src: type for representing file:line positions

Updates #5

src/src.go

@@ -0,0 +1,28 @@
+package src
+
+import "strconv"
+
+// Position represents a position in a source file.
+type Position struct {
+	Filename string
+	Line     int // 1-up
+}
+
+// IsValid reports whether the position is valid: Line must be positive, but
+// Filename may be empty.
+func (p Position) IsValid() bool {
+	return p.Line > 0
+}
+
+// String represents Position as a string.
+func (p Position) String() string {
+	if !p.IsValid() {
+		return "-"
+	}
+	var s string
+	if p.Filename != "" {
+		s += p.Filename + ":"
+	}
+	s += strconv.Itoa(p.Line)
+	return s
+}