Added meaningful error messages for Starlark code

internal/compiler/compiler.go

@@ -42,11 +42,13 @@ func (c *Compiler) Registry() *CommandRegistry {
 func (c *Compiler) Compile(lines []preproc.Line) ([]string, error) {
 	var codeOutput []string
 	var lastKind = preproc.Source
-	var scriptBuffer []string
+	var scriptBuffer []preproc.Line
 	var scriptIsLibrary bool
 	var macroBuffer []string
 	var currentMacroName string
 	var currentMacroParams []string
+	var currentMacroSourceFile string
+	var currentMacroStartLine int
 	var currentAsmTarget *[]string // nil = no active ASM block, or points to target slice
 
 	// Reset deferred ASM storage for this compilation
@@ -74,15 +76,19 @@ func (c *Compiler) Compile(lines []preproc.Line) ([]string, error) {
 			if lastKind == preproc.ScriptMacroDef {
 				if currentMacroName != "" {
 					c.ctx.ScriptMacros[currentMacroName] = &ScriptMacro{
-						Name:   currentMacroName,
+						Name:       currentMacroName,
-						Params: currentMacroParams,
+						Params:     currentMacroParams,
-						Body:   macroBuffer,
+						Body:       macroBuffer,
+						SourceFile: currentMacroSourceFile,
+						StartLine:  currentMacroStartLine,
 					}
 					codeOutput = append(codeOutput, fmt.Sprintf("; ENDSCRIPT MACRO %s", currentMacroName))
 				}
 				macroBuffer = nil
 				currentMacroName = ""
 				currentMacroParams = nil
+				currentMacroSourceFile = ""
+				currentMacroStartLine = 0
 			}
 
 			// Close previous Assembler block
@@ -199,12 +205,17 @@ func (c *Compiler) Compile(lines []preproc.Line) ([]string, error) {
 				*currentAsmTarget = append(*currentAsmTarget, codePart+commentPart)
 			} else if line.Kind == preproc.Script || line.Kind == preproc.ScriptLibrary {
 				// Collect script lines for execution
-				scriptBuffer = append(scriptBuffer, line.Text)
+				scriptBuffer = append(scriptBuffer, line)
 			} else if line.Kind == preproc.ScriptMacroDef {
 				// Skip the header line (already parsed in transition)
 				if strings.HasPrefix(strings.TrimSpace(line.Text), "SCRIPT MACRO ") {
 					continue
 				}
+				// Capture source provenance from first body line
+				if len(macroBuffer) == 0 {
+					currentMacroSourceFile = line.Filename
+					currentMacroStartLine = line.LineNo
+				}
 				// Collect macro body lines
 				macroBuffer = append(macroBuffer, line.Text)
 			}

internal/compiler/compiler_test.go

@@ -165,7 +165,7 @@ func TestExecuteScript_BasicPrint(t *testing.T) {
 		"  print('  nop')",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("executeScript failed: %v", err)
 	}
@@ -189,7 +189,7 @@ func TestExecuteScript_EmptyOutput(t *testing.T) {
 		"x = 1 + 1",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("executeScript failed: %v", err)
 	}
@@ -210,7 +210,7 @@ func TestExecuteScript_Library_DefinesFunction(t *testing.T) {
 		"    print('  nop')",
 	}
 
-	_, err := executeScript(libraryLines, ctx, true)
+	_, err := testExecuteScript(libraryLines, ctx, true)
 	if err != nil {
 		t.Fatalf("library executeScript failed: %v", err)
 	}
@@ -232,7 +232,7 @@ func TestExecuteScript_Library_FunctionCallableFromScript(t *testing.T) {
 		"    print('  nop')",
 	}
 
-	_, err := executeScript(libraryLines, ctx, true)
+	_, err := testExecuteScript(libraryLines, ctx, true)
 	if err != nil {
 		t.Fatalf("library executeScript failed: %v", err)
 	}
@@ -242,7 +242,7 @@ func TestExecuteScript_Library_FunctionCallableFromScript(t *testing.T) {
 		"emit_nops(2)",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("script executeScript failed: %v", err)
 	}
@@ -267,7 +267,7 @@ func TestExecuteScript_MultipleLibraries_Accumulate(t *testing.T) {
 		"def func_a():",
 		"  print('  ; from a')",
 	}
-	_, err := executeScript(lib1, ctx, true)
+	_, err := testExecuteScript(lib1, ctx, true)
 	if err != nil {
 		t.Fatalf("lib1 failed: %v", err)
 	}
@@ -277,7 +277,7 @@ func TestExecuteScript_MultipleLibraries_Accumulate(t *testing.T) {
 		"def func_b():",
 		"  print('  ; from b')",
 	}
-	_, err = executeScript(lib2, ctx, true)
+	_, err = testExecuteScript(lib2, ctx, true)
 	if err != nil {
 		t.Fatalf("lib2 failed: %v", err)
 	}
@@ -295,7 +295,7 @@ func TestExecuteScript_MultipleLibraries_Accumulate(t *testing.T) {
 		"func_a()",
 		"func_b()",
 	}
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("script failed: %v", err)
 	}
@@ -322,7 +322,7 @@ func TestExecuteScript_RegularScript_DoesNotPersist(t *testing.T) {
 		"local_func()",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("script failed: %v", err)
 	}
@@ -377,7 +377,7 @@ func TestExecuteMacro_WithLibraryFunction(t *testing.T) {
 		"def emit_nop():",
 		"  print('  nop')",
 	}
-	_, err := executeScript(lib, ctx, true)
+	_, err := testExecuteScript(lib, ctx, true)
 	if err != nil {
 		t.Fatalf("library failed: %v", err)
 	}
@@ -540,7 +540,7 @@ func TestExecuteScript_LocalVariableExpansion(t *testing.T) {
 		"print('  inc |counter|')",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("executeScript failed: %v", err)
 	}
@@ -569,7 +569,7 @@ func TestExecuteScript_Library_GlobalVariableExpansion(t *testing.T) {
 		"  print('  inc |global_counter|')",
 	}
 
-	_, err := executeScript(libraryLines, ctx, true)
+	_, err := testExecuteScript(libraryLines, ctx, true)
 	if err != nil {
 		t.Fatalf("library script failed: %v", err)
 	}
@@ -579,7 +579,7 @@ func TestExecuteScript_Library_GlobalVariableExpansion(t *testing.T) {
 		"inc_global()",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("executeScript failed: %v", err)
 	}
@@ -610,7 +610,7 @@ func TestExecuteScript_Library_VariableExpansionAtDefinitionTime(t *testing.T) {
 	}
 
 	// Library defined at global scope - |local_var| won't find caller's local
-	_, err := executeScript(libraryLines, ctx, true)
+	_, err := testExecuteScript(libraryLines, ctx, true)
 	if err != nil {
 		t.Fatalf("library script failed: %v", err)
 	}
@@ -625,7 +625,7 @@ func TestExecuteScript_Library_VariableExpansionAtDefinitionTime(t *testing.T) {
 		"use_local()",
 	}
 
-	output, err := executeScript(scriptLines, ctx, false)
+	output, err := testExecuteScript(scriptLines, ctx, false)
 	if err != nil {
 		t.Fatalf("executeScript failed: %v", err)
 	}
@@ -1219,3 +1219,16 @@ func TestAsmAfterVarsWithVariables(t *testing.T) {
 		t.Errorf("expected ASM block in deferred section")
 	}
 }
+
+// testExecuteScript is a test helper that wraps executeScript with convenient types
+func testExecuteScript(scriptLines []string, ctx *CompilerContext, isLibrary bool) ([]string, error) {
+	lines := make([]preproc.Line, len(scriptLines))
+	for i, text := range scriptLines {
+		lines[i] = preproc.Line{
+			Text:     text,
+			Filename: "test.c65",
+			LineNo:   i + 1,
+		}
+	}
+	return executeScript(lines, ctx, isLibrary)
+}

internal/compiler/context.go

@@ -8,9 +8,11 @@ import (
 
 // ScriptMacro represents a named, parameterized script macro
 type ScriptMacro struct {
-	Name   string   // macro name
+	Name       string   // macro name
-	Params []string // parameter names
+	Params     []string // parameter names
-	Body   []string // Starlark code lines (the macro body)
+	Body       []string // Starlark code lines (the macro body)
+	SourceFile string   // source file where macro is defined
+	StartLine  int      // 1-based line number in source file of first body line
 }
 
 // CompilerContext holds all shared resources needed by commands during compilation

internal/compiler/scriptexec.go

@@ -2,29 +2,195 @@ package compiler
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
+	"os"
 	"strings"
 
+	"c65gm/internal/preproc"
 	"c65gm/internal/utils"
 
 	"go.starlark.net/lib/math"
 	"go.starlark.net/starlark"
 )
 
+// mapStarlarkLine maps a Starlark 1-based line number to an index into scriptLines
+// (a slice of preproc.Line). Returns -1 if the line cannot be mapped.
+// For non-library scripts, the Starlark source is:
+//
+//	Line 1: def _main():
+//	Line 2..N+1: indented script lines
+//	Line N+2: _main()
+//
+// For library scripts, lines are used as-is.
+func mapStarlarkLine(starlarkLine int, numScriptLines int, isLibrary bool) int {
+	var idx int
+	if isLibrary {
+		idx = starlarkLine - 1
+	} else {
+		idx = starlarkLine - 2
+	}
+	if idx < 0 || idx >= numScriptLines {
+		return -1
+	}
+	return idx
+}
+
+// starlarkPosition extracts the source position from a Starlark error.
+// Returns the 1-based line number (0 if unknown).
+func starlarkPosition(err error) int {
+	var evalErr *starlark.EvalError
+	if errors.As(err, &evalErr) {
+		if len(evalErr.CallStack) > 0 {
+			top := evalErr.CallStack.At(0)
+			return int(top.Pos.Line)
+		}
+	}
+	return 0
+}
+
+// starlarkErrorMsg extracts just the message from a Starlark error.
+func starlarkErrorMsg(err error) string {
+	var evalErr *starlark.EvalError
+	if errors.As(err, &evalErr) {
+		return evalErr.Msg
+	}
+	return err.Error()
+}
+
+// printScriptErrorContext prints a Starlark error with script-block-bounded source context,
+// synthesizing SCRIPT/ENDSCRIPT boundary lines that the preprocessor discards.
+// When the original Starlark error has a call stack with frames beyond the script block
+// (e.g. a library function), those frames are shown as a backtrace below the source context.
+func printScriptErrorContext(err error, errMsg string, scriptLines []preproc.Line, errorIdx int, blockType string) {
+	if len(scriptLines) == 0 || errorIdx < 0 || errorIdx >= len(scriptLines) {
+		fmt.Fprintf(os.Stderr, "\nError: Starlark error: %s\n\n", errMsg)
+		return
+	}
+
+	line := scriptLines[errorIdx]
+	filename := line.Filename
+
+	const contextLines = 3
+
+	fmt.Fprintf(os.Stderr, "\nError: Starlark error: %s\n", errMsg)
+	fmt.Fprintf(os.Stderr, "  --> %s:%d\n\n", filename, line.LineNo)
+
+	startIdx := errorIdx - contextLines
+	if startIdx < 0 {
+		startIdx = 0
+	}
+	endIdx := errorIdx + contextLines
+	if endIdx >= len(scriptLines) {
+		endIdx = len(scriptLines) - 1
+	}
+
+	scriptMarkerLineNo := scriptLines[0].LineNo - 1
+	endScriptLineNo := scriptLines[len(scriptLines)-1].LineNo + 1
+
+	maxLineNo := endScriptLineNo
+	if scriptLines[endIdx].LineNo > maxLineNo {
+		maxLineNo = scriptLines[endIdx].LineNo
+	}
+	if scriptMarkerLineNo > maxLineNo {
+		maxLineNo = scriptMarkerLineNo
+	}
+	lineNumWidth := len(fmt.Sprintf("%d", maxLineNo))
+
+	if startIdx == 0 {
+		fmt.Fprintf(os.Stderr, "   %*d | %s\n", lineNumWidth, scriptMarkerLineNo, blockType)
+	}
+
+	for i := startIdx; i <= endIdx; i++ {
+		l := scriptLines[i]
+		marker := "   "
+		if i == errorIdx {
+			marker = ">> "
+		}
+		fmt.Fprintf(os.Stderr, "%s%*d | %s\n", marker, lineNumWidth, l.LineNo, l.Text)
+	}
+
+	if endIdx == len(scriptLines)-1 {
+		fmt.Fprintf(os.Stderr, "   %*d | END%s\n", lineNumWidth, endScriptLineNo, blockType)
+	}
+
+	// Show call stack if the error has additional frames beyond the script block
+	printStarlarkBacktrace(err, line.LineNo, filename)
+
+	fmt.Fprintf(os.Stderr, "\n")
+}
+
+// printStarlarkBacktrace prints a concise call stack from a Starlark error,
+// filtering out frames from the given scriptBlockLine and <toplevel>/_main wrappers.
+func printStarlarkBacktrace(err error, scriptBlockLine int, scriptFile string) {
+	var evalErr *starlark.EvalError
+	if !errors.As(err, &evalErr) || len(evalErr.CallStack) < 2 {
+		return
+	}
+
+	var frames []string
+	for _, cf := range evalErr.CallStack {
+		name := cf.Name
+		pos := cf.Pos
+		// Skip frames that are already shown in the bounded source context
+		if name == "<toplevel>" || name == "_main" {
+			continue
+		}
+		if int(pos.Line) == scriptBlockLine && pos.Filename() == scriptFile {
+			continue
+		}
+		if pos.Filename() != "" && int(pos.Line) > 0 {
+			frames = append(frames, fmt.Sprintf("  %s: in %s", pos.String(), name))
+		}
+	}
+	if len(frames) > 0 {
+		fmt.Fprintf(os.Stderr, "Call stack:\n%s\n", strings.Join(frames, "\n"))
+	}
+}
+
+// printScriptErrorFallback prints what we can from a Starlark error when
+// we couldn't map it to a specific source line within a script block.
+func printScriptErrorFallback(err error) {
+	fmt.Fprintf(os.Stderr, "\nError: Starlark error: %s\n", err)
+	var evalErr *starlark.EvalError
+	if errors.As(err, &evalErr) && len(evalErr.CallStack) > 0 {
+		fmt.Fprintf(os.Stderr, "Call stack:\n")
+		for _, cf := range evalErr.CallStack {
+			name := cf.Name
+			pos := cf.Pos
+			if pos.Filename() != "" && int(pos.Line) > 0 {
+				fmt.Fprintf(os.Stderr, "  %s: in %s\n", pos.String(), name)
+			}
+		}
+	}
+	fmt.Fprintf(os.Stderr, "\n")
+}
+
 // executeScript runs a Starlark script and returns the output lines.
 // If isLibrary is true, the script is executed at top level (no _main wrapper)
 // and resulting globals are persisted to ctx.ScriptLibraryGlobals.
-func executeScript(scriptLines []string, ctx *CompilerContext, isLibrary bool) ([]string, error) {
+func executeScript(scriptLines []preproc.Line, ctx *CompilerContext, isLibrary bool) ([]string, error) {
+	// Extract text from preproc.Lines
+	texts := make([]string, len(scriptLines))
+	for i, l := range scriptLines {
+		texts[i] = l.Text
+	}
+
 	// Join script lines
-	scriptText := strings.Join(scriptLines, "\n")
+	scriptText := strings.Join(texts, "\n")
 
 	// Expand |varname| -> actual variable names
 	scriptText = expandVariables(scriptText, ctx)
 
+	// Determine the source filename for Starlark
+	sourceFile := scriptLines[0].Filename
+
 	var finalScript string
+	var starlarkFilename string
 	if isLibrary {
 		// LIBRARY: execute at top level so defs become globals
 		finalScript = scriptText
+		starlarkFilename = sourceFile
 	} else {
 		// Regular SCRIPT: wrap in function (Starlark requires control flow inside functions)
 		finalScript = "def _main():\n"
@@ -32,6 +198,7 @@ func executeScript(scriptLines []string, ctx *CompilerContext, isLibrary bool) (
 			finalScript += "    " + line + "\n"
 		}
 		finalScript += "_main()\n"
+		starlarkFilename = sourceFile
 	}
 
 	// Capture print output
@@ -55,9 +222,25 @@ func executeScript(scriptLines []string, ctx *CompilerContext, isLibrary bool) (
 	}
 
 	// Execute
-	globals, err := starlark.ExecFile(thread, "script.star", finalScript, predeclared)
+	globals, err := starlark.ExecFile(thread, starlarkFilename, finalScript, predeclared)
 	if err != nil {
-		return nil, err
+		// Map Starlark error position back to source
+		starLine := starlarkPosition(err)
+		if starLine > 0 {
+			idx := mapStarlarkLine(starLine, len(scriptLines), isLibrary)
+			if idx >= 0 {
+				msg := starlarkErrorMsg(err)
+				blockType := "SCRIPT"
+				if isLibrary {
+					blockType = "SCRIPT LIBRARY"
+				}
+				printScriptErrorContext(err, msg, scriptLines, idx, blockType)
+				return nil, fmt.Errorf("Starlark error: %s", msg)
+			}
+		}
+		// Fallback: print whatever info we can extract
+		printScriptErrorFallback(err)
+		return nil, fmt.Errorf("Starlark error: %w", err)
 	}
 
 	// For LIBRARY: persist new globals (functions, variables defined at top level)
@@ -130,6 +313,12 @@ func ExecuteMacro(macroName string, args []string, ctx *CompilerContext) ([]stri
 	}
 	finalScript += "_macro()\n"
 
+	// Use the source file where the macro was defined
+	starlarkFilename := macro.SourceFile
+	if starlarkFilename == "" {
+		starlarkFilename = "macro.star"
+	}
+
 	// Capture print output
 	var output bytes.Buffer
 	thread := &starlark.Thread{
@@ -154,9 +343,19 @@ func ExecuteMacro(macroName string, args []string, ctx *CompilerContext) ([]stri
 	}
 
 	// Execute
-	_, err := starlark.ExecFile(thread, "macro.star", finalScript, predeclared)
+	_, err := starlark.ExecFile(thread, starlarkFilename, finalScript, predeclared)
 	if err != nil {
-		return nil, err
+		// Map error position back to macro definition site
+		starLine := starlarkPosition(err)
+		if starLine > 0 && macro.SourceFile != "" {
+			idx := mapStarlarkLine(starLine, len(macro.Body), false) // macro is always wrapped
+			if idx >= 0 {
+				sourceLine := macro.StartLine + idx
+				msg := starlarkErrorMsg(err)
+				return nil, fmt.Errorf("Starlark error: at %s:%d: %s", macro.SourceFile, sourceLine, msg)
+			}
+		}
+		return nil, fmt.Errorf("Starlark error: %w", err)
 	}
 
 	// Split output into lines

internal/compiler/scriptexec_test.go

@@ -0,0 +1,439 @@
+package compiler
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+
+	"c65gm/internal/preproc"
+
+	"go.starlark.net/lib/math"
+	"go.starlark.net/starlark"
+)
+
+// --- Unit tests for mapStarlarkLine ---
+
+func TestMapStarlarkLine_Regular(t *testing.T) {
+	tests := []struct {
+		starlarkLine  int
+		numLines      int
+		isLibrary     bool
+		want          int
+	}{
+		{2, 5, false, 0},   // first script line
+		{3, 5, false, 1},   // second script line
+		{6, 5, false, 4},   // last script line
+		{1, 5, false, -1},  // def _main(): line — out of range
+		{7, 5, false, -1},  // _main() call — out of range
+		{0, 5, false, -1},  // before start
+		{2, 1, false, 0},   // single line script
+		{3, 1, false, -1},  // single line, past it
+	}
+
+	for _, tt := range tests {
+		got := mapStarlarkLine(tt.starlarkLine, tt.numLines, tt.isLibrary)
+		if got != tt.want {
+			t.Errorf("mapStarlarkLine(%d, %d, %v) = %d, want %d",
+				tt.starlarkLine, tt.numLines, tt.isLibrary, got, tt.want)
+		}
+	}
+}
+
+func TestMapStarlarkLine_Library(t *testing.T) {
+	tests := []struct {
+		starlarkLine int
+		numLines     int
+		want         int
+	}{
+		{1, 5, 0},   // first script line
+		{2, 5, 1},   // second script line
+		{5, 5, 4},   // last script line
+		{0, 5, -1},  // before start
+		{6, 5, -1},  // past end
+		{1, 1, 0},   // single line
+		{2, 1, -1},  // single line, past it
+	}
+
+	for _, tt := range tests {
+		got := mapStarlarkLine(tt.starlarkLine, tt.numLines, true)
+		if got != tt.want {
+			t.Errorf("mapStarlarkLine(%d, %d, true) = %d, want %d",
+				tt.starlarkLine, tt.numLines, got, tt.want)
+		}
+	}
+}
+
+// --- Tests for starlarkPosition and starlarkErrorMsg ---
+
+func TestStarlarkPosition_NoEvalError(t *testing.T) {
+	got := starlarkPosition(fmt.Errorf("plain error"))
+	if got != 0 {
+		t.Errorf("starlarkPosition(plain error) = %d, want 0", got)
+	}
+}
+
+func TestStarlarkPosition_WithCallStack(t *testing.T) {
+	// Create an EvalError by executing Starlark code with a runtime error
+	thread := &starlark.Thread{}
+	predeclared := starlark.StringDict{"math": math.Module}
+	_, err := starlark.ExecFile(thread, "test.star",
+		`def f():
+  x = 1 // 0
+f()`,
+		predeclared)
+	if err == nil {
+		t.Fatal("expected Starlark error")
+	}
+
+	pos := starlarkPosition(err)
+	if pos <= 0 {
+		t.Errorf("starlarkPosition() = %d, want > 0 (error: %v)", pos, err)
+	}
+}
+
+func TestStarlarkErrorMsg_StripsCallStack(t *testing.T) {
+	thread := &starlark.Thread{}
+	predeclared := starlark.StringDict{"math": math.Module}
+	_, err := starlark.ExecFile(thread, "test.star", `x = 1 // 0`, predeclared)
+	if err == nil {
+		t.Fatal("expected Starlark error")
+	}
+
+	msg := starlarkErrorMsg(err)
+	if !strings.Contains(msg, "division by zero") {
+		t.Errorf("starlarkErrorMsg() = %q, want division by zero", msg)
+	}
+	// Should NOT contain traceback prefixes
+	if strings.Contains(msg, "Traceback") {
+		t.Errorf("starlarkErrorMsg() should not contain Traceback, got %q", msg)
+	}
+}
+
+func TestStarlarkErrorMsg_PlainError(t *testing.T) {
+	msg := starlarkErrorMsg(fmt.Errorf("hello"))
+	if msg != "hello" {
+		t.Errorf("starlarkErrorMsg(plain) = %q, want 'hello'", msg)
+	}
+}
+
+// --- Tests for printScriptErrorContext ---
+
+func TestPrintScriptErrorContext_Basic(t *testing.T) {
+	scriptLines := []preproc.Line{
+		{Text: "  for i in range(3):", Filename: "test.c65", LineNo: 7, Kind: preproc.Script},
+		{Text: "    print(i)", Filename: "test.c65", LineNo: 8, Kind: preproc.Script},
+		{Text: "  end", Filename: "test.c65", LineNo: 9, Kind: preproc.Script},
+	}
+
+	// Capture stderr
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	err := fmt.Errorf("test error")
+	printScriptErrorContext(err, "test error", scriptLines, 1, "SCRIPT")
+
+	w.Close()
+	os.Stderr = old
+
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+	output := buf.String()
+
+	// Should contain SCRIPT marker
+	if !strings.Contains(output, "SCRIPT") {
+		t.Errorf("output should contain SCRIPT marker, got:\n%s", output)
+	}
+	// Should contain ENDSCRIPT
+	if !strings.Contains(output, "ENDSCRIPT") {
+		t.Errorf("output should contain ENDSCRIPT, got:\n%s", output)
+	}
+	// Should contain the error line
+	if !strings.Contains(output, "print(i)") {
+		t.Errorf("output should contain the error line, got:\n%s", output)
+	}
+	// Should contain Starlark error header
+	if !strings.Contains(output, "Starlark error:") {
+		t.Errorf("output should contain 'Starlark error:', got:\n%s", output)
+	}
+	// Should contain source marker
+	if !strings.Contains(output, "-->") {
+		t.Errorf("output should contain '-->', got:\n%s", output)
+	}
+}
+
+func TestPrintScriptErrorContext_EdgeCases(t *testing.T) {
+	tests := []struct {
+		name       string
+		lines      []preproc.Line
+		errorIdx   int
+		blockType  string
+		wantMarker bool
+	}{
+		{
+			name:       "empty lines",
+			lines:      []preproc.Line{},
+			errorIdx:   0,
+			blockType:  "SCRIPT",
+			wantMarker: false,
+		},
+		{
+			name:       "invalid index",
+			lines:      []preproc.Line{{Text: "x", Filename: "t", LineNo: 1}},
+			errorIdx:   5,
+			blockType:  "SCRIPT",
+			wantMarker: false,
+		},
+		{
+			name:       "first line",
+			lines:      []preproc.Line{{Text: "x", Filename: "t", LineNo: 1}},
+			errorIdx:   0,
+			blockType:  "SCRIPT LIBRARY",
+			wantMarker: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			old := os.Stderr
+			r, w, _ := os.Pipe()
+			os.Stderr = w
+
+			printScriptErrorContext(fmt.Errorf("err"), "err", tt.lines, tt.errorIdx, tt.blockType)
+
+			w.Close()
+			os.Stderr = old
+
+			var buf bytes.Buffer
+			buf.ReadFrom(r)
+			output := buf.String()
+
+			if tt.wantMarker && !strings.Contains(output, tt.blockType) {
+				t.Errorf("output should contain %q, got:\n%s", tt.blockType, output)
+			}
+			if !tt.wantMarker && !strings.Contains(output, "Starlark error") {
+				// Should at minimum print the error
+				t.Errorf("output should contain error even in edge case, got:\n%s", output)
+			}
+		})
+	}
+}
+
+// --- Tests for executeScript error prefix ---
+
+func TestExecuteScript_ErrorPrefix(t *testing.T) {
+	pragma := preproc.NewPragma()
+	ctx := NewCompilerContext(pragma)
+
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	scriptLines := []preproc.Line{
+		{Text: "def f():\n  return 1 // 0\nf()", Filename: "test.c65", LineNo: 1, Kind: preproc.Script},
+	}
+
+	_, err := executeScript(scriptLines, ctx, false)
+
+	w.Close()
+	os.Stderr = old
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+
+	if err == nil {
+		t.Fatal("expected error")
+	}
+
+	if !strings.Contains(err.Error(), "Starlark error:") {
+		t.Errorf("error should contain 'Starlark error:', got: %v", err)
+	}
+}
+
+func TestExecuteScript_LibraryRuntimeError(t *testing.T) {
+	pragma := preproc.NewPragma()
+	ctx := NewCompilerContext(pragma)
+
+	// Define a library function that errors
+	libLines := []preproc.Line{
+		{Text: "def bad():\n  return 1 // 0", Filename: "test.c65", LineNo: 5, Kind: preproc.ScriptLibrary},
+	}
+	_, err := executeScript(libLines, ctx, true)
+	if err != nil {
+		t.Fatalf("library definition should succeed: %v", err)
+	}
+
+	// Capture stderr
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	// Call the library function — should error
+	callLines := []preproc.Line{
+		{Text: "bad()", Filename: "test.c65", LineNo: 10, Kind: preproc.Script},
+	}
+	_, err = executeScript(callLines, ctx, false)
+
+	w.Close()
+	os.Stderr = old
+
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+	stderr := buf.String()
+
+	if err == nil {
+		t.Fatal("expected error from library call")
+	}
+
+	if !strings.Contains(err.Error(), "Starlark error:") {
+		t.Errorf("error should contain 'Starlark error:', got: %v", err)
+	}
+
+	// Should show bounded context for the calling SCRIPT block
+	if !strings.Contains(stderr, "SCRIPT") {
+		t.Errorf("stderr should contain SCRIPT marker, got:\n%s", stderr)
+	}
+	if !strings.Contains(stderr, "ENDSCRIPT") {
+		t.Errorf("stderr should contain ENDSCRIPT, got:\n%s", stderr)
+	}
+}
+
+func TestExecuteScript_DirectRuntimeError(t *testing.T) {
+	pragma := preproc.NewPragma()
+	ctx := NewCompilerContext(pragma)
+
+	// Capture stderr
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	scriptLines := []preproc.Line{
+		{Text: "  print(1 // 0)", Filename: "test.c65", LineNo: 3, Kind: preproc.Script},
+	}
+	_, err := executeScript(scriptLines, ctx, false)
+
+	w.Close()
+	os.Stderr = old
+
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+	stderr := buf.String()
+
+	if err == nil {
+		t.Fatal("expected error")
+	}
+
+	if !strings.Contains(err.Error(), "Starlark error:") {
+		t.Errorf("error should contain 'Starlark error:', got: %v", err)
+	}
+
+	// Should show the error source line in stderr context
+	if !strings.Contains(stderr, "print(1 // 0)") {
+		t.Errorf("stderr should contain the error line, got:\n%s", stderr)
+	}
+}
+
+// --- Tests for ExecuteMacro error mapping ---
+
+func TestExecuteMacro_ErrorDefinitionSite(t *testing.T) {
+	pragma := preproc.NewPragma()
+	ctx := NewCompilerContext(pragma)
+
+	// Register a macro with source provenance
+	ctx.ScriptMacros["div_macro"] = &ScriptMacro{
+		Name:       "div_macro",
+		Params:     []string{"a", "b"},
+		Body:       []string{"result = a // b"},
+		SourceFile: "defs.c65",
+		StartLine:  12,
+	}
+
+	// Capture stderr
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	_, err := ExecuteMacro("div_macro", []string{"10", "0"}, ctx)
+
+	w.Close()
+	os.Stderr = old
+
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+
+	if err == nil {
+		t.Fatal("expected error")
+	}
+
+	// Error should mention the definition site
+	if !strings.Contains(err.Error(), "defs.c65:12") {
+		t.Errorf("error should reference definition site 'defs.c65:12', got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "Starlark error:") {
+		t.Errorf("error should contain 'Starlark error:', got: %v", err)
+	}
+}
+
+func TestExecuteMacro_ErrorFallbackNoSource(t *testing.T) {
+	pragma := preproc.NewPragma()
+	ctx := NewCompilerContext(pragma)
+
+	// Register a macro WITHOUT source provenance (backward compatibility)
+	ctx.ScriptMacros["m"] = &ScriptMacro{
+		Name:   "m",
+		Params: []string{},
+		Body:   []string{"1 // 0"},
+	}
+
+	_, err := ExecuteMacro("m", []string{}, ctx)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "Starlark error:") {
+		t.Errorf("error should contain 'Starlark error:', got: %v", err)
+	}
+}
+
+// --- Tests for printStarlarkBacktrace ---
+
+func TestPrintStarlarkBacktrace_NoFrames(t *testing.T) {
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	printStarlarkBacktrace(fmt.Errorf("plain error"), 10, "test.c65")
+
+	w.Close()
+	os.Stderr = old
+
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+	output := buf.String()
+
+	if output != "" {
+		t.Errorf("expected empty output for plain error, got: %q", output)
+	}
+}
+
+func TestPrintScriptErrorFallback(t *testing.T) {
+	old := os.Stderr
+	r, w, _ := os.Pipe()
+	os.Stderr = w
+
+	printScriptErrorFallback(fmt.Errorf("test message"))
+
+	w.Close()
+	os.Stderr = old
+
+	var buf bytes.Buffer
+	buf.ReadFrom(r)
+	output := buf.String()
+
+	if !strings.Contains(output, "Starlark error:") {
+		t.Errorf("output should contain 'Starlark error:', got: %q", output)
+	}
+	if !strings.Contains(output, "test message") {
+		t.Errorf("output should contain the error message, got: %q", output)
+	}
+}