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1 point by evanrmurphy 2396 days ago | link | parent

Indeed. One might say the real problem with Lisp is that it's not concatenative. ;)

I think the OP's point about cons cells is excellent. I've spent a long time thinking cons cells were some ultimate abstraction only to realize they're more of a pretty hack.

2 points by rocketnia 2396 days ago | link

I don't think cons cells are a hack, but I do think it's a hack to use them for things other than sequences. Since we almost always want len(x)=len(cdr(x))+1 for cons cells, rather than len(x)=2, they aren't really useful as their own two-element collection type.


1 point by Pauan 2395 days ago | link

Yeah, I'm tempted to agree with you. In the arc.arc source code, pg even mentions a solution to improper lists: allow any symbol to terminate a list, rather than just nil.

Of course, an easier fix would be to change `cons` so it throws an error if the second argument isn't a cons or nil. Honestly, are improper lists useful often enough to warrant overloading the meaning of cons? We could have a separate data-type for B-trees.

That's one area that I can actually agree with the article, but that has nothing to do with conses in general (when used to create proper lists), only with improper lists. And contrary to what he says, it's not an "unfixable problem", instead it would probably take only 2 lines of Python code to fix it.

One thing though... function argument lists:

  (fn (a b . c))
Of course I can special-case this in PyArc, so . has special meaning only in the argument list. This is, in fact, what I do right now. But that may make it harder for implementations in say... Common Lisp or Scheme (assuming you're piggybacking, rather than writing a full-on parser/reader/interpreter).

If so... then you may end up with the odd situation that it's possible to create improper lists, using the (a . b) syntax, but not possible to create improper lists using `cons`


By the way... how about this: proper lists would have a type of 'list and improper lists would have a type of 'cons. Yes, it would break backwards compatibility, but it might be a good idea in the long run. Or we could have lists have a type of 'cons and improper lists have a type of 'pair.


2 points by rocketnia 2395 days ago | link

I don't know if improper lists are really a problem, just hackish. :) My "solution" would be to remove the need for them by changing the rest parameter syntax (both in parameter lists and in destructuring patterns).


"how about this: proper lists would have a type of 'list and improper lists would have a type of 'cons."

I don't think I like the idea of something's type changing when it's modified. But then that's mostly because I don't think the 'type function is useful on a high level; it generally seems more flexible to do (if afoo.x ...) rather than (case type.x foo ..), 'cause that way something can have more than one "type." Because of this approach, I end up using the 'type type just to identify the kind of concrete implementation a value has, and the way I think about it, the concrete implementation is whatever invariants are preserved under mutation.

That's just my take on it. Your experience with 'type may vary. ^_^