Document and add iterator panic recovery.

formatter.go

@@ -7,10 +7,37 @@ import (
 // A Formatter for Chroma lexers.
 type Formatter interface {
 	// Format returns a formatting function for tokens.
+	//
+	// If the iterator panics, the Formatter should recover.
 	Format(w io.Writer, style *Style, iterator Iterator) error
 }
 
 // A FormatterFunc is a Formatter implemented as a function.
+//
+// Guards against iterator panics.
 type FormatterFunc func(w io.Writer, style *Style, iterator Iterator) error
 
-func (f FormatterFunc) Format(w io.Writer, s *Style, it Iterator) error { return f(w, s, it) }
+func (f FormatterFunc) Format(w io.Writer, s *Style, it Iterator) (err error) {
+	defer func() {
+		if perr := recover(); perr != nil {
+			err = perr.(error)
+		}
+	}()
+	return f(w, s, it)
+}
+
+type recoveringFormatter struct {
+	Formatter
+}
+
+func (r recoveringFormatter) Format(w io.Writer, s *Style, it Iterator) (err error) {
+	defer func() {
+		if perr := recover(); perr != nil {
+			err = perr.(error)
+		}
+	}()
+	return r.Formatter.Format(w, s, it)
+}
+
+// RecoveringFormatter wraps a formatter with panic recovery.
+func RecoveringFormatter(formatter Formatter) Formatter { return recoveringFormatter{formatter} }