DSL in Swift: How to Create Your Own Programming Language in an Evening

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Introduction: When Versatility is the Enemy

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Imagine you are writing code for network configuration, log parsing, or describing business rules. Every time — repetitive constructs, tons of brackets, nested conditional statements, and painful reading of someone else's code. Sound familiar?

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In such situations, domain-specific languages (DSL) come to the rescue. This is not magic or reinventing the wheel — it's a way to make code so readable that it documents itself. In this article, we'll break down how to create your own DSL in Swift — powerful, elegant, and yet not requiring a compiler.

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A DSL in Swift is not just a trendy iOS development fad. It's a tool used in SwiftUI, Combine, and even in backend libraries. You will learn to design languages that solve specific tasks: from data parsing to UI description. Let's go!

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What is a DSL and Why Does a Developer Need It?

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A Domain-Specific Language is a language tailored to one narrow area. Unlike a general-purpose language (like Swift or Python), a DSL speaks the language of your subject domain. For example, instead of:

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let config = NetworkConfig()\nconfig.host = "api.example.com"\nconfig.port = 443\nconfig.timeout = 30

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You write:

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let config = networkConfig {\n    host("api.example.com")\n    port(443)\n    timeout(30)\n}

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The second option reads like instructions in a human language. This is the goal of a DSL — to reduce cognitive load and make code self-documenting.

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In Swift, creating a DSL most often uses:

Result Builders — a new feature in Swift 5.4+ that allows building nested structures using declarative syntax.
Functional chains — the classic builder pattern.
Operators — operator overloading to create fluent interfaces.

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But let's get straight to the point. We will create a DSL for parsing CSV files — a task that torments every developer at least once in their life.

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Example 1: A Simple DSL for Network Configuration

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Let's start with a basic example that demonstrates the mechanics. Suppose we need to describe HTTP requests. Instead of a bunch of parameters — a concise DSL.

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// Define the request structure\nstruct HTTPRequest {\n    var method: String = "GET"\n    var path: String = "/"\n    var headers: [String: String] = [:]\n    var body: Data? = nil\n}\n\n// Create a DSL using Result Builder\n@resultBuilder\nstruct RequestBuilder {\n    static func buildBlock(_ components: HTTPRequestModifier...) -> HTTPRequest {\n        var request = HTTPRequest()\n        for mod in components {\n            mod.apply(&request)\n        }\n        return request\n    }\n}\n\n// Request modifiers\nprotocol HTTPRequestModifier {\n    func apply(_ request: inout HTTPRequest)\n}\n\nstruct MethodModifier: HTTPRequestModifier {\n    let method: String\n    func apply(_ request: inout HTTPRequest) {\n        request.method = method\n    }\n}\n\nstruct PathModifier: HTTPRequestModifier {\n    let path: String\n    func apply(_ request: inout HTTPRequest) {\n        request.path = path\n    }\n}\n\n// Helper functions\nfunc method(_ m: String) -> HTTPRequestModifier { MethodModifier(method: m) }\nfunc path(_ p: String) -> HTTPRequestModifier { PathModifier(path: p) }\n\n// Usage\n@RequestBuilder\nfunc buildGetRequest() -> HTTPRequest {\n    method("GET")\n    path("/users")\n}\n\nlet request = buildGetRequest()\nprint(request.method, request.path) // GET /users

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This code is the foundation of any DSL in Swift. @resultBuilder collects all expressions inside the block and transforms them into a single structure. Important: each element is a modifier that changes the state.

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Now let's complicate the task: add nesting and conditions.