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Result builders

Table of Contents

Changes from the first revision

  • The feature is now called result builders (rather than "function builders"). James Dempsey provided some exploration and rationale for naming that led to this choice.
  • Although not part of the proposal itself, the implementation quality has been improved to help guide users in writing their result builders, with code completions and Fix-Its to help define the builder methods.
  • Added a section on dropping Void/Never values to the list of alternatives considered.
  • Clarified the role of each of the result-building methods, and provided declarations for each that are easier to understand and copy/paste.

Introduction

This proposal describes result builders, a new feature which allows certain functions (specially-annotated, often via context) to implicitly build up a result value from a sequence of components.

The basic idea is that the results of the function's statements are collected using a builder type, like so:

// Original source code:
@TupleBuilder
func build() -> (Int, Int, Int) {
  1
  2
  3
}

// This code is interpreted exactly as if it were this code:
func build() -> (Int, Int, Int) {
  let _a = TupleBuilder.buildExpression(1)
  let _b = TupleBuilder.buildExpression(2)
  let _c = TupleBuilder.buildExpression(3)
  return TupleBuilder.buildBlock(_a, _b, _c)
}

In this example, all the statements are expressions and so produce a single value apiece. Other statements, like let, if, and while, are variously either handled differently or prohibited; see the proposal details below.

In effect, this proposal allows the creation of a new class of embedded domain-specific languages in Swift by applying builder transformations to the statements of a function. The power of these builder transformations is intentionally limited so that the result preserves the dynamic semantics of the original code: the original statements of the function are still executed as normal, it's just that values which would be ignored under normal semantics are in fact collected into the result. The use of an ad hoc protocol for the builder transformation leaves room for a wide variety of future extension, whether to support new kinds of statements or to customize the details of the transformation. A similar builder pattern was used successfully for string interpolation in SE-0228.

Result builders have been a "hidden" feature since Swift 5.1, under the name "function builder", and the implementation (and its capabilities) have evolved since then. They are used most famously by SwiftUI to declaratively describe user interfaces, but others have also experimented with building Swift syntax trees, testing, a Shortcuts DSL, a CSS DSL, and an alternative SwiftPM manifest format. There's a GitHub repository dedicated to awesome function builders with more applications.

Motivation

It's always been a core goal of Swift to allow the creation of great libraries. A lot of what makes a library great is its interface, and Swift is designed with rich affordances for building expressive, type-safe interfaces for libraries. In some cases, a library's interface is distinct enough and rich enough to form its own miniature language within Swift. We refer to this as a Domain Specific Language (DSL), because it lets you better describe solutions within a particular problem domain.

Result builders target a specific kind of interface that involves the declaration of list and tree structures, which are useful in many problem domains, including generating structured data (e.g. XML or JSON), UI view hierarchies (notably including Apple's SwiftUI framework, mentioned above), and similar use cases. In this proposal, we will be primarily working with code which generates an HTML DOM hierarchy, somewhat like a web templating language except in code; credit goes to Harlan Haskins for this example.

Suppose that you have a program, part of which generates HTML. You could, of course, directly generate a String, but that's error-prone (you have to make sure you're handling escapes and closing tags correctly throughout your code) and would make it hard to structurally process the HTML before sending it out. An alternative approach is to generate a DOM-like representation of the HTML and then render it to a String as a separate pass:

protocol HTML {
  func renderAsHTML(into stream: HTMLOutputStream)
}

extension String: HTML {
  func renderAsHTML(into stream: HTMLOutputStream) {
    stream.writeEscaped(self)
  }
}

struct HTMLNode: HTML {
  var tag: String
  var attributes: [String: String] = [:]
  var children: [HTML] = []

  func renderAsHTML(into stream: HTMLOutputStream) {
    stream.write("<")
    stream.write(tag)
    for (k, v) in attributes.sort { $0.0 < $1.0 } {
      stream.write(" ")
      stream.write(k)
      stream.write("=")
      stream.writeDoubleQuoted(v)
    }
    if children.isEmpty {
      stream.write("/>")
    } else {
      stream.write(">")
      for child in children {
        child.renderAsHTML(into: stream)
      }
      stream.write("</")
      stream.write(tag)
      stream.write(">")
    }
  }
}

To make it easier to build these HTML hierarchies, we can define a bunch of convenient node-builder functions corresponding to common nodes:

func body(_ children: [HTML]) -> HTMLNode { ... }
func division(_ children: [HTML]) -> HTMLNode { ... }
func paragraph(_ children: [HTML]) -> HTMLNode { ... }
func header1(_ text: String) -> HTMLNode { ... }

Unfortunately, even with these helper functions, it's still pretty awkward to actually produce a complex hierarchy because of all the lists of children:

return body([
  division([
    header1("Chapter 1. Loomings."),
    paragraph(["Call me Ishmael. Some years ago"]),
    paragraph(["There is now your insular city"])
  ]),
  division([
    header1("Chapter 2. The Carpet-Bag."),
    paragraph(["I stuffed a shirt or two"])
  ])
])

The first problem is that there's a lot of punctuation here: commas, parentheses, and brackets. This is a pretty superficial problem, and it's probably not a showstopper by itself, but it is something that it'd be nice to avoid, because it does distract a little from the content.

The second problem is that, because we're using array literals for the children, the type-checker is going to require the elements to have a homogeneous type. That's fine for our HTML example, but it's limiting in general, because some trees are more generic and would benefit from propagating the types of the children into the type of the node. For example, SwiftUI uses this for various optimizations within the view hierarchy.

The biggest problem, though, is that it's awkward to change the structure of this hierarchy. That's fine if our hierarchy is just the static contents of Moby Dick, but in reality, we're probably generating HTML that should vary significantly based on dynamic information. For example, if we wanted to allow chapter titles to be turned off in the output, we'd have to restructure that part of the code like this:

division((useChapterTitles ? [header1("Chapter 1. Loomings.")] : []) +
    [paragraph(["Call me Ishmael. Some years ago"]),
     paragraph(["There is now your insular city"])])

It's also harder to use good coding practices within this hierarchy. For example, suppose there's a common string we want to use many times, and so we want to create a variable for it:

let chapter = spellOutChapter ? "Chapter " : ""
  ...
header1(chapter + "1. Loomings.")
  ...
header1(chapter + "2. The Carpet-Bag.")

Most programmers would advise declaring this variable in as narrow a scope as possible, and as close as possible to where it's going to be used. But because the entire hierarchy is an expression, and there's no easy way to declare variables within expressions, every variable like this has to be declared above the entire hierarchy. (Now, it's true that there's a trick for declaring locals within expressions: you can start a closure, which gives you a local scope that you can use to declare whatever you want, and then immediately call it. But this is awkward in its own way and significantly adds to the punctuation problem.)

Some of these problems would be solved, at least in part, if the hierarchy was built up by separate statements:

let chapter = spellOutChapter ? "Chapter " : ""

let d1header = useChapterTitles ? [header1(chapter + "1. Loomings.")] : []
let d1p1 = paragraph(["Call me Ishmael. Some years ago"])
let d1p2 = paragraph(["There is now your insular city"])
let d1 = division(d1header + [d1p1, d1p2])

let d2header = useChapterTitles ? [header1(chapter + "2. The Carpet-Bag.")] : []
let d2p1 = paragraph(["I stuffed a shirt or two"])
let d2 = division(d2header + [d2p1])

return body([d1, d2])

But in most ways, this is substantially worse. There's quite a lot of extra code that's made it much more difficult to track what's really going on. That's especially true with all the explicit data flow, where it can be tough to piece together what nodes are children of others; moreover, that code is as tedious to write as it is to read, making it very prone to copy-paste bugs. Furthermore, the basic structure of the hierarchy used to be clear from the code, and that's been completely lost: all of the nicely-nested calls to node builders have been flattened into one sequence. Also, while optional children are a common problem for this hierarchy, the actual code to handle them has to be repeated over and over again, leading to boilerplate and bugs. Overall, this is not a good approach at all.

What we really want is a compromise between these two approaches:

  • We want the programming flexibility that comes from building things up with ordinary blocks of code: the ability to have local declarations and explicit control flow.

  • We want the explicit recursive structure and implicit data flow that comes from building things up with expression operands.

This suggests a straightforward resolution: allow certain blocks of code to have implicit data flow out of the block and into the context which entered the block. The goal here is to allow this node hierarchy to be declared something like this:

return body {
  let chapter = spellOutChapter ? "Chapter " : ""
  division {
    if useChapterTitles {
      header1(chapter + "1. Loomings.")
    }
    paragraph {
      "Call me Ishmael. Some years ago"
    }
    paragraph {
      "There is now your insular city"
    }
  }
  division {
    if useChapterTitles {
      header1(chapter + "2. The Carpet-Bag.")
    }
    paragraph {
      "I stuffed a shirt or two"
    }
  }
}

The above has to be embedded into the ordinary language somehow, which means that at least the outermost layer must obey something like ordinary language rules. Under ordinary language rules, this is a function call to body passing a trailing closure. It makes sense, then, that what we're doing is taking the body of the anonymous function and applying some sort of transformation to it. This raises a series of separate questions:

  1. What is it about this source code that triggers the transformation? We have chosen not to require an explicit annotation on every closure that needs transformation; see Alternatives Considered for a discussion. So somehow this must get picked up from the fact that we're passing the closure to body.

  2. Given that the transformation is going to collect a bunch of information, how does that information get back to body? Since the transformation is working on a function body, this one's straightforward: the collected information will just be the return value of the function. There's no requirement to support this transformation simultaneously with ordinary return.

  3. Given that the transformation has collected a sequence of partial results, how do they get combined to produce a return value? We could simply always produce a tuple, but that isn't necessarily what the caller actually wants. In particular, it may be useful to allow the DSL to do different things for different partial results. The logical answer is that there should be a function somehow provided by the DSL to combine the partial results, and this function might need to be generic or overloaded.

  4. Given that the transformation might collect a partial result within optionally-executed code (e.g. in an if statement), what should be passed to the combining function? The transformation can be taught to produce an optional value of the partial result type, but the DSL needs to be able to distinguish between a partial result that is optionally produced and a partial result that coincidentally happens to be of optional type. The logical answer is that there should be a function provided by the DSL to translate optional partial results into "ordinary" partial results that can be collected normally.

These last two points (and some other considerations) strongly suggest that the DSL should be identified with a type that can provide an arbitrary namespace of functions that can be used as ad hoc customization points for the transformation.

Detailed design

SE-0258 introduced the concept of custom attributes to Swift, an approach we build on here.

Result builder types

A result builder type is a type that can be used as a result builder, which is to say, as an embedded DSL for collecting partial results from the expression-statements of a function and combining them into a return value.

A result builder type must satisfy two basic requirements:

  • It must be annotated with the @resultBuilder attribute, which indicates that it is intended to be used as a result builder type and allows it to be used as a custom attribute.

  • It must supply at least one static buildBlock result-building method.

In addition, it may supply a sufficient set of other result-building methods to translate the kinds of functions which the DSL desires to translate.

Result builder attributes

A result builder type can be used as an attribute in two different syntactic positions. The first position is on a func, var, or subscript declaration. For the var and subscript, the declaration must define a getter, and the attribute is treated as if it were an attribute on that getter. A result builder attribute used in this way causes the result builder transform to be applied to the body of the function; it is not considered part of the interface of the function and does not affect its ABI.

A result builder type can also be used as an attribute on a parameter of function type, including on parameters of protocol requirements. A result builder attribute used in this way causes the result builder transform to be applied to the body of any explicit closures that are passed as the corresponding argument, unless the closure contains a return statement. This is considered part of the interface of the function and can affect source compatibility, although it does not affect its ABI.

Result-building methods

To be useful as a result builder, the result builder type must provide a sufficient subset of the result-building methods. The protocol between the compiler's generated code and the result builder type is intended to be ad hoc and arbitrarily extensible in the future.

Result-building methods are static methods that can be called on the result builder type. Calls to result-building methods are type-checked as if a programmer had written BuilderType.<methodName>(<arguments>), where the arguments (including labels) are as described below; therefore, all the ordinary overload resolution rules apply. However, in some cases the result builder transform changes its behavior based on whether a result builder type declares a certain method at all; it is important to note that this is a weaker check, and it may be satisfied by a method that cannot in fact ever be successfully called on the given builder type.

This is a quick reference for the result-building methods currently proposed. The typing here is subtle, as it often is in macro-like features. In the following descriptions, Expression stands for any type that is acceptable for an expression-statement to have (that is, a raw partial result), Component stands for any type that is acceptable for a partial or combined result to have, and FinalResult stands for any type that is acceptable to be ultimately returned by the transformed function.

  • buildBlock(_ components: Component...) -> Component is used to build combined results for statement blocks. It is required to be a static method in every result builder.

  • buildOptional(_ component: Component?) -> Component is used to handle a partial result that may or may not be available in a given execution. When a result builder provides buildOptional(_:), the transformed function can include if statements without an else.

  • buildEither(first: Component) -> Component and buildEither(second: Component) -> Component are used to build partial results when a selection statement produces a different result from different paths. When a result builder provides these methods, the transformed function can include if statements with an else statement as well as switch statements.

  • buildArray(_ components: [Component]) -> Component is used to build a partial result given the partial results collected from all of the iterations of a loop. When a result builder provides buildArray(_:), the transformed function can include for..in statements.

  • buildExpression(_ expression: Expression) -> Component is used to lift the results of expression-statements into the Component internal currency type. It is optional, but when provided it allows a result builder to distinguish Expression types from Component types or to provide contextual type information for statement-expressions.

  • buildFinalResult(_ component: Component) -> FinalResult is used to finalize the result produced by the outermost buildBlock call for top-level function bodies. It is optional, and allows a result builder to distinguish Component types from FinalResult types, e.g. if it wants builders to internally traffic in some type that it doesn't really want to expose to clients.

  • buildLimitedAvailability(_ component: Component) -> Component is used to transform the partial result produced by buildBlock in a limited-availability context (such as if #available) into one suitable for any context. It is optional, and is only needed by result builders that might carry type information from inside an if #available outside it.

The set of requirements can be summarized by the following example result builder:

@resultBuilder
struct ExampleResultBuilder {
  /// The type of individual statement expressions in the transformed function,
  /// which defaults to Component if buildExpression() is not provided.
  typealias Expression = ...

  /// The type of a partial result, which will be carried through all of the
  /// build methods.
  typealias Component = ...

  /// The type of the final returned result, which defaults to Component if
  /// buildFinalResult() is not provided.
  typealias FinalResult = ...

  /// Required by every result builder to build combined results from
  /// statement blocks.
  static func buildBlock(_ components: Component...) -> Component { ... }

  /// If declared, provides contextual type information for statement
  /// expressions to translate them into partial results.
  static func buildExpression(_ expression: Expression) -> Component { ... }

  /// Enables support for `if` statements that do not have an `else`.
  static func buildOptional(_ component: Component?) -> Component { ... }

  /// With buildEither(second:), enables support for 'if-else' and 'switch'
  /// statements by folding conditional results into a single result.
  static func buildEither(first component: Component) -> Component { ... }

  /// With buildEither(first:), enables support for 'if-else' and 'switch'
  /// statements by folding conditional results into a single result.
  static func buildEither(second component: Component) -> Component { ... }

  /// Enables support for 'for..in' loops by combining the
  /// results of all iterations into a single result.
  static func buildArray(_ components: [Component]) -> Component { ... }

  /// If declared, this will be called on the partial result of an 'if
  /// #available' block to allow the result builder to erase type
  /// information.
  static func buildLimitedAvailability(_ component: Component) -> Component { ... }

  /// If declared, this will be called on the partial result from the outermost
  /// block statement to produce the final returned result.
  static func buildFinalResult(_ component: Component) -> FinalResult { ... }
}

The result builder transform

The result builder transform is a recursive transformation operating on statement blocks and the individual statements within them.

Statement blocks

Within a statement block, the individual statements are separately transformed into sequences of statements which are then concatenated. Each such sequence may optionally produce a single partial result, which is an expression (typically a reference to a unique local variable defined as part of the transformation) which can be used later in the block.

After the transformation has been applied to all the statements, a call to buildBlock is generated to form a combined result, with all the partial results as unlabelled arguments.

If the statement block is the top-level body of the function being transformed, the final statement in the transformed block is a return with the combined result expression as its operand, wrapped in buildFinalResult if it was provided by the builder. Otherwise, the combined result is propagated outwards, typically by assigning it (possibly after a transformation) to a local variable in the enclosing block; the exact rule is specified in the section for the enclosing statement below.

Declaration statements

Local declarations are left alone by the transformation. This allows developers to factor out subexpressions freely to clarify their code, without affecting the result builder transformation.

Expression statements

An expression statement which does not perform an assignment is transformed as follows:

  • If the result builder type declares the buildExpression result-building method, the transformation calls it, passing the expression-statement as a single unlabeled argument. This call expression is hereafter used as the expression statement. This call is type-checked together with the statement-expression and may influence its type.

  • The statement-expression is used to initialize a unique variable of the statement-expression's result type, as if by let v = <expression>. This variable is treated as a partial result of the containing block. References to this variable are type-checked independently from it so that they do not affect the type of the expression.

The ability to control how expressions are embedded into partial results is an important advantage for certain kinds of DSL, including our HTML example. In the original HTML example, we have an HTML protocol with a very small (and essentially fixed) number of implementing types. This would probably be more natural to represent in Swift as an enum rather than a protocol, but that would prevent a String from being written directly wherever an HTML was required, which would make complex HTML literals even more onerous to write in our original, pre-DSL solution:

return body([
  division([
    header1("Chapter 1. Loomings."),
    paragraph([.text("Call me Ishmael. Some years ago")]),
    paragraph([.text("There is now your insular city")])
  ]),
  division([
    header1("Chapter 2. The Carpet-Bag."),
    paragraph([.text("I stuffed a shirt or two")])
  ])
])

By using a DSL with buildExpression, however, we can use an enum for its natural representational, pattern-matching, and other advantages, then just add overloads to buildExpression to make it easier to build common cases within the DSL:

static func buildExpression(_ text: String) -> [HTML] {
  return [.text(text)]
}

static func buildExpression(_ node: HTMLNode) -> [HTML] {
  return [.node(node)]
}

static func buildExpression(_ value: HTML) -> [HTML] {
  return [value]
}

Assignments

An expression statement which performs an assignment is treated in the same manner as all other expression statements, although it will always return (). A result builder could choose to handle ()-returning expression statements specially by overloading buildExpression, e.g.,

static func buildExpression(_: ()) -> Component { ... }

Selection statements

if/else and switch statements produce values conditionally depending on their cases. There are two basic transformation patterns which can be used, depending on the selection statement itself; we'll show examples of each, then explain the details of the transformation.

Consider a simple "if" statement without an "else" block:

if i == 0 {
  "0"
}

The first transformation pattern for selection statements turns the case into its own optional partial result in the enclosing block. This is a simple transformation handling code that is optionally executed:

var vCase0: String?
if i == 0 {
  var thenVar = "0"
  var thenBlock = BuilderType.buildBlock(thenVar)
  vCase0 = BuilderType.buildOptional(.some(thenBlock))
}

If if statement doesn't have a corresponding else block, like in our example, the result builder transform is going create one implicitly and inject a call to buildOptional(.none) as follows:

var vCase0: String?
if i == 0 {
  var thenVar = "0"
  var thenBlock = BuilderType.buildBlock(thenVar)
  vCase0 = BuilderType.buildOptional(.some(thenBlock))
} else {
  vCase0 = BuilderType.buildOptional(.none)
}

The second transformation pattern produces a balanced binary tree of injections into a single partial result in the enclosing block. It supports if-else and switch. Consider the following code:

if i == 0 {
  "0"
} else if i == 1 {
  "1"
} else {
  generateFibTree(i)
}

Under this pattern, the example code becomes something like the following:

let vMerged: PartialResult
if i == 0 {
  var firstVar = "0"
  var firstBlock = BuilderType.buildBlock(firstVar)
  vMerged = BuilderType.buildEither(first: firstBlock)
} else if i == 1 {
  var secondVar = "1"
  var secondBlock = BuilderType.buildBlock(secondVar)
  vMerged = BuilderType.buildEither(second:
        BuilderType.buildEither(first: secondBlock))
} else {
  var elseVar = generateFibTree(i)
  var elseBlock = BuilderType.buildBlock(elseVar)
  vMerged = BuilderType.buildEither(second:
        BuilderType.buildEither(second: elseBlock))
}

The detailed transformation of selection statements proceeds as follows. The child blocks of the statement are first analyzed to determine the number N of cases that can produce results and whether there are any cases that don't. The implementation is permitted to analyze multiple nesting levels of statements at once; e.g. if a case consists solely of an if chain, the cases of the if can be treated recursively as cases of the switch at the implementation's discretion. A missing else is a separate case for the purposes of this analysis, and will be handled by buildOptional(_:).

If N = 0, the statement is ignored by the transformation. Otherwise, an injection strategy is chosen:

  • If the result builder type declares the buildEither(first:) and buildEither(second:) result-building methods, a full binary tree with N leaves (the injection tree) is chosen, and each result-producing case is uniquely assigned a leaf in it; these decisions are implementation-defined. A unique variable vMerged of fresh type is declared before the statement.

  • Otherwise, unique variables vCase are declared before the statement for each result-producing case.

The transformation then proceeds as follows:

  • In each result-producing case, the transformation is applied recursively. As the final statement in the case, the combined result is injected and assigned outwards:

    • If the statement is not using an injection tree, the combined result is wrapped in Optional.Some and assigned to the appropriate vCase.

    • Otherwise, the path from the root of the injection tree to the appropriate leaf is considered. An expression is formed by the following rules and then assigned to vMerged:

      • For an empty path, the original combined result from the case.

      • For a left branch through the tree, a call to the result-building method buildEither(first:) with the argument being the injection expression for the remainder of the path.

      • For a right branch through the tree, the same but with buildEither(second:).

      • Finally, if there are any non-result-producing cases, the expression is wrapped in Optional.some.

      For example, if the path to the case's leaf is left, left, right, and there are non-result-producing cases, and the original combined result is E, then the injection expression assigned to vMerged is

      Optional.some(
  BuilderType.buildEither(first:
    BuilderType.buildEither(first:
      BuilderType.buildEither(second: E))))

      Note that all of the assignments to vMerged will be type-checked together, which should allow any free generic arguments in the result types of the injections to be unified.

  • After the statement, if there is an if that does not have a corresponding else, a new unique variable v2 is initialized by calling the result-building method buildOptional(_:) with v as the argument, and v2 is then a partial result of the surrounding block. Otherwise, there is a unique variable vMerged, and vMerged is a partial result of the surrounding block.

Imperative control-flow statements

return statements are ill-formed when they appear within a transformed function. However, note that the transformation is suppressed in closures that contain a return statement, so this rule is only applicable in funcs and getters that explicitly provide the attribute.

break and continue statements are ill-formed when they appear within a transformed function. These statements may be supported in some situations in the future, for example by treating all potentially-skipped partial results as optional.

guard is provisionally ill-formed when it appears within a transformed function. Situations in which this statement may appear may be supported in the future, such as when the statement does not produce a partial result.

Exception-handling statements

throw statements are left alone by the transformation.

defer statements are ill-formed when encountered in transformed functions.

do statements with catch blocks are ill-formed when encountered in transformed functions.

do statements

do statements without catch blocks are effectively wrappers around a block statement, and are transformed accordingly:

  • A unique variable v is declared immediately prior to the do.

  • The transformation is applied recursively to the child statement block.

  • The combined result is assigned to v as the final statement in the child block, and v becomes a partial result of the containing block.

for..in loops

for...in statements execute each of the iterations of the loop, collecting the partial results from all iterations into an array. That array is then passed into buildArray. Specifically:

  • A unique variable v is declared immediately prior to the for.
  • A unique variable vArray is declared immediately prior to the for, is given Array type (with as-yet-undetermined element type), and is initialized to [].
  • The transformation is applied recursively to the body of the for..in loop, except that the partial result produced by the body is appended to the array via a call to vArray.append.
  • The result of calling buildArray(vArray) is assigned to v, and v becomes a partial result of the containing block.

If no buildArray is provided, for..in loops are not supported in the body.

Compiler Diagnostic Directives

#warning and #error have no run-time impact and are left unchanged by the result builder transformation.

Availability

Statements that introduce limited availability contexts, such as if #available(...), allow the use of newer APIs while still making the code backward-deployable to older versions of the libraries. A result builder that carries complete type information (such as SwiftUI's ViewBuilder) may need to "erase" type information from a limited availability context using buildLimitedAvailability. Here is a SwiftUI example borrowed from Paul Hudson:

@available(macOS 10.15, iOS 13.0, *)
struct ContentView: View {
    var body: some View {
        ScrollView {
            if #available(macOS 11.0, iOS 14.0, *) {
                LazyVStack {
                    ForEach(1...1000, id: \.self) { value in
                        Text("Row \(value)")
                    }
                }
            } else {
                VStack {
                    ForEach(1...1000, id: \.self) { value in
                        Text("Row \(value)")
                    }
                }
            }
        }
    }
}

LazyVStack was introduced in macOS 11/iOS 14.0, but this view is also available on macOS 10.15/iOS 13.0, so it uses if #available. SwiftUI carries complete type information throughout the view builder closure, including conditional branches:

static func buildEither<TrueContent, FalseContent>(first: TrueContent) -> _ConditionalContent<TrueContent, FalseContent>

This means that the type of the ScrollView will refer to LazyVStack, even on macOS 10.15/iOS 13.0, which results in a compilation error. buildLimitedAvailability provides a way for the result builder to "erase" type information it would normally keep, specifically in these situations:

static func buildLimitedAvailability<Content: View>(_ content: Content) -> AnyView { .init(content) }

Consider a cut-down example focusing on the if #available:

if #available(macOS 11.0, iOS 14.0, *) {
    LazyVStack { }
} else {
    VStack { }
}

This will be transformed as:

let vMerged: *inferred type*
if #available(macOS 11.0, iOS 14.0, *) {
    let v0 = LazyVStack { }
    let v1 = ViewBuilder.buildBlock(v0)
    let v2 = ViewBuilder.buildLimitedAvailability(v1)
    vMerged = ViewBuilder.buildEither(first: v2)
} else {
    let v3 = VStack { }
    let v4 = ViewBuilder.buildBlock(v3)
    vMerged = ViewBuilder.buildEither(second: v4)
}

Example

Let's return to our earlier example and work out how to define a result-builder DSL for it. First, we need to define a basic result builder type:

@resultBuilder
struct HTMLBuilder {
  // We'll use these typealiases to make the lifting rules clearer in this example.
  // Result builders don't really require these to be specific types that can
  // be named this way!  For example, Expression could be "either a String or an
  // HTMLNode", and we could just overload buildExpression to accept either.
  // Similarly, Component could be "any Collection of HTML", and we'd just have
  // to make buildBlock et al generic functions to support that.  But we'll keep
  // it simple so that we can write these typealiases.

  // Expression-statements in the DSL should always produce an HTML value.
  typealias Expression = HTML

  // "Internally" to the DSL, we'll just build up flattened arrays of HTML
  // values, immediately flattening any optionality or nested array structure.
  typealias Component = [HTML]

  // Given an expression result, "lift" it into a Component.
  //
  // If Component were "any Collection of HTML", we could have this return
  // CollectionOfOne to avoid an array allocation.
  static func buildExpression(_ expression: Expression) -> Component {
    return [expression]
  }

  // Build a combined result from a list of partial results by concatenating.
  //
  // If Component were "any Collection of HTML", we could avoid some unnecessary
  // reallocation work here by just calling joined().
  static func buildBlock(_ children: Component...) -> Component {
    return children.flatMap { $0 }
  }

  // We can provide this overload as a micro-optimization for the common case
  // where there's only one partial result in a block.  This shows the flexibility
  // of using an ad-hoc builder pattern.
  static func buildBlock(_ component: Component) -> Component {
    return component
  }
  
  // Handle optionality by turning nil into the empty list.  
  static func buildOptional(_ children: Component?) -> Component {
    return children ?? []
  }

  // Handle optionally-executed blocks.
  static func buildEither(first child: Component) -> Component {
    return child
  }
  
  // Handle optionally-executed blocks.
  static func buildEither(second child: Component) -> Component {
    return child
  }
}

Next, we need to adjust our convenience functions to use it:

func body(@HTMLBuilder makeChildren: () -> [HTML]) -> HTMLNode {
  return HTMLNode(tag: "body", attributes: [:], children: makeChildren())
}
func division(@HTMLBuilder makeChildren: () -> [HTML]) -> HTMLNode { ... }
func paragraph(@HTMLBuilder makeChildren: () -> [HTML]) -> HTMLNode { ... }

Now we can go back to the example code and see how the transformation acts on a small part of it:

division {
  if useChapterTitles {
    header1(chapter + "1. Loomings.")
  }
  paragraph {
    "Call me Ishmael. Some years ago"
  }
  paragraph {
    "There is now your insular city"
  }
}

The transformation proceeds one-by-one through the top-level statements of the closure body passed to division.

For the if statement, we see that there are two cases: the “then” case and the implicit “else” case. The first produces a result (because it has a non-assignment expression-statement), the second does not. We apply the recursive transformation to the “then” case:

  if useChapterTitles {
    let v0: [HTML] = HTMLBuilder.buildExpression(header1(chapter + "1. Loomings."))
    // combined result is HTMLBuilder.buildBlock(v0)
  }

We're not using an injection tree because this is a single if without an else:

  var v0_opt: [HTML]?
  if useChapterTitles {
    let v0: [HTML] = HTMLBuilder.buildExpression(header1(chapter + "1. Loomings."))
    v0_opt = v0
  }
  let v0_result = HTMLBuilder.buildOptional(v0_opt)
  // partial result is v0_result

The two calls to paragraph happen to involve arguments which are also transformed blocks; we'll leave those alone for now, but suffice it to say that they'll also get transformed in time when the type-checker gets around to checking those calls. These are just non-assignment expression-statements, so we just lift them into the Component type:

division {
  var v0_opt: [HTML]?
  if useChapterTitles {
    let v0: [HTML] = HTMLBuilder.buildExpression(header1(chapter + "1. Loomings."))
    v0_opt = v0
  }
  let v0_result = HTMLBuilder.buildOptional(v0_opt)
  
  let v1 = HTMLBuilder.buildExpression(paragraph {
    "Call me Ishmael. Some years ago"
  })
  
  let v2 = HTMLBuilder.buildExpression(paragraph {
    "There is now your insular city"
  })
  
  // partial results are v0_result, v1, v2
}

Finally, we finish this block with a call to buildBlock. This is the top-level body of the function, but the result builder type doesn't declare buildFinalResult, that result is returned directly:

division {
  var v0_opt: [HTML]?
  if useChapterTitles {
    let v0: [HTML] = HTMLBuilder.buildExpression(header1(chapter + "1. Loomings."))
    v0_opt = v0
  }
  let v0_result = HTMLBuilder.buildOptional(v0_opt)
  
  let v1 = HTMLBuilder.buildExpression(paragraph {
    "Call me Ishmael. Some years ago"
  })
  
  let v2 = HTMLBuilder.buildExpression(paragraph {
    "There is now your insular city"
  })
  
  return HTMLBuilder.buildBlock(v0_result, v1, v2)
}

This closure has now been completely transformed (except for the nested closures passed to paragraph).

Type inference

Result builder bodies

Type inference in result builder bodies follows from the syntactic effects of the result builder transformation. For example, when applying the result builder to the following closure:

{
  42
  3.14159
}

the result builder transformation produces

let v1 = 42
let v2 = 3.14159
return Builder.buildBlock(v1, v2)

The types for v1 and v2 are determined independently by the normal type inference rules to Int and Double, respectively, then buildBlock can operate on both types to produce the final result of the closure. However, the type of buildBlock cannot have any effect on how the types of v1 and v2 are computed. For example, if the builder contained a buildBlock like the following:

func buildBlock<T>(_ a: T, _ b: T) -> T { ... }

Then the call to buildBlock(v1, v2) will fail because Int and Double have different types, even though the integer literal 42 could have been treated as a Double if type inference were permitted to propagate information "backward" to affect v1.

Note that the first implementation of result builders in Swift 5.1 used a different syntactic transform that did allow such backward propagation, e.g.,

return Builder.buildBlock(42, 3.14159)  // not proposed; example only

in which case the 42 would be treated as a Double. There are several reasons why allowing such "backward" propagation of type information is undesirable for result builders:

  • The type inference model would be different from normal closures or function bodies, which is a divergence that makes the mental model more complicated
  • Type checker performance with moderate-to-large result builder bodies was unacceptable, because backward propagation introduced exponential behavior. The implementation of one-way constraints for result builders (which introduced the current behavior) resolved most reported "expression too complex to be solved in a reasonable time" issues with SwiftUI code.

Inferring result builders from protocol requirements

Most result builder transformations are applied implicitly, without the client of the API writing the name of the result builder. For example, given the following API:

func paragraph(@HTMLBuilder makeChildren: () -> [HTML]) -> HTMLNode { ... }

The result builder HTMLBuilder is applied at each call site, implicitly, when the closure argument is matched to the parameter that has a result builder attribute:

paragraph {
  "Call me Ishmael. Some years ago"
}

Most function declarations are standalone, so only the explicit result builder annotation can enable the transformation. However, result builder DSLs like SwiftUI tend to have a central protocol to which many different types conform. A typical SwiftUI view might look something like this:

struct ContentView: View {
  @ViewBuilder var body: some View {
    Image(named: "swift")
    Text("Hello, Swift!")
  }
}

Nearly every body for a SwiftUI view can use @ViewBuilder, because body defines a View, and those are best built with a ViewBuilder. To eliminate the boilerplate from writing @ViewBuilder on each, one can annotate body with @ViewBuilder in the View protocol itself:

protocol View {
  associatedtype Body: View
  @ViewBuilder var body: Body { get }
}

When a View-conforming type defines its body, the @ViewBuilder attribute is inferred from the protocol requirement it satisfies, implicitly applying the result builder transform. This inference occurs unless:

  • The function or property already has a result builder attribute explicitly written on it, or
  • The body of the function or property getter contains an explicit return statement.

Implicit memberwise initializer

Result builders are designed with composition in mind, and it is common to have a number of small structures that use result builders to describe their child content. For example, a custom VStack in SwiftUI might look like this:

struct CustomVStack<Content: View>: View {
    let content: () -> Content

    var body: some View {
        VStack {
            // custom stuff here
            content()
        }
    }
}

However, this custom VStack doesn't work with result builder syntax without writing an initializer to introduce the @ViewBuilder attribute:

init(@ViewBuilder content: @escaping () -> Content) {
    self.content = content
}

A result builder attribute can be placed on a stored property. This introduces the result builder attribute on the corresponding parameter of the implicit memberwise initializer. In other words, changing the CustomVStack definition to the following:

struct CustomVStack<Content: View>: View {
    @ViewBuilder let content: () -> Content

    var body: some View {
        VStack {
            // custom stuff here
            content()
        }
    }
}

implicitly produces the memberwise initializer shown above.

A result builder attribute can also be placed on a stored property whose type does not structurally resemble function type. In this case, the implicit memberwise initializer will have a corresponding function parameter that is a result-builder-attributed closure returning the property's type, and the body of the initializer will call the function. For example, given:

struct CustomHStack<Content: View>: View {
    @ViewBuilder let content: Content

    var body: some View {
        HStack {
            // custom stuff here
            content
        }
    }
}

the implicit memberwise initializer would have the following definition:

init(@ViewBuilder content: () -> Content) {
    self.content = content()
}

This idea was reported as SR-13188, and the examples are pulled from there.

Source compatibility

Result builders are an additive feature which should not affect existing source code.

Because some decisions with result builders are implementation-defined, e.g. the structure of the injection tree for switch statements, it is possible that certain DSLs will observe differences in behavior across future compiler versions if they are written to observe such differences.

Effect on ABI stability and API resilience

Result builders are based on compile-time code generation and do not require support from the language runtime or standard library.

Because result builders are essentially a kind of macro system, where the details of expansion are basically an aspect of the current implementation rather than necessarily a stable interface, library authors are encouraged to make as much as possible inlinable and, if possible, non-ABI.

Future Directions

There are a number of future directions that could be layered on top of this proposal without compromising its basic design. Several of them are covered here.

"Simple" result builder protocol

On the Swift forums, @anreitersimon demonstrated the ability to use protocols to make it easier to define new result builders that support all of the syntax, and for which all expressions have the same type. Their example (slightly tuned) follows. The basic idea is to form a tree describing the result:

enum Either<T,U> {
  case first(T)
  case second(U)
}

indirect enum ResultBuilderTerm<Expression> {
    case expression(Expression)
    case block([ResultBuilderTerm])
    case either(Either<ResultBuilderTerm, ResultBuilderTerm>)
    case optional(ResultBuilderTerm?)
}

and then define a ResultBuilder protocol with only a single requirement, buildFinalResult, to take all of the values and form the final result:

protocol ResultBuilder {
    associatedtype Expression
    typealias Component = ResultBuilderTerm<Expression>
    associatedtype FinalResult

    static func buildFinalResult(_ component: Component) -> FinalResult
}

All of the other build methods---to enable if, switch, and so on---are implemented in an extension of ResultBuilder:

extension ResultBuilder {
    static func buildExpression(_ expression: Expression) -> Component { .expression(expression) }
    static func buildBlock(_ components: Component...) -> Component { .block(components) }
    static func buildOptional(_ component: Component?) -> Component { .optional(component) }
    static func buildArray(_ components: [Component]) -> Component { .block(components) }
    static func buildLimitedAvailability(_ component: Component) -> Component { component }
}

It then becomes possible to define new version builders with only a small amount of code, e.g., here is a builder that flattens the result term into an array:

@resultBuilder
enum ArrayBuilder<E>: ResultBuilder {
    typealias Expression = E
    typealias FinalResult = [E]

    static func buildFinalResult(_ component: Component) -> FinalResult {
        switch component {
        case .expression(let e): return [e]
        case .block(let children): return children.flatMap(buildFinalResult)
        case .either(.first(let child)): return buildFinalResult(child)
        case .either(.second(let child)): return buildFinalResult(child)
        case .optional(let child?): return buildFinalResult(child)
        case .optional(nil): return []
        }
    }
}

With some more experience, a facility like this could become part of the standard library.

Stateful result builders

All of the build* methods for a result builder type are defined to be static, and no instances of the result builder type are created by the result builder transform. However, the transform could create an instance of the result builder type at the beginning of the transformed function, then call build* instance methods on it. For example, this code adapted from Constantino Tsarouhas):

struct Heading<Content : Element> : Element {
    init(@ElementBuilder(containerStyle: .inline) content: () -> Content) { … }
}

Heading {
  Text("Loomings")
  Text("Call me Ishmael. Some years ago")
}

would be translated as:

Heading {
  var builder = ElementBuilder(containerStyle: .inline)
  let v0 = builder.buildExpression(Text("Loomings"))
  let v1 = builder.buildExpression(Text("Call me Ishmael. Some years ago"))
  let v2 = builder.buildBlock(v0, v1)
  return builder.buildFinalResult(v2)
}

The introduction of stateful result builders would be a pure extension. However, the same effect can be achieved by the "simple result builder protocol" described above, so it is not clear that this facility belongs in the language.

Transforming declarations

Result builders leave all declaration statements unmodified. However, some DSLs might want to incorporate declarations, notifying the builder of such declarations so it can incorporate them. Here's an abstracted example based on one provided by Konrad Malawski:

Definition {
  Thing {
    // "import" the Def people, in order to be able to use it in this Thing
    // there may be many Defs, but this Thing only uses people
    let people = Defs.people // <1>
    let other = Def.otherThings // <2>

    show {
      people.name // <1>
      people.surname // <1>
      other.information // <2>
    }
    filter {
      equals(people.age, 42)
    }
  }
}

The builder could be informed of each let declaration, allowing it to produce a partial result, e.g., the innermost Thing could be transformed to:

let people = Defs.people
let v0 = ThingBuilder.buildDeclaration(people)
let other = Def.otherThings
let v1 = ThingBuilder.buildDeclaration(other)

let v2 = <result of translating show expression>
let v3 = <result of translation filter expression>
let v4 = ThingBuilder.buildBlock(v0, v1, v2, v3)

Such DSLs would not change the way declarations are type checked, but would have the option to produce partial results for them, which could further eliminate boilerplate. On the other hand, without this feature one can freely use let to pull out subexpressions and partial computations without changing the code. For example, today one expects to be able to refactor

a.doSomething().doSomethingElse()

into

let b = a.doSomething()
b.doSomethingElse()

without changing semantics. That would no longer be the case for result builders that opt in to this feature by providing buildDeclaration.

Virtualized Abstract Syntax Trees (ASTs)

The result builder model executes the transformed function directly, collecting the partial results that get passed into the builder. Some DSLs might prefer to "virtualize" the structure of the transformed function, such that the DSL can determine how the evaluation happens. A simple form of this can be achieved by using escaping autoclosures in buildExpression::

typealias DelayedValue = () -> Any

static func buildExpression(_ value: @autoclosure @escaping () -> Any) -> DelayedValue {
  return value
}

Here, the partial results provided to other build methods (e.g., buildBlock) will be functions that produce the value. The DSL is free to call those functions whenever it wants to produce the values.

However, virtualizing any kind of control flow would require a new kind of build method that describes more of the structure of the AST. For example, consider a for..in loop:

for person in employees {
  "Hello, \(person.preferredName)"
}

Result builders currently will execute all iterations of the loop. Virtualizing the execution means passing along the means to execute the loop to the result builder, e.g., via a buildVirtualFor operation:

Builder.buildVirtualFor(employees, { person in
  let v0 = Builder.buildExpression("Hello, \(person.preferredName)")
  return Builder.buildBlock(v0)
}

The builder's buildVirtualFor would have a signature such as:

static func buildVirtualFor<S: Sequence, T>(_ sequence: S, @escaping (S.Element) -> T) -> ForEach<S, T> { ... }

Such a facility could be used to map the language's for..in syntax to a lazily-evaluated construct like SwiftUI's ForEach. Similar builder methods would need to be developed for each supported syntax, e.g., if statements where the condition and then/else blocks are provided via closures.

Virtualized ASTs would be a powerful extension to result builders. However, they will require an additional set of builder methods that match more closely with the syntax of the function being transformed.

Alternatives considered

Additional control-flow statements

The set of statements that are permitted within a transformed function are intentionally limited to those that are "strictly structural", and could reasonably be thought of as being part of a single, functional expression, aggregating values but without complicated control flow. However, one could go beyond this model to accept additional statements in a transformed function:

  • Local control flow statements that aren't “strictly structural”, like break, continue, and do/catch, could be handled by treating subsequent partial results as optional, as if they appeared within an if.
  • Iteration statements other than for..in (i.e., while and repeat..while) could be supported via buildArray.

Support for additional control-flow statements would weaken the declarative feel of result builders, and makes the "tree" structure of the DSL harder to reason about.

It has been suggested that there could be two "forms" of result builders, one that matches the design in this proposal and a second, "simpler" one that handles the full breadth of the statement grammar (including all loops, break, continue, and so on) but sees only the partial results (e.g., via buildExpression) and not the structure (buildBlock, buildEither(first:), etc. would not get called). The "simple result builder protocol" described above illustrates how one can get the second part of this--defining a simple result builder that receives all of the values without the structure--by building on top of this proposal. However, we should not have two forms of result builders in the language itself, with different capabilities, because it leads to confusion. If result builders gain support for additional control-flow statements (as a general feature), that should be reflected in the "simple result builder protocol" to extend the feature set for result builders that don't want the structure.

Builder-scoped name lookup

It is common for DSLs to want to introduce shorthands which might be unreasonable to introduce into the global scope. For example, p might be a reasonable name in the context of our HTMLBuilder DSL (rather than paragraph), but actually introducing a global function named p just for DSL use is quite unfortunate. Contextual lookups like .p will generally not work at the top level in DSLs because they will be interpreted as continuations of the previous statement. One could imagine having some way for the DSL to affect lexical lookup within transformed functions so that, e.g., within the transformed function one could use short names like p, div, and h1:

return body {
  let chapter = spellOutChapter ? "Chapter " : ""
  div {
    if useChapterTitles {
      h1(chapter + "1. Loomings.")
    }
    p {
      "Call me Ishmael. Some years ago"
    }
    p {
      "There is now your insular city"
    }
  }
}

which are defined in the result builder type itself, e.g.,

extension HTMLBuilder {
  static func body(@HTMLBuilder _ children: () -> [HTML]) -> HTMLNode { ... }
  static func div(@HTMLBuilder _ children: () -> [HTML]) -> HTMLNode { ... }
  static func p(@HTMLBuilder _ children: () -> [HTML]) -> HTMLNode { ... }
  static func h1(_ text: String) -> HTMLNode { ... }
}

Name lookup that doesn't find the names p, div, or h1 within the closure would look into the result builder being used to transform the closure before continuing lexical name lookup.

Note that one can simulate this kind of effect by following the common pattern set out by SwiftUI, where the transformed function is usually within a type that conforms to some common protocol. For example, if we were to always say that HTML documents are types that conform to an HTMLDocument protocol, like this:

protocol HTMLDocument {
  @HTMLBuilder var body: HTML { get }
}

struct MobyDick: HTMLDocument {
  var body: HTML {
    let chapter = spellOutChapter ? "Chapter " : ""
    div {
      if useChapterTitles {
        h1(chapter + "1. Loomings.")
      }
      p {
        "Call me Ishmael. Some years ago"
      }
      p {
        "There is now your insular city"
      }
    }
  }
}

Here, one can put the shorthand names (or, indeed, everything defined for the DSL) into an extension of the protocol:

extension HTMLDocument {
  static func body(@HTMLBuilder _ children: () -> [HTML]) -> HTMLNode { ... }
  static func div(@HTMLBuilder _ children: () -> [HTML]) -> HTMLNode { ... }
  static func p(@HTMLBuilder _ children: () -> [HTML]) -> HTMLNode { ... }
  static func h1(_ text: String) -> HTMLNode { ... }
}

Dropping Void/Never values

During the first review, it was suggested that a Void or Never-returning buildExpression method should cause the corresponding value to be dropped from the result builder itself. For example, the following result builder collects string values and puts them into a comma-separated string:

@resultBuilder
struct StringConcatenator {
  static func buildExpression(_ string: String) -> String { string }

  static func buildBlock(_ strings: String...) -> String { 
    strings.joined(separator: ", ")
  }
}

However, it will reject any statements that don't produce a String. What if the builder wanted to allow other statements, but without collecting the values? One could re-work the builder like this:

@resultBuilder
struct StringConcatenator {
  static func buildExpression(_ string: String) -> String? { string }
  static func buildExpression<T>(_: T) -> String? { nil }

  static func buildBlock(_ strings: String?...) -> String { 
    strings.compactMap({$0}).joined(separator: ", ")
  }
}

However, it's a non-obvious change to the result builder. The suggestion is to allow the original builder to be extended with a Void-producing buildExpression, e.g.,

static func buildExpression<T>(_: T) { }

and have any Void-producing partial results be dropped. For example, applying the result builder to this closure:

func applyStringConcatenator(@StringConcatenator _: () -> String) { ... }

applyStringConcatenator { 
  "hello"
  3.14159
  "world"
}

would produce "hello, world". While this is perhaps easier for the the author of the result builder, it makes the translation of the closure much less predictable. The basic translation of that closure would be to:

applyStringConcatenator {
  let a = StringConcatenator.buildExpression("hello")
  let b = StringConcatenator.buildExpression(3.14159)
  let c = StringConcatenator.buildExpression("world")
  return StringConcatenator.buildBlock(a, b, c)
}

With this proposal, the formation of the buildBlock call would depend on the type inference for the partial result variables a, b, and c. In other words, because b will be inferred to type Void, the actual buildBlock call would end up being StringConcatenator.buildBlock(a, c). This complication to the mental model outweighs the benefits to authors of result builders, because this feature isn't adding any expressive power--it's a shortcut to make it more convenient to address this case.