55 lines
1.3 KiB
Markdown
55 lines
1.3 KiB
Markdown
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---
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slug: /1233/default-values-cue
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displayed_sidebar: '0.2'
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---
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# Default values and optional fields
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When writing a Cue config, you will sometimes want to set default values in your package.
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The most common way you'll encounter in our codebase is: `key: type | *value`:
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```cue
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defaultValue: string | *"foo"
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defaultValue: bool | *false
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```
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You'll also encounter the `*null` default value, which is self explanatory:
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```cue
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// here, defaultValue either accepts a #PersonalDefinition, or stays null by default
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defaultValue: #PersonalDefinition | *null
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```
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To test the type of `defaultValue`, you can directly do such assertion:
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```cue
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if defaultValue != "foo" | if defaultValue != false | if defaultValue != null {
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...
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}
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if defaultValue == "foo" | if defaultValue == false | if defaultValue == null {
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...
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}
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```
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However, don't get confused with the optional fields. Optional fields check whether a key is concrete at the given scope in the DAG. You declare them with `?` at the end of their name: `foo?`.
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```cue
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foo?: string // 1. declare foo. It remains undefined for now
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foo: "bar" // 2. Now, `foo` gets concrete. The field isn't undefined anymore
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```
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To check on a field's concreteness, use the bottom value `_|_`:
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```cue
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if foo != _|_ { // if foo is not `undefined`
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...
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}
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if foo == _|_ { // if foo is `undefined`
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...
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}
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```
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