Support slices of same type

Fixes #209

issue209_test.go

@@ -0,0 +1,26 @@
+package mergo_test
+
+import (
+	"testing"
+
+	"github.com/imdario/mergo"
+)
+
+func TestIssue209(t *testing.T) {
+	dst := []string{"a", "b"}
+	src := []string{"c", "d"}
+
+	if err := mergo.Merge(&dst, src, mergo.WithAppendSlice); err != nil {
+		t.Error(err)
+	}
+
+	expected := []string{"a", "b", "c", "d"}
+	if len(dst) != len(expected) {
+		t.Errorf("arrays not equal length")
+	}
+	for i := range expected {
+		if dst[i] != expected[i] {
+			t.Errorf("array elements at %d are not equal", i)
+		}
+	}
+}

mergo.go

@@ -17,7 +17,7 @@ import (
 var (
 	ErrNilArguments                = errors.New("src and dst must not be nil")
 	ErrDifferentArgumentsTypes     = errors.New("src and dst must be of same type")
-	ErrNotSupported                = errors.New("only structs and maps are supported")
+	ErrNotSupported                = errors.New("only structs, maps, and slices are supported")
 	ErrExpectedMapAsDestination    = errors.New("dst was expected to be a map")
 	ErrExpectedStructAsDestination = errors.New("dst was expected to be a struct")
 	ErrNonPointerAgument           = errors.New("dst must be a pointer")
@@ -65,7 +65,7 @@ func resolveValues(dst, src interface{}) (vDst, vSrc reflect.Value, err error) {
 		return
 	}
 	vDst = reflect.ValueOf(dst).Elem()
-	if vDst.Kind() != reflect.Struct && vDst.Kind() != reflect.Map {
+	if vDst.Kind() != reflect.Struct && vDst.Kind() != reflect.Map && vDst.Kind() != reflect.Slice {
 		err = ErrNotSupported
 		return
 	}

mergo_test.go

@@ -930,11 +930,11 @@ func TestMergeMapWithInnerSliceOfDifferentType(t *testing.T) {
 	}
 }
 
-func TestMergeSlicesIsNotSupported(t *testing.T) {
+func TestMergeDifferentSlicesIsNotSupported(t *testing.T) {
 	src := []string{"a", "b"}
 	dst := []int{1, 2}
 
-	if err := mergo.Merge(&src, &dst, mergo.WithOverride, mergo.WithAppendSlice); err != mergo.ErrNotSupported {
+	if err := mergo.Merge(&src, &dst, mergo.WithOverride, mergo.WithAppendSlice); err != mergo.ErrDifferentArgumentsTypes {
 		t.Errorf("expected %q, got %q", mergo.ErrNotSupported, err)
 	}
 }