lyng/docs/lyng_d_files.md

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# `.lyng.d` Definition Files
`.lyng.d` files declare Lyng symbols for tooling without shipping runtime implementations. The IntelliJ IDEA plugin merges
all `*.lyng.d` files from the current directory and its parent directories into the active file’s analysis, enabling:
- completion
- navigation
- error checking for declared symbols
- Quick Docs for declarations defined in `.lyng.d`
Place `*.lyng.d` files next to the code they describe (or in a parent folder). The plugin will pick them up automatically.
## Writing `.lyng.d` Files
You can declare any language-level symbol in a `.lyng.d` file. Use doc comments before declarations to make Quick Docs
work in the IDE. The doc parser accepts standard comments (`/** ... */` or `// ...`) and supports tags like `@param`.
### Full Example
```lyng
/** Library entry point */
extern fun connect(url: String, timeoutMs: Int = 5000): Client
/** Type alias with generics */
type NameMap = Map<String, String>
/** Multiple inheritance via interfaces */
interface A { abstract fun a(): Int }
interface B { abstract fun b(): Int }
/** A concrete class implementing both */
class Multi(name: String) : A, B {
/** Public field */
val id: Int = 0
/** Mutable property with accessors */
var size: Int
get() = 0
set(v) { }
/** Instance method */
fun a(): Int = 1
fun b(): Int = 2
}
/** Nullable and dynamic types */
extern val dynValue: dynamic
extern var dynVar: dynamic?
/** Delegated property */
class LazyBox(val create) {
fun getValue(thisRef, name) = create()
}
val cached by LazyBox { 42 }
/** Delegated function */
object RpcDelegate {
fun invoke(thisRef, name, args...) = Unset
}
fun remoteCall by RpcDelegate
/** Singleton object */
object Settings {
val version: String = "1.0"
}
/** Class with documented members */
class Client {
/** Returns a greeting. */
fun greet(name: String): String = "hi " + name
}
```
See a runnable sample file in `docs/samples/definitions.lyng.d`.
Notes:
- Use real bodies if the declaration is not `extern` or `abstract`.
- If you need purely declarative stubs, prefer `extern` members (see `embedding.md`).
## Doc Comment Format
Doc comments are picked up when they immediately precede a declaration.
```lyng
/**
* A sample function.
* @param name user name
* @return greeting string
*/
fun greet(name: String): String = "hi " + name
```
## Generating `.lyng.d` Files
You can generate `.lyng.d` as part of your build. A common approach is to write a Gradle task that emits a file from a
template or a Kotlin data model.
Example (pseudo-code):
```kotlin
tasks.register("generateLyngDefs") {
doLast {
val out = file("src/main/lyng/api.lyng.d")
out.writeText(
"""
/** Generated API */
fun ping(): Int
""".trimIndent()
)
}
}
```
Place the generated file in your source tree, and the IDE will load it automatically.