Array as map key

[jinyus]

Please describe the bug

Is this possible with a mutable array? somehow it's not detecting that the new array shouldn't be dropped.

Sample repo: https://github.com/jinyus/inko_demo

Code:

  let mut tag_map: Map[String, mut Array[Int]] = Map.with_capacity(128)

  posts.iter.each_with_index fn (idx,p){

    p.tags.iter.each fn (tag){
      # rust equivalent: post_tags_map.entry(tag).or_default().push(post_idx);

      let tag_posts = tag_map.opt_mut(tag).unwrap_or(mut Array.with_capacity(128))

      
      if tag_posts.size == 0 {
        tag_map.set(tag, tag_posts)
      }

      tag_posts.push(idx)
    }

  }

Error:

Stack trace (the most recent call comes last):
  /home/dev/projects/inko/demo/inko_demo/demo.inko:29 in main.Main.main
  /usr/lib/inko/std/std/iter.inko:69 in std.iter.Stream.each_with_index
  /usr/lib/inko/std/std/iter.inko:50 in std.iter.Stream.each
  /usr/lib/inko/std/std/iter.inko:71 in std.iter.Closure75.call
 /projects/inko/demo/inko_demo/demo.inko:31 in main.Closure53.call
  /usr/lib/inko/std/std/iter.inko:50 in std.iter.Stream.each
  /projects/inko/demo/inko_demo/demo.inko:34 in main.Closure54.call
  /usr/lib/inko/std/std/array.inko:104 in std.array.Array.$dropper
Process 'Main' (0x55d19d1878b0) panicked: can't drop a value of type 'Array' as it still has 1 reference(s)

ps: I'm trying to add inko to my benchmark: https://github.com/jinyus/related_post_gen

Please list the exact steps necessary to reproduce the bug

git clone https://github.com/jinyus/inko_demo.git

cd inko_demo

inko run demo.inko

Operating system

Linux (6.6.1-arch1-1)

Inko version

0.13.1

Rust version

1.73.0 (cc66ad468 2023-10-03)

[yorickpeterse]

The problem is this line:

let tag_posts = tag_map.opt_mut(tag).unwrap_or(mut Array.with_capacity(128))

Within a method call chain, temporary values that aren't moved are dropped at the end of the chain. In this case the mut Array is moved, leaving behind the owned value. This value is then dropped, but the mut Array is still stored in tag_posts.

To better understand this, the line in question is essentially the same as this:

let tag_posts = {
  let array = Array.with_capacity(128)
  let ret = tag_map.opt_mut(tag).unwrap_or(mut array)

  drop(array)
  ret
}

What you want to do instead here is something like this:

match tag_map.opt_mut(tag) {
  case Some(v) -> v
  case _ -> {
    let array = Array.with_capacity(128)
    let ret = mut array

    tag_map.set(tag, array)
    array
  }
}

However, in doing so you'll most likely run into the bug outlined in #636. You can however work around that by adding an explicit type annotation to tag_map.

Looking at the documentation, I realized I didn't document how drops work for method call chains. In fact, the only documentation on method calls is the syntax guide, which isn't great. I'll create an issue for that.

[yorickpeterse]

The issue in question is #658.

[yorickpeterse]

Since this isn't an actual bug but rather the result of lacking documentation, I'll close this in favour of #658

[jinyus]

Thanks for the guidance. I guess this can be closed since it's expected behavior..

[jinyus]

@yorickpeterse take a look if you have the time: jinyus/related_post_gen#440