Add primary associated type for MarkupVisitor

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.5
+// swift-tools-version:5.7
 /*
  This source file is part of the Swift.org open source project
 

Sources/Markdown/Visitor/MarkupVisitor.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2023 Apple Inc. and the Swift project authors
  Licensed under Apache License v2.0 with Runtime Library Exception
 
  See https://swift.org/LICENSE.txt for license information
@@ -13,7 +13,7 @@
 /// - note: This interface only provides requirements for visiting each kind of element. It does not require each visit method to descend into child elements.
 ///
 /// Generally, ``MarkupWalker`` is best for walking a ``Markup`` tree if the ``Result`` type is `Void` or is built up some other way, or ``MarkupRewriter`` for recursively changing a tree's structure. This type serves as a common interface to both. However, for building up other structured result types you can implement ``MarkupVisitor`` directly.
-public protocol MarkupVisitor {
+public protocol MarkupVisitor<Result> {
 
     /**
      The result type returned when visiting a element.

Tests/MarkdownTests/Visitors/MarkupVisitorTests.swift

@@ -0,0 +1,56 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2023 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import XCTest
+import Markdown
+
+class MarkupVisitorTests: XCTestCase {
+    struct IntegerConverter: MarkupVisitor {
+        var value: Int
+
+        mutating func defaultVisit(_: Markdown.Markup) -> Int {
+            defer { value += 1 }
+            return value
+        }
+    }
+
+
+    // A compile time check for PAT support
+    func testMarkupVisitorPrimaryAssociatedType() {
+        var visitor: some MarkupVisitor<Int> = IntegerConverter(value: 1)
+        let markup = Text("")
+        XCTAssertEqual(visitor.visit(markup), 1)
+        XCTAssertEqual(visitor.visit(markup), 2)
+        var mappedVisitor: some MarkupVisitor<Int> = visitor.map { $0 * $0 }  
+        XCTAssertEqual(mappedVisitor.visit(markup), 9)
+        XCTAssertEqual(mappedVisitor.visit(markup), 16)
+        XCTAssertEqual(visitor.visit(markup), 3)
+    }
+}
+
+struct _MappVisitor<A: MarkupVisitor, B>: MarkupVisitor {
+    typealias Result = B
+    init(visitor: A, _ transform: @escaping (A.Result) -> B) {
+        self.visitor = visitor
+        self.transform = transform
+    }
+    private var visitor: A
+    private let transform: (A.Result) -> B
+
+    mutating func defaultVisit(_ markup: Markdown.Markup) -> B {
+        transform(visitor.defaultVisit(markup))
+    }
+}
+
+extension MarkupVisitor {
+    func map<U>(_ transform: @escaping (Self.Result) -> U) -> some MarkupVisitor<U> {
+        _MappVisitor(visitor: self, transform)
+    }
+}