Add hello world example

Makefile

@@ -1,4 +1,4 @@
-examples: profile session
+examples: profile session helloworld
 
 .PHONY: examples
 
@@ -7,3 +7,6 @@ profile:
 
 session:
 	bash examples/compile.bash examples/session
+
+helloworld:
+	bash examples/compile.bash examples/helloworld

README.md

@@ -38,6 +38,7 @@ Original motivation was to create a simple templating renderer for USSD clients,
 
 * Breakpoints.
 * Key/value database reference example.
+* Same-page catch with dedicated error string to prepend to template
 
 
 ## Opcodes

dev/interactive/main.go

@@ -23,11 +23,20 @@ func main() {
 
 	ctx := context.Background()
 	en := engine.NewSizedEngine(dir, uint32(size))
-	err := en.Init(ctx)
+	cont, err := en.Init(ctx)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "engine init exited with error: %v\n", err)
 		os.Exit(1)
 	}
+	if !cont {
+		_, err = en.WriteResult(os.Stdout, ctx)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "dead init write error: %v\n", err)
+			os.Exit(1)
+		}
+		os.Stdout.Write([]byte{0x0a})
+		os.Exit(0)
+	}
 	err = engine.Loop(&en, os.Stdin, os.Stdout, ctx)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "loop exited with error: %v\n", err)

engine/engine.go

@@ -53,37 +53,34 @@ func NewEngine(cfg Config, st *state.State, rs resource.Resource, ca cache.Memor
 // Init must be explicitly called before using the Engine instance.
 //
 // It loads and executes code for the start node.
-func(en *Engine) Init(ctx context.Context) error {
+func(en *Engine) Init(ctx context.Context) (bool, error) {
 	if en.initd {
 		log.Printf("already initialized")
-		return nil
+		return true, nil
 	}
 	sym := en.root
 	if sym == "" {
-		return fmt.Errorf("start sym empty")
+		return false, fmt.Errorf("start sym empty")
 	}
 	inSave, _ := en.st.GetInput()
 	err := en.st.SetInput([]byte{})
 	if err != nil {
-		return err
+		return false, err
 	}
 	b := vm.NewLine(nil, vm.MOVE, []string{sym}, nil, nil)
 	log.Printf("start new init VM run with code %x", b)
 	b, err = en.vm.Run(b, ctx)
 	if err != nil {
-		return err
-	}
-	if len(b) == 0 {
-		return fmt.Errorf("no code left after init, that's just useless and sad")
+		return false, err
 	}
+	
 	log.Printf("ended init VM run with code %x", b)
 	en.st.SetCode(b)
 	err = en.st.SetInput(inSave)
 	if err != nil {
-		return err
+		return false, err
 	}
-	en.initd = true
-	return nil
+	return len(b) > 0, nil
 }
 
 // Exec processes user input against the current state of the virtual machine environment.
@@ -99,13 +96,11 @@ func(en *Engine) Init(ctx context.Context) error {
 func (en *Engine) Exec(input []byte, ctx context.Context) (bool, error) {
 	var err error
 	if en.st.Moves == 0 {
-		err = en.Init(ctx)
+		cont, err := en.Init(ctx)
 		if err != nil {
 			return false, err
 		}
-		if len(input) == 0 {
-			return true, nil
-		}
+		return cont, nil
 	}
 	err = vm.ValidInput(input)
 	if err != nil {
@@ -146,7 +141,7 @@ func (en *Engine) Exec(input []byte, ctx context.Context) (bool, error) {
 	en.st.SetCode(code)
 	if len(code) == 0 {
 		log.Printf("runner finished with no remaining code")
-		err = en.reset(ctx)
+		_, err = en.reset(ctx)
 		return false, err
 	}
 
@@ -168,17 +163,17 @@ func(en *Engine) WriteResult(w io.Writer, ctx context.Context) (int, error) {
 }
 
 // start execution over at top node while keeping current state of client error flags.
-func(en *Engine) reset(ctx context.Context) error {
+func(en *Engine) reset(ctx context.Context) (bool, error) {
 	var err error
 	var isTop bool
 	for !isTop {
 		isTop, err = en.st.Top()
 		if err != nil {
-			return err
+			return false, err
 		}
 		_, err = en.st.Up()
 		if err != nil {
-			return err
+			return false, err
 		}
 		en.ca.Pop()
 	}

engine/engine_test.go

@@ -85,7 +85,7 @@ func TestEngineInit(t *testing.T) {
 		Root: "root",
 	}, &st, &rs, ca, ctx)
 
-	err = en.Init(ctx)
+	_, err = en.Init(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -141,7 +141,8 @@ func TestEngineExecInvalidInput(t *testing.T) {
 	en := NewEngine(Config{
 		Root: "root",
 	}, &st, &rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -161,7 +162,8 @@ func TestEngineResumeTerminated(t *testing.T) {
 	en := NewEngine(Config{
 		Root: "root",
 	}, &st, &rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}

engine/loop_test.go

@@ -24,7 +24,8 @@ func TestLoopTop(t *testing.T) {
 		Root: "root",
 	}
 	en := NewEngine(cfg, &st, &rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -60,7 +61,8 @@ func TestLoopBackForth(t *testing.T) {
 		Root: "root",
 	}
 	en := NewEngine(cfg, &st, &rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -94,7 +96,8 @@ func TestLoopBrowse(t *testing.T) {
 		Root: "root",
 	}
 	en := NewEngine(cfg, &st, &rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}

examples/helloworld/que.txt.orig

@@ -0,0 +1 @@
+world

examples/helloworld/root

@@ -0,0 +1 @@
+hello, {{.que}}!

examples/helloworld/root.vis

@@ -0,0 +1,2 @@
+LOAD que 5
+MAP que

examples/profile/main.go

@@ -124,7 +124,8 @@ func main() {
 	}
 	ctx := context.Background()
 	en := engine.NewEngine(cfg, &st, rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "engine init fail: %v\n", err)
 		os.Exit(1)

examples/session/main.go

@@ -72,7 +72,8 @@ func main() {
 	ctx := context.Background()
 	ctx = context.WithValue(ctx, "SessionId", sessionId)
 	en := engine.NewEngine(cfg, &st, rs, ca, ctx)
-	err := en.Init(ctx)
+	var err error
+	_, err = en.Init(ctx)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "engine init fail: %v\n", err)
 		os.Exit(1)