YAML is a human friendly data serialization standard for all programming languages.
— https://yaml.org
I don't know precisely which humans YAML is friendly to but I'm not one of them.
And it looks like the people who run in my circles are also excluded.
Mapping to JSON
Between Eleventy, GitHub Actions, and a recent foray into serverless, I'm using a more YAML. And I don't understand it.
This is mapping for my JSON-acquainted brain.
- Comments
- Array of strings
- Array of objects — one value
- Array of objects — many values
- Object of keyed arrays of strings
- Object of keyed objects with mixed values
- Array of arrays with mixed values
- Nodes (variables)
- Sample GitHub Action
- Takeaways
- Resources and further reading
Comments
YAML
# you can put these p much anywhere
JSON
JSON doesn't support comments.
Array of strings
Sequence of Scalars
YAML
- Evermore
- Folklore
- Lover
JSON
["Evermore", "Folklore", "Love"]
Array of objects — single value
Mapping Scalars to Scalars
YAML
- Evermore: 2020
- Folklore: 2020
- Lover: 2019
JSON
[
{
"Evermore": 2020
},
{
"Folklore": 2020
},
{
"Lover": 2019
}
]
Array of objects — many values
Sequence of Mappings
YAML
- name: Taylor Swift
album_count: 9
label: Republic
- name: The National
album_count: 8
label: 4AD
JSON
[
{
"name": "Taylor Swift",
"album_count": 9,
"label": "Republic"
},
{
"name": "The National",
"album_count": 8,
"label": "4AD"
}
]
This syntax confused the shit out of me.
This code is auto-formatted by prettier putting the dash and the first property on the same line.
YAML docs show the dash and first property on different lines which is much more legible.
-
name: Taylor Swift
album_count: 9
label: Republic
-
name: The National
album_count: 8
label: 4AD
Object of keyed arrays of strings
Mapping Scalars to Sequences
YAML
Taylor Swift:
- Evermore
- Folklore
- Lover
The National:
- I Am Easy to Find
- Sleep Well Beast
- Trouble Will Find Me
JSON
{
"Taylor Swift": ["Evermore", "Folklore", "Lover"],
"The National": [
"I Am Easy to Find",
"Sleep Well Beast",
"Trouble Will Find Me"
]
}
Object of keyed objects with mixed values
Mapping of Mappings
YAML
Taylor Swift:
album_count: 9
label: Republic
The National:
album_count: 8
label: 4AD
JSON
{
"Taylor Swift": {
"album_count": 9,
"label": "Republic"
},
"The National": {
"album_count": 8,
"label": "4AD"
}
}
Array of arrays with mixed values
Sequence of Sequences
YAML
- [name, album_count, label]
- [Taylor Swift, 9, Republic]
- [The National, 8, 4AD]
JSON
[
["name", "album_count", "label"],
["Taylor Swift", 9, "Republic"],
["The National", 8, "4AD"]
]
Nodes
YAML
Folklore:
# Following node labeled TS and AD
- &TS Taylor Swift
- &AD Aaron Dressner
- Jack Antonoff
Lover:
- *TS # Subsequent occurrence
I Am Easy to Find:
- *AD
JSON
No JSON equivalent.
But this is the output from the above YAML.
{
"Folklore": ["Taylor Swift", "Aaron Dressner", "Jack Antonoff"],
"Lover": ["Taylor Swift"],
"I Am Easy to Find": ["Aaron Dressner"]
}
Sample GitHub Action
YAML
name: Netlify Rebuild
on:
schedule:
- cron: "0 21 * * MON-FRI"
jobs:
build:
name: Netlify Rebuild
runs-on: ubuntu-latest
steps:
- name: Curl request
run: curl -X POST -d {} https://api.netlify.com/build_hooks/601321b7879709a8b8874175
JSON
{
"name": "Netlify Rebuild",
"on": {
"schedule": [
{
"cron": "0 21 * * MON-FRI"
}
]
},
"jobs": {
"build": {
"name": "Netlify Rebuild",
"runs-on": "ubuntu-latest",
"steps": [
{
"name": "Curl request",
"run": "curl -X POST -d {} https://api.netlify.com/build_hooks/601321b7879709a8b8874175"
}
]
}
}
}
Takeaways
Complex types are inferred
The structures of complex types are hidden in YAML.
They get inferred by the structure of included data.
Keep a look out for -
and :
which indicate the containing structure.
Dash -
If you see a dash -
, it means you're describing a single array item.
This means you're in an array.
Colon :
If you see a color :
separating two values, it means your describing a key-value pair.
This means you're in an object.
In the wild
Since YAML is used often for configuration, it's likely that your root type is object
.
At least that's the case for Markdown Frontmatter, GitHub Actions, and CloudFormation templates.
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