opt

text, err := json.MarshalIndent(StructExample{}, "", "  ")
if err != nil {
	fmt.Println(err)
} else {
	os.Stdout.Write(text)
}

Output:

{
  "Scalar": {
    "Bool": false,
    "String": "",
    "Int": 0,
    "Float": 0,
    "Addr": "invalid IP",
    "AddrPort": "invalid AddrPort",
    "Prefix": "invalid Prefix",
    "Duration": "0s",
    "URL": ""
  },
  "Slice": {
    "Ints": [],
    "Floats": [],
    "Strings": [],
    "Prefixes": [],
    "Structs": null
  }
}

var x StructExample
_, err := toml.Decode(`
[[opt]]

[scalar]
bool = true
string = "hello world"
int = 789
float = 7.89
addr = "192.168.2.2"
addr_port = "192.168.2.2:80"
prefix = "192.168.2.2/24"
duration = "15s"
url = "https://golang.org/pkg/flag"

[slice]
ints = [ 3, 2, 1 ]
floats = [ 3.3, 2.2, 1.1 ]
strings = [
        "hello world",
        "bonjour le monde",
        "こんにちは世界",
]
prefixes = [
        "192.168.1.1/32",
        "192.168.1.2/32",
        "192.168.1.3/32",
    ]
[[slice.structs]]
number = 123
string = "hello world"
[[slice.structs]]
number = 456
string = "bonjour le monde"
[[slice.structs]]
number = 789
string = "こんにちは世界"
`, &x)
if err != nil {
	fmt.Println(err)
	return
}
text, err := json.MarshalIndent(&x, "", "  ")
if err != nil {
	fmt.Println(err)
} else {
	os.Stdout.Write(text)
}

Output:

{
  "Scalar": {
    "Bool": true,
    "String": "hello world",
    "Int": 789,
    "Float": 7.89,
    "Addr": "192.168.2.2",
    "AddrPort": "192.168.2.2:80",
    "Prefix": "192.168.2.2/24",
    "Duration": "15s",
    "URL": "https://golang.org/pkg/flag"
  },
  "Slice": {
    "Ints": [
      3,
      2,
      1
    ],
    "Floats": [
      3.3,
      2.2,
      1.1
    ],
    "Strings": [
      "hello world",
      "bonjour le monde",
      "こんにちは世界"
    ],
    "Prefixes": [
      "192.168.1.1/32",
      "192.168.1.2/32",
      "192.168.1.3/32"
    ],
    "Structs": [
      {
        "Number": 123,
        "String": "hello world"
      },
      {
        "Number": 456,
        "String": "bonjour le monde"
      },
      {
        "Number": 789,
        "String": "こんにちは世界"
      }
    ]
  }
}

var x StructExample
err := yaml.Unmarshal([]byte(`
scalar:
  bool: true
  string: hello world
  int: 789
  float: 7.89
  addr: 192.168.2.2
  addr_port: 192.168.2.2:80
  prefix: 192.168.2.2/24
  duration: "55s"
  url: https://golang.org/pkg/flag

slice:
  ints:
    - 3
    - 2
    - 1
  floats:
    - 3.3
    - 2.2
    - 1.1
  strings:
    - hello world
    - bonjour le monde
    - こんにちは世界
  prefixes:
    - 192.168.1.1/32
    - 192.168.1.2/32
    - 192.168.1.3/32
  structs:
    - number: 123
      string: hello world
    - number: 456
      string: bonjour le monde
    - number: 789
      string: こんにちは世界
`), &x)
if err != nil {
	fmt.Println(err)
	return
}
text, err := yaml.Marshal(&x)
if err != nil {
	fmt.Println(err)
} else {
	os.Stdout.Write(text)
}

Output:

scalar:
  bool: true
  string: hello world
  int: 789
  float: 7.89
  addr: 192.168.2.2
  addr_port: 192.168.2.2:80
  prefix: 192.168.2.2/24
  duration: 55s
  url: https://golang.org/pkg/flag
slice:
  ints:
  - 3
  - 2
  - 1
  floats:
  - 3.3
  - 2.2
  - 1.1
  strings:
  - hello world
  - bonjour le monde
  - こんにちは世界
  prefixes:
  - 192.168.1.1/32
  - 192.168.1.2/32
  - 192.168.1.3/32
  structs:
  - number: 123
    string: hello world
  - number: 456
    string: bonjour le monde
  - number: 789
    string: こんにちは世界