Dumping fails on nested mappings

[kklingenberg]

Hello. Thank you for making this egg. I've been using it to parse YAML and like it a lot :)

I found an issue with dumping nested mappings, like the following:

foo:
  bar: 123

As shown by the following snippet, which fails with an "unknown" exn:

(yaml-dump (yaml-load "--- {foo: {bar: 123}}"))

Other cases of weird behaviour is when sequences are nested in maps, like the following:

(yaml-dump (yaml-load "--- {foo: [123, 456]}"))

which yields:

---
- - foo
  - 123
  - 456

I think the source of the problem is here, which I interpret as a predicate to check object is an alist:

https://github.com/tenderlove/chicken-yaml/blob/5c9d4fbf9e62af0c67def86198d7ebf7d92d0074/yaml.scm#L67-L69

In it, the final (list? (car object)) is actually #t because an association to a list is also a list :cry:

I couldn't think of a clean predicate for this, since plain lists are mapped to yaml sequences. I think that's probably the reason why medea uses vectors for JSON lists, and other JSON eggs use hash-tables for mappings. It can get quite ugly to implement a proper alist? predicate. This is my attempt, which sadly forsakes correctness for nested lists, and is tied to the type mappings for strings.

(define (yaml-alist? object)
  (and (list? object)
       (not (null? object))
       (pair? (car object))
       (or (string? (caar object))
           (symbol? (caar object)))))