Tools for DSL

You already know about Domain Specific Languages (DSLs), and why Kotlin is a good choice to build DSLs. Today, we will see the types of DSLs - Internal and External. Then we will learn to build our own DSLs, which you can use when building your own library or in your next project.

Types of DSLs in Kotlin

Kotlin, as a modern programming language, offers unique features that facilitate Domain-Specific Language (DSL) creation. These DSLs can be split into two main types: internal DSLs and external DSLs.

Internal DSLs

Internal DSLs are directly built within Kotlin. They leverage Kotlin's syntax to create a fluent and intuitive language-like API. Kotlin's support for higher-order functions, extension functions, and infix notation makes it especially suitable for internal DSL creation.

Here's a simple example of an internal DSL in Kotlin:

fun greetingDSL(block: GreetingBuilder.() -> Unit): String {
    val builder = GreetingBuilder()
    builder.block()
    return builder.build()
}

class GreetingBuilder {
    var name: String = ""
    
    fun build(): String = "Hello, $name!"
}

// Usage
val greeting = greetingDSL {
    name = "Alice"
}
println(greeting) // Prints: Hello, Alice!

In the above snippet, greetingDSL is a function that receives a lambda with receiver, allowing us to scope functions and properties to the GreetingBuilder class, creating DSL-like syntax.

External DSLs

On the other hand, external DSLs are not limited by Kotlin's syntax. They are usually defined in a custom language and parsed separately. However, Kotlin can be used to write the parser and interpreter for the DSL, taking advantage of Kotlin's expressiveness and brevity.

Here's a conceptual snippet for an external DSL parser written in Kotlin:

class DSLParser(val input: String) {
    fun parse(): DSLAst {
        // Parsing logic to convert input to an AST
    }
}

// Assume DSLAst is an abstract syntax tree representation of our DSL
// Usage
val parser = DSLParser("dsl-code-here")
val ast = parser.parse()
// Further processing of ast

In this case, the DSL code is a string ("dsl-code-here") that gets parsed by DSLParser to produce an abstract syntax tree (AST), which could then be interpreted or compiled.

In summary, Kotlin's flexibility and concise syntax makes it an excellent choice for creating both internal and external DSLs, allowing developers to create tailored languages for specific domains. In this topic, we'll be focusing specifically on internal DSLs.

Basic Building Blocks for Creating a DSL in Kotlin

When creating a Domain-Specific Language (DSL) in Kotlin, there are certain basic components and techniques at play. These building blocks are crucial in creating a fluent and intuitive API that feels like a natural extension of the language.

1. Lambda with Receiver:

This technique allows you to scope functions and properties within a lambda expression, providing a DSL context. It's like extending the lambda's scope with added functionality.

class HtmlBuilder {
    fun body(content: String) {}
}

fun html(init: HtmlBuilder.() -> Unit): HtmlBuilder {
    val builder = HtmlBuilder()
    builder.init()
    return builder
}

val homePage = html {
    body("Hello, DSL!")
}

2. Invoking Functions with Conventions:

Kotlin supports conventions like invoke that can be used to simulate calling an object as a function, which is a neat way to structure a DSL.

class HtmlBuilder {
    operator fun invoke(route: String, builder: () -> Unit) {}
}

homePage("/home") {
    // handle the route
}

3. Standard Library Functions:

Kotlin's standard library is rich with functions that help reduce boilerplate code. Functions like apply, with, and also can be used to configure objects within a DSL.

data class ServerConfig(var host: String = "", var port: Int = 0)

fun serverConfig(init: ServerConfig.() -> Unit) = ServerConfig().apply(init)

val config = serverConfig {
    host = "localhost"
    port = 8080
}

By using these components, Kotlin developers can create powerful and expressive DSLs that blend seamlessly with the language features, offering a high abstraction level and readability for specific domains.

Conclusion

Kotlin’s abilities for DSL development can be harnessed to create both internal and external DSLs. Internal DSLs utilize Kotlin's syntax to provide a fluent API within the language, while external DSLs use Kotlin's expressiveness for parsing and interpreting custom language constructs. In summary, Kotlin's language features, the capacity to facilitate creation of both internal and external DSLs make it an excellent choice for developers looking to build tailored languages for their domains, thereby improving the clarity and efficiency of their code.