 'pushnew actually does traverse the input list to find out whether the element is new. I was thinking of 'push. XD

I disagree with the notion of membership you're using for dotted lists. If I 'pushnew nil onto (1 2 3 . nil), I want to get (nil 1 2 3 . nil), and it won't work that way if nil is already considered to be a member.

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1 point by akkartik 3518 days ago | link

Hmm, it seems to work:

  arc> (= x '(1 2 3))
  (1 2 3)
  arc> (pushnew nil x)
  (nil 1 2 3)

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2 points by rocketnia 3517 days ago | link

I think the membership check you're using is like this:

  (The final cdr is an element iff it isn't nil.)
  If the list is...
    A cons cell:
      If the car is the value we're looking for, succeed. Otherwise,
      continue by searching the cdr.
    Nil:
      Fail.
    A non-cons, non-nil value:
      Continue by comparing the list to the value we're looking for.
I feel we could simplify this specification by rewriting the last two cases in one of these ways, ordered from my least to most favorite:

  (The final cdr is an element.)
  (This breaks (pushnew nil ...) in existing Arc code.)
  ...
    A non-cons value:
      Continue by comparing the list to the value we're looking for.
  
  (The final cdr is not an element.)
  ...
    A non-cons value:
      Fail.
  
  (The final cdr is always nil and is not an element.)
  (Arc 3.1 already uses this.)
  ...
    Nil:
      Fail.
    A non-cons, non-nil value:
      Raise an exception.
By using the "element iff it isn't nil" approach, you're able to use 'pushnew to traverse the simple argument lists you build as intermediate results of that 'make-br-fn implementation. But I don't know if it's worthwhile to complicate the notion of "list" just to accommodate a special case of argument lists.

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1 point by akkartik 3516 days ago | link

Yeah, that's a valid summary of what I've done.

"I don't know if it's worthwhile to complicate the notion of "list" just to accommodate a special case of argument lists."

Yeah I see your point. My aim was to extend the correspondence between the syntax for vararg params, rest params and lists of regular params. But those features merely match a (proper) list of args to some template. I'm entangling the template with the notion of a list. Hmm..

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