Diffable interface

diff.go

@@ -37,6 +37,11 @@ type Printfer interface {
 	Printf(format string, a ...interface{})
 }
 
+// Diffable is the interface that gives a chance to return custom value for diff
+type Diffable interface {
+	DiffValue() interface{}
+}
+
 // Pdiff prints to p a description of the differences between a and b.
 // It calls Printf once for each difference, with no trailing newline.
 // The standard library log.Logger is a Printfer.
@@ -77,6 +82,17 @@ func (w diffPrinter) printf(f string, a ...interface{}) {
 }
 
 func (w diffPrinter) diff(av, bv reflect.Value) {
+	if av.IsValid() && av.CanInterface() {
+		if diffable, ok := av.Interface().(Diffable); ok {
+			av = reflect.ValueOf(diffable.DiffValue())
+		}
+	}
+	if bv.IsValid() && bv.CanInterface() {
+		if diffable, ok := bv.Interface().(Diffable); ok {
+			bv = reflect.ValueOf(diffable.DiffValue())
+		}
+	}
+
 	if !av.IsValid() && bv.IsValid() {
 		w.printf("nil != %# v", formatter{v: bv, quote: true})
 		return

diff_test.go

@@ -31,8 +31,13 @@ type S struct {
 type (
 	N struct{ N int }
 	E interface{}
+	D struct{ a string }
 )
 
+func (d D) DiffValue() interface{} {
+	return d.a
+}
+
 var (
 	c0 = make(chan int)
 	c1 = make(chan int)
@@ -59,6 +64,8 @@ var diffs = []difftest{
 	{S{C: []int{}}, S{C: []int{1}}, []string{`C: []int[0] != []int[1]`}},
 	{S{C: []int{1, 2, 3}}, S{C: []int{1, 2, 4}}, []string{`C[2]: 3 != 4`}},
 	{S{}, S{A: 1, S: new(S)}, []string{`A: 0 != 1`, `S: nil != &pretty.S{}`}},
+	{D{a: "a"}, D{a: "b"}, []string{`"a" != "b"`}},
+	{struct{ D D }{D{"a"}}, struct{ D D }{D{"b"}}, []string{`D: "a" != "b"`}},
 
 	// unexported fields of every reflect.Kind (both equal and unequal)
 	{struct{ x bool }{false}, struct{ x bool }{false}, nil},