Arc Forum | possible bug involving len and nil?

Arc Forum

possible bug involving len and nil?

2 points by dido 4866 days ago | 8 comments

This, I think, is very wrong if one considers () and nil to be the same thing:

  arc> (len nil)
  1
  arc> (len '())
  1

Shouldn't both expressions evaluate to 0? Under what possible interpretation can the empty list have the same length as a list with one element?

  arc> (len '(1))
  1

Something is very wrong here.

1 point by dido 4866 days ago | link

Well, it might possibly be interpreted as 1 because the length of a symbol is 1, but this then raises the question as to why (len 'symbol) should not produce an error the way taking the len of other atoms does, e.g.:

  arc> (len 1)
  Error: "length: expects argument of type <proper list>; given 1"
  arc> (len 'symbol)
  1

The way I see it the most logical value for (len nil) is 0, that is if you want it to be the same as the empty list, and taking the len of any other symbol should produce an error.

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1 point by akkartik 4866 days ago | link

What arc are you using? On anarki and arc3.1 len.nil evaluates to 0.

Ah, I see rocketnia recently fixed it. Perhaps you just need to git pull?

http://github.com/nex3/arc/commit/9c0f4fb

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1 point by dido 4866 days ago | link

I just did a git pull on Anarki and indeed now (len nil) returns 0 as it should. However, (len 'sym) still returns 1, and I have to ask why this is so and not an error the way all other non-nil/string atomic values are.

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1 point by rocketnia 4866 days ago | link

Apparently Samuel Christie added it in order for Anarki's ppr.arc to work: https://github.com/nex3/arc/commit/fccf160fee607373ac15105e3...

I don't like len!sym being 1 either. I think ppr.arc is the thing that should be fixed, perhaps by having it use its own spinoff of 'len. But I didn't try to fix that; I just did the minimal change needed to get len.nil to work properly again.

It's kinda funny... I just noticed that both of my len.nil fixes (https://github.com/nex3/arc/commits/9c0f4fb2b54f4ceb11d0a2d2...) are currently living alongside each other, the older one just being dead code. :-p

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1 point by akkartik 4864 days ago | link

Perhaps he just wanted dotted lists to count the final element. I fixed that without breaking len!sym.

http://github.com/nex3/arc/commit/2d86c3

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2 points by aw 4866 days ago | link

Which Arc implementation are you using? This is not the behavior of Arc 3.1

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2 points by jsgrahamus 4857 days ago | link

Jarc and rainbow also return 0 for these.

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[dead]

2 points by zck 4854 days ago | link

Interesting...we're getting comments plagiarized from elsewhere in the page similar to how reddit was spammed: http://www.reddit.com/r/reportthespammers/comments/i78c2/red...

However, these aren't amazon affiliate links.

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