Convert panics to errors for public interface

main.go

@@ -25,19 +25,22 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	r, out := bytes.NewReader(bs), ""
+	r, out, err := bytes.NewReader(bs), "", nil
 	switch strings.ToLower(os.Args[2]) {
 	case "org":
-		out = org.NewDocument().Parse(r).Write(org.NewOrgWriter()).String()
+		out, err = org.NewDocument().Parse(r).Write(org.NewOrgWriter())
 	case "html":
-		out = org.NewDocument().Parse(r).Write(org.NewHTMLWriter()).String()
+		out, err = org.NewDocument().Parse(r).Write(org.NewHTMLWriter())
 	case "html-chroma":
 		writer := org.NewHTMLWriter()
 		writer.HighlightCodeBlock = highlightCodeBlock
-		out = org.NewDocument().Parse(r).Write(writer).String()
+		out, err = org.NewDocument().Parse(r).Write(writer)
 	default:
 		log.Fatal("Unsupported output format")
 	}
+	if err != nil {
+		log.Fatal(err)
+	}
 	log.Print(out)
 }
 

org/document.go

@@ -9,6 +9,7 @@ import (
 )
 
 type Document struct {
+	Path                string
 	tokens              []token
 	Nodes               []Node
 	Footnotes           *Footnotes
@@ -17,6 +18,7 @@ type Document struct {
 	AutoLink            bool
 	BufferSettings      map[string]string
 	DefaultSettings     map[string]string
+	Error               error
 }
 
 type Writer interface {
@@ -77,24 +79,45 @@ func NewDocument() *Document {
 	}
 }
 
-func (d *Document) Write(w Writer) Writer {
+func (d *Document) Write(w Writer) (out string, err error) {
-	if d.Nodes == nil {
+	defer func() {
-		panic("cannot Write() empty document: you must call Parse() first")
+		if recovered := recover(); recovered != nil {
+			err = fmt.Errorf("could not write output: %s", recovered)
+		}
+	}()
+	if d.Error != nil {
+		return "", d.Error
+	} else if d.Nodes == nil {
+		return "", fmt.Errorf("could not write output: parse was not called")
 	}
 	w.before(d)
 	w.writeNodes(d.Nodes...)
 	w.after(d)
-	return w
+	return w.String(), err
 }
 
 func (d *Document) Parse(input io.Reader) *Document {
+	defer func() {
+		if err := recover(); err != nil {
+			d.Error = fmt.Errorf("could not parse input: %s", err)
+		}
+	}()
+	if d.tokens != nil {
+		d.Error = fmt.Errorf("parse was called multiple times")
+	}
 	d.tokenize(input)
 	_, nodes := d.parseMany(0, func(d *Document, i int) bool { return !(i < len(d.tokens)) })
 	d.Nodes = nodes
 	return d
 }
 
-func (d *Document) FrontMatter(input io.Reader, f func(string, string) interface{}) map[string]interface{} {
+func (d *Document) FrontMatter(input io.Reader, f func(string, string) interface{}) (_ map[string]interface{}, err error) {
+	defer func() {
+		d.tokens = nil
+		if recovered := recover(); recovered != nil {
+			err = fmt.Errorf("could not parse input: %s", recovered)
+		}
+	}()
 	d.tokenize(input)
 	d.parseMany(0, func(d *Document, i int) bool {
 		if !(i < len(d.tokens)) {
@@ -107,7 +130,7 @@ func (d *Document) FrontMatter(input io.Reader, f func(string, string) interface
 	for k, v := range d.BufferSettings {
 		frontMatter[k] = f(k, v)
 	}
-	return frontMatter
+	return frontMatter, err
 }
 
 func (d *Document) tokenize(input io.Reader) {
@@ -117,7 +140,7 @@ func (d *Document) tokenize(input io.Reader) {
 		d.tokens = append(d.tokens, tokenize(scanner.Text()))
 	}
 	if err := scanner.Err(); err != nil {
-		panic(err)
+		d.Error = fmt.Errorf("could not tokenize input: %s", err)
 	}
 }
 

org/document_test.go

@@ -64,7 +64,11 @@ var frontMatterTests = []frontMatterTest{
 
 func TestParseFrontMatter(t *testing.T) {
 	for _, test := range frontMatterTests {
-		actual := NewDocument().FrontMatter(strings.NewReader(test.input), test.handler)
+		actual, err := NewDocument().FrontMatter(strings.NewReader(test.input), test.handler)
+		if err != nil {
+			t.Errorf("%s\n got error: %s", test.name, err)
+			continue
+		}
 		if !reflect.DeepEqual(test.expected, actual) {
 			t.Errorf("%s\n got: %#v\nexpected: %#v\n%s'", test.name, actual, test.expected, test.input)
 		}

org/html_test.go

@@ -8,9 +8,12 @@ import (
 func TestHTMLWriter(t *testing.T) {
 	for _, path := range orgTestFiles() {
 		reader, writer := strings.NewReader(fileString(path)), NewHTMLWriter()
-		actual := NewDocument().Parse(reader).Write(writer).String()
+		actual, err := NewDocument().Parse(reader).Write(writer)
+		if err != nil {
+			t.Errorf("%s\n got error: %s", path, err)
+			continue
+		}
 		expected := fileString(path[:len(path)-len(".org")] + ".html")
-
 		if expected != actual {
 			t.Errorf("%s:\n%s'", path, diff(actual, expected))
 		} else {

org/org_test.go

@@ -14,7 +14,11 @@ func TestOrgWriter(t *testing.T) {
 	for _, path := range orgTestFiles() {
 		expected := fileString(path)
 		reader, writer := strings.NewReader(expected), NewOrgWriter()
-		actual := NewDocument().Parse(reader).Write(writer).String()
+		actual, err := NewDocument().Parse(reader).Write(writer)
+		if err != nil {
+			t.Errorf("%s\n got error: %s", path, err)
+			continue
+		}
 		if actual != expected {
 			t.Errorf("%s:\n%s'", path, diff(actual, expected))
 		} else {