DSL Building

Introduction#

Focus on the syntax details to design internal DSLs in Kotlin.

Infix approach to build DSL

If you have:

infix fun <T> T?.shouldBe(expected: T?) = assertEquals(expected, this)

you can write the following DSL-like code in your tests:

@Test
fun test() {
  100.plusOne() shouldBe 101
}

Overriding invoke method to build DSL

If you have:

class MyExample(val i: Int) {
  operator fun <R> invoke(block: MyExample.() -> R) = block()
  fun Int.bigger() = this > i
}

you can write the following DSL-like code in your production code:

fun main2(args: Array<String>) {
    val ex = MyExample(233)
    ex {
        // bigger is defined in the context of `ex`
        // you can only call this method inside this context
        if (777.bigger()) kotlin.io.println("why")
    }
}

Using operators with lambdas

If you have:

val r = Random(233)
infix inline operator fun Int.rem(block: () -> Unit) {
  if (r.nextInt(100) < this) block()
}

You can write the following DSL-like code:

20 % { println("The possibility you see this message is 20%") }

Using extensions with lambdas

If you have:

operator fun <R> String.invoke(block: () -> R) = {
  try { block.invoke() }
  catch (e: AssertException) { System.err.println("$this\n${e.message}") }
}

You can write the following DSL-like code:

"it should return 2" {
   parse("1 + 1").buildAST().evaluate() shouldBe 2
}

If you feel confused with shouldBe above, see the example Infix approach to build DSL.