Swift Result Builders Cheat Sheet
Covers writing @resultBuilder types, buildBlock/buildOptional/buildEither methods, and how SwiftUI's ViewBuilder and DSLs like this work under the hood.
2 PagesAdvancedJan 22, 2026
Defining a Result Builder
A result builder is a type with static `buildBlock`-family methods, applied via `@resultBuilder`.
swift
@resultBuilderstruct HTMLBuilder { static func buildBlock(_ components: String...) -> String { components.joined(separator: "\n") }}@HTMLBuilderfunc page() -> String { "<h1>Title</h1>" "<p>Body text</p>"}print(page())// <h1>Title</h1>// <p>Body text</p>
Supporting `if`/`else` and Optionals
Implement `buildEither` and `buildOptional` to allow control flow inside the builder closure.
swift
@resultBuilderstruct HTMLBuilder { static func buildBlock(_ components: String...) -> String { components.joined(separator: "\n") } static func buildEither(first component: String) -> String { component } static func buildEither(second component: String) -> String { component } static func buildOptional(_ component: String?) -> String { component ?? "" } static func buildArray(_ components: [String]) -> String { components.joined(separator: "\n") }}@HTMLBuilderfunc page(isAdmin: Bool) -> String { "<h1>Dashboard</h1>" if isAdmin { "<button>Admin Panel</button>" } else { "<p>Standard user</p>" } for i in 1...3 { "<li>Item \(i)</li>" }}
SwiftUI-style Usage
This is exactly the mechanism behind `@ViewBuilder`, `@SceneBuilder`, and `Regex` builders.
swift
struct ContentView: View { var body: some View { VStack { // VStack's init takes @ViewBuilder content: () -> Content Text("Hello") if isLoggedIn { Text("Welcome back") } } }}// Applying a builder to a closure parameter:func container(@HTMLBuilder content: () -> String) -> String { "<div>\(content())</div>"}
Result Builder Static Methods
The methods the compiler looks for, and when each is invoked.
- buildBlock(_:)- combines a sequence of statements into one result (required)
- buildOptional(_:)- handles `if` without `else` (result is optional)
- buildEither(first:)/(second:)- handles `if/else` and `switch` branches
- buildArray(_:)- handles `for` loops inside the builder
- buildExpression(_:)- transforms each individual expression before blocking
- buildLimitedAvailability(_:)- handles `if #available` type erasure
- buildFinalResult(_:)- wraps/transforms the final built value before returning
Pro Tip
Implement buildExpression(_:) to accept multiple input types (e.g. String and Image) and normalize them to one internal representation — that's how SwiftUI lets ViewBuilder mix wildly different View types in one block.
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