From 647619a7f60f369b7f60c9e3542031dd40bbb67b Mon Sep 17 00:00:00 2001
From: Sasha Koshka <sashakoshka@tebibyte.media>
Date: Wed, 19 Nov 2025 16:25:31 -0500
Subject: [PATCH] generate: It is no longer possible to make impossible type
 asserts with Receive

---
 generate/generate.go | 44 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/generate/generate.go b/generate/generate.go
index b8d1b82..02959f5 100644
--- a/generate/generate.go
+++ b/generate/generate.go
@@ -114,6 +114,8 @@ func (this *Generator) Generate(protocol *Protocol) (n int, err error) {
 	n += nn; if err != nil { return n, err }
 
 	// type definitions
+	nn, err = this.generateMessageValueInterface()
+	n += nn; if err != nil { return n, err }
 	for _, name := range slices.Sorted(maps.Keys(protocol.Types)) {
 		nn, err := this.generateTypedef(name, protocol.Types[name])
 		n += nn; if err != nil { return n, err }
@@ -140,6 +142,42 @@ func (this *Generator) Generate(protocol *Protocol) (n int, err error) {
 	return n, nil
 }
 
+func (this *Generator) generateMessageValueInterface() (n int, err error) {
+	keys := slices.Sorted(maps.Keys(this.protocol.Messages))
+	nn, err := this.iprint(
+		"\n// ReceivedMessage is a sealed interface representing the value of a\n" +
+		"// message in this package. To determine what kind of message it is,\n" +
+		"// use a type switch like this:\n" +
+		"// \n" +
+		"//   switch message := message.(type) {\n")
+	n += nn; if err != nil { return n, err }
+	for index, method := range keys {
+		if index > 2 {
+			nn, err := this.iprint("// \n//   ...\n// \n")
+			n += nn; if err != nil { return n, err }
+			break
+		}
+		nn, err := this.iprintf("//   case %s:\n",
+			this.resolveMessageName(this.protocol.Messages[method].Name))
+		n += nn; if err != nil { return n, err }
+		nn, err = this.iprintf("//           doSomething()\n")
+		n += nn; if err != nil { return n, err }
+	}
+	nn, err = this.iprintf("//   }\n")
+	n += nn; if err != nil { return n, err }
+	
+	nn, err = this.iprintf("type ReceivedMessage interface {\n")
+	n += nn; if err != nil { return n, err }
+	this.push()
+		nn, err = this.iprintf("isReceivedMessage()\n")
+		n += nn; if err != nil { return n, err }
+	this.pop()
+	nn, err = this.iprintf("}\n")
+	n += nn; if err != nil { return n, err }
+	
+	return n, nil
+}
+
 func (this *Generator) generateTypedef(name string, typedef Typedef) (n int, err error) {
 	typ := typedef.Type
 
@@ -327,6 +365,10 @@ func (this *Generator) generateMessage(method uint16, message Message) (n int, e
 	this.pop()
 	nn, err = this.iprintf("}\n")
 	n += nn; if err != nil { return n, err }
+
+	// isReceivedMessage method
+	nn, err = this.iprintf("func (this %s) isReceivedMessage() { }\n", this.resolveMessageName(message.Name))
+	n += nn; if err != nil { return n, err }
 	
 	return n, nil
 }
@@ -1207,7 +1249,7 @@ func (this *Generator) generateReceive() (n int, err error) {
 		"// Use a type switch to determine what type of message it is.\n")
 	n += nn; if err != nil { return n, err }
 	nn, err = this.iprintf(
-		"func Receive(trans hopp.Trans) (message any, n int, err error) {\n")
+		"func Receive(trans hopp.Trans) (message ReceivedMessage, n int, err error) {\n")
 	n += nn; if err != nil { return n, err }
 	this.push()
 		nn, err = this.iprintf("method, reader, err := trans.ReceiveReader()\n")