Handle printing of cyclic structures

[alex-snezhko]

Currently attempting to print data that contains cyclic references results in a stack overflow. A minimal example:

record Node {
  val: Number,
  mut next: Option<Node>
}

let a = { val: 1, next: None }
let aOpt = Some(a)
a.next = Some({ val: 2, next: aOpt })
// cycle between 1 and 2
print(a) // RangeError: Maximum call stack size exceeded

Perhaps an error value of "<data with cyclic reference>" (or some other more clever way of displaying this data) can be printed instead if a cycle is detected.

[spotandjake]

Hmms, this could get confusing
Python i guess just resorts to not printing any more then 2 levels, when i tested it it just logged the next value as {...}, I like js's approach better where it says [circular object Object]. but this doesnt really indicate what its circular too for instance if you have like a cycle 3 levels down that might look confusing.

What if we did something instead like <Cyclic Data, Node> and said the type name, not an error but we just serialized it as a cycle so you can still print.

Notes on this: So a few important notes on this are one in grain given we want good dev x and print anything goes along with that we should try to print it at all costs imo rather then error. two i think any fix to this requires chanign the way toString works atm because i dont think that stringifying a child knows anything about the parent. secondly do we want to allow a few levels of cycles maybe like one level or do we think the first cycle is enough and three I think another question / fix for this is if we had an option on print for print depth though that would again change the way toString works and i dont think we really firmly decided on if we want the newline behaviour because it breaks passing it into stuff like forEach, that seems like a slightly seperate dicussion but might be a user experience improvment that would also help to solve this problem.

[alex-snezhko]

@spotandjake I don't think having to specify a depth (or having an implicit depth) is great from a DX perspective.

I also just tried doing this in node and it gave me

<ref *1> { val: 1, next: { val: 2, next: [Circular *1] } }

This seems like a pretty good way of handling it imo, you get a reference for what there is a cycle to. Perhaps we could do something similar?

[spotandjake]

@spotandjake I don't think having to specify a depth (or having an implicit depth) is great from a DX perspective.

I also just tried doing this in node and it gave me

<ref *1> { val: 1, next: { val: 2, next: [Circular *1] } }

This seems like a pretty good way of handling it imo, you get a reference for what there is a cycle to. Perhaps we could do something similar?

I like this approach actually.