Add `AtIndex` method to deal to attach index at the current error. (#257)

* Add `AtIndex` method to deal to attach index at the current error.

This is useful when we are dealing with primite leaf list (e.g.
a list of strings).
The alternative are
a) creating dummy error and then executing ViaFieldIndex(), which is
plain ugly
b) implementing Validate() on the primitive type via type aliasing and
then collecting errors, which seems an overkill.

In addition it seems ViaFieldIndex was not covered by unit tests,
so I've added one to take care of that.

* implement suggestions from the review

* cleanup

* cosmetic

* cosmetic

apis/field_error.go

@@ -192,7 +192,7 @@ func asKey(key string) string {
 //   err([0]).ViaField(bar).ViaField(foo) -> foo.bar.[0] converts to foo.bar[0]
 //   err(bar).ViaIndex(0).ViaField(foo) -> foo.[0].bar converts to foo[0].bar
 //   err(bar).ViaField(foo).ViaIndex(0) -> [0].foo.bar converts to [0].foo.bar
-//   err(bar).ViaIndex(0).ViaIndex[1].ViaField(foo) -> foo.[1].[0].bar converts to foo[1][0].bar
+//   err(bar).ViaIndex(0).ViaIndex(1).ViaField(foo) -> foo.[1].[0].bar converts to foo[1][0].bar
 func flatten(path []string) string {
 	var newPath []string
 	for _, part := range path {
@@ -300,6 +300,12 @@ func ErrDisallowedFields(fieldPaths ...string) *FieldError {
 	}
 }
 
+// ErrInvalidArrayValue consturcts a FieldError for a repetetive `field`
+// at `index` that has received an invalid string value.
+func ErrInvalidArrayValue(value, field string, index int) *FieldError {
+	return ErrInvalidValue(value, CurrentField).ViaFieldIndex(field, index)
+}
+
 // ErrInvalidValue constructs a FieldError for a field that has received an
 // invalid string value.
 func ErrInvalidValue(value, fieldPath string) *FieldError {

apis/field_error_test.go

@@ -21,6 +21,8 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 func TestFieldError(t *testing.T) {
@@ -389,6 +391,12 @@ can not use @, do not try`,
 			return err
 		}(),
 		want: "invalid field(s): jar[0][E].baz[1].bar[A].faa, jar[0][E].baz[1].bar[A].foo",
+	}, {
+		name: "leaf field error with index",
+		err: func() *FieldError {
+			return ErrInvalidArrayValue("kapot", "indexed", 5)
+		}(),
+		want: `invalid value "kapot": indexed[5]`,
 	}, {
 		name:     "nil propagation",
 		err:      nil,
@@ -421,7 +429,7 @@ can not use @, do not try`,
 
 			if test.want != "" {
 				if got, want := fe.Error(), test.want; got != want {
-					t.Errorf("%s: Error() = %v, wanted %v", test.name, got, want)
+t.Errorf("%s: Error() = %q, wanted %q, diff: %s", test.name, got, want, cmp.Diff(got, want))
 				}
 			} else if fe != nil {
 				t.Errorf("%s: ViaField() = %v, wanted nil", test.name, fe)