What Constants Are

Constants define reusable literal values.

const apiVersion = "1.0.0"
const maxRetries = 3
const featureEnabled = true

Constants do not have explicit type annotations. VDL infers their value kind from the literal.

Correct:

const timeoutMs = 2500

Invalid:

const timeoutMs int = 2500

Scalar Literals

VDL supports these scalar literal forms:

const textValue = "hello"
const intValue = 42
const floatValue = 3.14
const trueValue = true
const falseValue = false
const belowZero = -42
const negativeRatio = -0.5

Numbers accept an optional leading minus with no space (-42, -0.5).

Integers must fit in 64-bit signed range and floats in 64-bit range; out-of-range literals are rejected instead of silently mistyped.

Strings use double quotes. A string may span multiple lines; its content is verbatim, so line breaks, spaces, tabs, and blank lines are preserved exactly.

Only \" (a quote) and \\ (a backslash) are decoded. Any other backslash sequence stays as written: \n typed as two characters is not a line break. Use \" to include a quote and \\ to include a backslash:

const quote = "say \"hi\""
const backslash = "back\\slash"

Multiline Authoring Pattern

Because strings are verbatim, source indentation becomes part of the value. When the value is whitespace-sensitive, place content at column zero:

const prompt = "line one
line two"

When the consumer ignores whitespace, indent freely for readability. Policy expressions are one example: the policy parser ignores insignificant whitespace, so this is fine:

const expr = "
  principal.id == resource.ownerId &&
  principal.department == 'Finance'
"

Object Literals

Object literals use braces and space-separated key/value entries.

const serverConfig = {
  host "localhost"
  port 8080
  tls false
}

There are no colons and no commas.

Object keys must be unique inside the same object.

Nested Object Literals

const appConfig = {
  server {
    host "localhost"
    port 8080
  }
  logging {
    level "info"
  }
}

Array Literals

Array items are separated by whitespace.

const roles = ["admin" "editor" "viewer"]
const retryBackoffMs = [100 250 500]

Array items merge by union: shared object keys must have the same type at every level, while extra keys are allowed (plugins read them as optional fields). Key order does not matter and scalar values are ignored, so [{a 1} {a 2}] and [{a 1} {b 2}] are both valid. Spreads are expanded before comparing.

Invalid:

const mixedValues = ["admin" 1 true]
const mixedKinds = [{a true b false} {a 1}]

Constant References

Constants can reference other constants.

const defaultTimeoutMs = 5000
const requestTimeoutMs = defaultTimeoutMs

References use the constant name directly.

Enum Member References

Constants can reference enum members with EnumName.MemberName.

enum ProductStatus {
  Draft = "draft"
  Published = "published"
}

const defaultStatus = ProductStatus.Draft

The referenced enum and member must exist.

Object Spreads

Object literals can spread another constant object.

const baseConfig = {
  host "localhost"
  port 8080
}

const productionConfig = {
  ...baseConfig
  host "api.example.com"
}

Only constants whose value is an object literal can be used in object spreads.

Spreads use the Name form. Name.Member is not valid for object spreads.

Constants In Annotations

Because annotation arguments are data literals, constants can also help share annotation values.

const owner = "platform"

@meta({ team owner })
type ServiceConfig {
  name string
}

Whether a plugin understands a particular annotation is plugin-specific.

Naming

Constant names should use camelCase.

const apiVersion = "1.0.0"
const maxPageSize = 100