does this work?

Author: griotspeak

Sources/DiscriminatedUnion/DiscriminatedUnion.swift

@@ -1,9 +1,12 @@
 
+@attached(
+    extension,
+    conformances: DiscriminatedUnion
+)
 @attached(
     member,
-    names: named(Discriminant), named(discriminant), prefixed(associatedValueFor)
+    names: arbitrary, named(Discriminant), named(discriminant), named(ExtractorError)
 )
-@attached(extension, conformances: DiscriminatedUnion)
 public macro discriminatedUnion() = #externalMacro(
     module: "DiscriminatedUnionMacros",
     type: "DiscriminatedUnionMacro")

Sources/DiscriminatedUnionClient/main.swift

@@ -8,6 +8,8 @@ enum Pet {
     case parrot(loud: Bool)
     case snake
     case turtle(snapper: Bool)
+//    case bird(name: String, Int)
+
 }
 
 Swift.print("usiyan::: Pet.Discriminant.dog == Pet.dog.discriminant: \(String(describing: Pet.Discriminant.dog == Pet.dog.discriminant))")

Sources/DiscriminatedUnionMacros/DiscriminatedUnionMacro.swift

@@ -54,17 +54,31 @@ extension DiscriminatedUnionMacro: MemberMacro {
 
         let unvalidatedPropertyDecl = try instance.declareDiscriminantProperty()
         let validatedPropertyDecl = try DeclSyntax(validating: unvalidatedPropertyDecl)
+        let validatedExtractors = try instance.doSomethingSpecial().map {
+            try DeclSyntax(validating: $0)
+        }
+
+        let validatedExtractorError = try DeclSyntax(validating: extractorErrorDecl())
 
         return try [
             DeclSyntax(validating: "\(raw: discriminantDecl)"),
-            validatedPropertyDecl
-        ]
+            validatedPropertyDecl,
+            validatedExtractorError
+        ] + validatedExtractors
     }
 
     enum Error: Swift.Error {
         case attemptPrint(String)
     }
 
+    static func extractorErrorDecl() -> DeclSyntax {
+        """
+        public enum ExtractorError: Swift.Error {
+            case invalidExtraction
+        }
+        """
+    }
+
     func declareDiscriminantType() throws -> EnumDeclSyntax {
         return try EnumDeclSyntax("public enum Discriminant: DiscriminantType") {
 
@@ -133,16 +147,16 @@ extension DiscriminatedUnionMacro: MemberMacro {
         }
 
         let theSomethings: [DeclSyntax] = theCases.map { caseName, tupleType, pBindings, returnValue in
-            let titleCasedName = "\(caseName.first!.uppercased())\(caseName.dropFirst())"
+//            let titleCasedName = "\(caseName.first!.uppercased())\(caseName.dropFirst())"
             return """
 
-                public var associatedValueFor\(raw: titleCasedName): \(raw: tupleType) {
-                    if case .\(raw: caseName)(raw: \(raw: pBindings)) = self {
-                        return \(raw: returnValue)
-                    } else {
-                        return nil
-                    }
+            public func \(raw: caseName)AssociatedValue() throws -> \(raw: tupleType) {
+                if case .\(raw: caseName)(\(raw: pBindings)) = self {
+                    return \(raw: returnValue)
+                } else {
+                    throw ExtractorError.invalidExtraction
                 }
+            }
 
             """
         }