Add reader

Sources/StructTransaction/Source.swift

@@ -1,24 +1,43 @@
 
-@attached(extension, conformances: StateModifyingType, names: named(Modifying), named(modify(source:modifier:)), named(ModifyingTarget))
-public macro Writing() = #externalMacro(module: "StructTransactionMacros", type: "WriterMacro")
+@attached(extension, conformances: DetectingType, names: named(Modifying), named(modify(source:modifier:)), named(read(source:reader:)), named(ModifyingTarget))
+public macro Detecting() = #externalMacro(module: "StructTransactionMacros", type: "WriterMacro")
 
-public protocol StateModifyingType {
+public protocol DetectingType {
 
   associatedtype Modifying
 
   @discardableResult
   static func modify(source: inout Self, modifier: (inout Modifying) throws -> Void) rethrows -> ModifyingResult
+
+  @discardableResult
+  static func read(source: Self, reader: (Modifying) throws -> Void) rethrows -> ReadResult
 }
 
-extension StateModifyingType {
+extension DetectingType {
 
   @discardableResult
   public mutating func modify(modifier: (inout Modifying) throws -> Void) rethrows -> ModifyingResult {
     try Self.modify(source: &self, modifier: modifier)
   }
 
+  @discardableResult
+  public mutating func read(reader: (Modifying) throws -> Void) rethrows -> ReadResult {
+    try Self.read(source: self, reader: reader)
+  }
+}
+
+public struct ReadResult {
+
+  public let readIdentifiers: Set<String>
+
+  public init(
+    readIdentifiers: Set<String>
+  ) {
+    self.readIdentifiers = readIdentifiers
+  }
 }
 
+
 public struct ModifyingResult {
 
   public let readIdentifiers: Set<String>

Sources/StructTransactionMacros/WriterMacro.swift

@@ -92,7 +92,7 @@ extension WriterMacro: ExtensionMacro {
 
     let modifyingDecl =
       ("""
-      extension \(type.trimmed): StateModifyingType {
+      extension \(type.trimmed): DetectingType {
 
         typealias ModifyingTarget = Self
 
@@ -109,6 +109,20 @@ extension WriterMacro: ExtensionMacro {
           }
         }
 
+        @discardableResult
+        public static func read(source: Self, reader: (Modifying) throws -> Void) rethrows -> ReadResult {
+          // FIXME: avoid copying
+          var reading = source
+
+          return try withUnsafeMutablePointer(to: &reading) { pointer in
+            let modifying = Modifying(pointer: pointer)
+            try reader(modifying)
+            return ReadResult(
+              readIdentifiers: modifying.$_readIdentifiers
+            )
+          }
+        }
+
       \(modifyingStructDecl)
       }
       """ as DeclSyntax)