Infinite recursion of lists containing themselves

[wren-lang]

Infinite recursion of lists containing themselves

[munificent]

Oh, good catch. I was aware of this but, like you, felt like it was too much of chore it implement correctly. Or, at least, I felt it was too much of a chore right now. There are certainly more important things to work on right now. :)

Fixing it correctly isn't too hard, actually. You just have to keep a list of the lists you've already visited, and if you encounter a list that's been visited already, just print "..." instead of recursing into it. If you'd like to take a stab at it, you'd just need to:

1. Write a little test case.
2. Fix it in corelib.wren.
3. Run make_corelib to slurp that code into wren_core.c.
4. Make sure the test passes. :)

If not, I'll get around to fixing it eventually.

You just have to keep a list of the lists you've already visited, and if you encounter a list that's been visited already, just print "..." instead of recursing into it.

Is this needed? I think the recursion only happens when a list contains itself. Is it sufficient to only check for this?

[kmarekspartz]

What if it contains a list which contains itself?

var x = []
var y = []
y.add(y)
x.add(y)
IO.print(x)

[munificent]

@zeckalpha is right. To be fully general, you have to handle deeper cycles too.

[kmarekspartz]

Now that I've got the concat operator, I did some work on handling toString for recursive lists:

https://github.com/zeckalpha/wren/compare/feature;toString-circular-lists

[kmarekspartz]

I've done a bit more on that branch, moving the extraneous helpers in as functions inside List.toString. Still not sure this is the right approach, and untested.

[kmarekspartz]

Should that branch be ported to C and/or updated to reflect the split of List into List and Sequence?

[munificent]

You won't be able to port it to C since it contains calls to toString. Right now, primitive methods cannot call user-defined ones.

Let's just sit on this for now. I might take a stab at it later. Either way, it's not a very big issue.