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1 point by shader 2521 days ago | link | parent

Good question.

I suppose that a list at the end of a list would have to be paired with nil, so (a b c (d e f)) would actually be the list (a b c (d e f) nil), but that should be possible to handle at the reader/'list level, and wouldn't make much of a difference in the use of the code. If that simple change is made, then your alist example shouldn't need to be explicitly terminated.



1 point by akkartik 2521 days ago | link

If we're relying on reader magic, what's the benefit of making atoms rather than lists nil-terminated?

It seems kinda futzy for the last element to be nil sometimes but not others.

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1 point by shader 2521 days ago | link

Yes, it does seem kind of fuzzy. And I doubt it would actually be the reader doing that, more likely the 'list function would be the source for the extra nils

However, you need some way to distinguish between a list in the car, and the next element in a chain. The only way to do that is to have it be in the car of a cons cell, and the only way to do that is to have something else in the cdr. If you don't have anything after the list, you have to fill it with nil.

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5 points by rocketnia 2521 days ago | link

The only way to do that is to have it be in the car...

Actually, you could have a special symbol '! in the car in order to identify that the cdr is actually supposed to be the last element. Then (a b c (d e f)) is represented as (a . (b . (c . (! . (d . (e . f)))))), i.e. (a b c ! d e f). Banged lists. ;)

In fact, having the symbol be a colon might even make it a nice optional style:

  (accum acc
    (each x args
      (acc (* 2 x))))
  
  (accum acc :
     each x args :
       acc : * 2 x)    ; reminiscent of Arc's (acc:* 2 x)
It's just that you never get to observe the ': from within code. There might be gotchas when generating code, too: The expression (quote : ) would evaluate to nil, huh? XD

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3 points by evanrmurphy 2521 days ago | link

Dear rocketnia,

Some of your ideas are very bizarre. I mean this as a compliment. Please keep 'em coming. :)

Regards, evanrmurphy

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