Comment by arijun | Hacker News Reader

Comment by arijun | Hacker News Reader

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Languages with immutable data structures are relatively rare. With its high level focus python could have been one of them, but went for different trade offs.

> In more sane languages, the expression [] means give me an empty list value, nothing more and nothing less

What is a “list value”? It’s either a pointer to some other space where the lists values are stored, or it’s the values stored themselves. If you say it’s the second (I can’t think of any language that does), then `a = […]; b = a` is an expensive copy by value assignment. Is that worth that trade off to get less surprising results in the articles case? I’m not saying the answer is “obviously no”, but it definitely isn’t “obviously yes”.

> If you say it’s the second (I can’t think of any language that does), then `a = […]; b = a` is an expensive copy by value assignment

In Swift collections are value types, so b = a conceptually is a copy. It’s not expensive, though, because Swift does copy-on-write.

You pay for that by a combination of having more expensive collection updates (they, at least conceptually, have to check whether to make that copy) and a more complex compiler (it tries to avoid having to actually do those checks at runtime)

  var a: [Int]=[]
  var b = [a,a,a,a]
  b[0].append(1)
  print(b)

will produce

  [[1],[],[],[]]

Just teaching myself Swift this week. Thanks, this is helpful!

Matlab is one language that chooses the second. Every assignment a=b where b is a matrix creates a distinct copy of the matrix.

The interpreter is slightly more clever and instead implements this as a copy-on-writr mechanism, but that's just an implementation detail .

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