Prefer indentation before flow style array nested in mapping.

[ra1028]

Hi, many thanks for Yams.

I'm using Yams to dump Encodable struct to file via YamlEncoder.
I found a little glitch that the malformed indentation before the flow style array elements that nested in mapping.
This isn't corrupted yaml syntax, but it's preferable to indent it.

Example

struct Test: Encodable {
    var foo: String
    var bar: Int
    var dict: [String: [String]]
}

let test = Test(
    foo: "foo",
    bar: 100,
    dict: [
        "baz": ["A", "B", "C"],
        "qux": ["1", "2"]
    ]
)

print(try YAMLEncoder().encode(test))

Current behavior

foo: foo
bar: 100
dict:
  baz:
  - A
  - B
  - C
  qux:
  - 1
  - 2

Expected

foo: foo
bar: 100
dict:
  baz:
    - A
    - B
    - C
  qux:
    - 1
    - 2

Environments

• Library version: 2.0.0

• Swift version: 5.1

• Xcode version: 11.2.1

[fmgcoelho]

yamllint output on the "Current behavior" above:

  1:1       warning  missing document start "---"  (document-start)
  5:3       error    wrong indentation: expected 4 but found 2  (indentation)
  9:3       error    wrong indentation: expected 4 but found 2  (indentation)

[CraigSiemens]

We've run into this as well. The main issue for us is that the YAML VSCode extension adds the indenting on save but then our tooling removed the indent when it updates the files.

It appears to be an issue with libyaml, which Yams wraps.

• Indentation of sequence elements yaml/libyaml#224