Circular Definitions in yaml - serialization

Question

I'm trying to use yaml to represent a train network with stations and lines; a minimum working example might be 3 stations, connected linearly, so A<->B<->C. I represent the three stations as follows:
---
stations:
- A
- B
- C
Now I want to store the different lines on the network, and where they start/end. To do this, I add a lines array and some anchors, as follows:
---
stations:
- &S-A A
- &S-B B
- &S-C C
lines:
- &L-A2C A to C:
from: *S-A
to: *S-C
- &L-C2A C to A:
from: *S-C
to: *S-A
and here's the part I'm having trouble with: I want to store the next stop each line at each station. Ideally something like this:
---
stations:
- &S-A A:
next:
- *L-A2C: *S-B
- &S-B B:
next:
- *L-A2C: *S-C
- *L-C2A: *S-A
- &S-C C:
next:
- *L-C2A: *S-B
(the lines array remains the same)
But this fails - at least in the Python yaml library, saying yaml.composer.ComposerError: found undefined alias 'L-A2C'. I know why this is - it's because I haven't defined the line yet. But I can't define the lines first, because they depend on the stations, but now the stations depend on the lines.
Is there a better way to implement this?

Answer 1

Congradulations! You found an issue in most (if not all) YAML implementations. I recently discovered this limitation too and I am investigating how to work around (in Ruby world). But that's not going to help you. What you are going to have to do is store the "next stops" as a separate set of data points.
next-stops:
*S-A:
- *L-A2C: *S-B
*S-B:
- *L-A2C: *S-C
- *L-C2A: *S-A
*S-C:
- *L-C2A: *S-B
Does that help?