Kapshot

Kapshot is a simple Kotlin compiler plugin for capturing source code text from closure blocks and declarations.

Usage

Include the io.koalaql.kapshot-plugin Gradle plugin in your build.gradle.kts:

plugins {
    /* ... */

    id("io.koalaql.kapshot-plugin") version "0.1.2"
}

Capturing Blocks

Now your Kotlin code can use CapturedBlock<T> as a source enriched replacement for () -> T. You can use the source property on any instance of CapturedBlock to access the source for that block.

import io.koalaql.kapshot.CapturedBlock

val captured = CapturedBlock {
    println("Hello!")
}

check(captured.source.text == """println("Hello!")""")

You can invoke the block similar to a regular function:

import io.koalaql.kapshot.CapturedBlock

fun equation(block: CapturedBlock<Int>): String {
    val result = block() // invoke the block

    return "${block.source} = $result"
}

check(equation { 2 + 2 } == "2 + 2 = 4")

Parameterized Blocks

The default CapturedBlock interface doesn't accept any arguments to invoke and is only generic on the return type. This means the captured source block must depend only on state from the enclosing scope. To write source capturing versions of builder blocks or common higher-order functions like map and filter you will need to define your own capture interface that extends Capturable.

/* must be a fun interface to support SAM conversion from blocks */
fun interface CustomCapturable<T, R> : Capturable<CustomCapturable<T, R>> {
    /* invoke is not special. this could be any single abstract method */
    operator fun invoke(arg: T): R

    /* withSource is called by the plugin to add source information */
    override fun withSource(source: Source): CustomCapturable<T, R> =
        object : CustomCapturable<T, R> by this { override val source = source }
}

Once you have declared your own Capturable you can use it in a similar way to CapturedBlock from above.

fun <T> List<T>.mapped(block: CustomCapturable<T, T>): String {
    return "$this.map { ${block.source} } = ${map { block(it) }}"
}

check(
    listOf(1, 2, 3).mapped { x -> x*2 } ==
    "[1, 2, 3].map { x -> x*2 } = [2, 4, 6]"
)

If it is present, the block's argument list is considered part of its source text.

Declarations

You can capture declaration sources using the @CaptureSource annotation. The source of annotated declarations can then be retrieved using sourceOf<T> for class declarations or sourceOf(::declaration) for method and property declarations. The source capture starts at the end of the @CaptureSource annotation.

@CaptureSource
class MyClass {
    @CaptureSource
    fun twelve() = 12
}

check(
    sourceOf<MyClass>().text ==
    """
    class MyClass {
        @CaptureSource
        fun twelve() = 12
    }
    """.trimIndent()
)

check(
    sourceOf(MyClass::twelve).text ==
    "fun twelve() = 12"
)

Source Location

The Source::location property contains information about the location of captured source code including the file path (relative to the project root directory) and the char, line and column offsets for both the start and end of the captured source. Offsets are 0-indexed.

val source = CapturedBlock { 2 + 2 }.source
val location = source.location

println(
    "`${source.text}`"
    + " found in ${location.path}"
    + " @ line ${location.from.line+1}"
)

The code above will print the following:

`2 + 2` found in src/main/kotlin/Main.kt @ line 176

Purpose

The purpose of this plugin is to support experimental literate programming and documentation generation techniques in Kotlin

An example of this is the code used to generate this README.md. Capturing source from blocks allows sample code to be run and tested during generation.

View the source here: readme/src/main/kotlin/Main.kt