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
+}

src/src_test.go

@@ -0,0 +1,27 @@
+package src_test
+
+import (
+	"fmt"
+
+	"github.com/mmcloughlin/avo/src"
+)
+
+func ExamplePosition_IsValid() {
+	fmt.Println(src.Position{"a.go", 42}.IsValid())
+	fmt.Println(src.Position{"", 42}.IsValid())
+	fmt.Println(src.Position{"a.go", -1}.IsValid())
+	// Output:
+	// true
+	// true
+	// false
+}
+
+func ExamplePosition_String() {
+	fmt.Println(src.Position{"a.go", 42})
+	fmt.Println(src.Position{"", 42})
+	fmt.Println(src.Position{"a.go", -1}) // invalid
+	// Output:
+	// a.go:42
+	// 42
+	// -
+}