Golang polymorphic parameters and returns - go

Question

Say I have functions:
func ToModelList(cats *[]*Cat) *[]*CatModel {
list := *cats
newModelList := []*CatModel{}
for i := range list {
obj := obj[i]
newModelList = append(newModelList, obj.ToModel())
}
return &newModelList
}
func ToModelList(dogs *[]*Dog) *[]*DogModel {
list := *dogs
newModelList := []*DogModel{}
for i := range list {
obj := obj[i]
newModelList = append(newModelList, obj.ToModel())
}
return &newModelList
}
Is there a way to combine those two so I can do something like
func ToModelList(objs *[]*interface{}) *[]*interface{} {
list := *objs
// figure out what type struct type objs/list are
newModelList := []*interface{}
// type cast newModelList to the correct array struct type
for i := range list {
obj := obj[i]
// type cast obj based on objs's type
newModelList = append(newModelList, obj.ToModel())
}
return &newModelList
}

Answer 1

First, slices are already a reference, unless you need to change the slice itself, you do not need to pass it as a pointer.
Second, an interface{} can be regardless an object or a pointer to an object. You do not need to have *interface{}.
I am not sure what you are trying to achieve but you could do something like this:
package main
// Interface for Cat, Dog
type Object interface {
ToModel() Model
}
// Interface for CatModel, DogModel
type Model interface {
Name() string
}
type Cat struct {
name string
}
func (c *Cat) ToModel() Model {
return &CatModel{
cat: c,
}
}
type CatModel struct {
cat *Cat
}
func (c *CatModel) Name() string {
return c.cat.name
}
type Dog struct {
name string
}
func (d *Dog) ToModel() Model {
return &DogModel{
dog: d,
}
}
type DogModel struct {
dog *Dog
}
func (d *DogModel) Name() string {
return d.dog.name
}
func ToModelList(objs []Object) []Model {
newModelList := []Model{}
for _, obj := range objs {
newModelList = append(newModelList, obj.ToModel())
}
return newModelList
}
func main() {
cats := []Object{
&Cat{name: "felix"},
&Cat{name: "leo"},
&Dog{name: "octave"},
}
modelList := ToModelList(cats)
for _, model := range modelList {
println(model.Name())
}
}
You define interfaces for your Cat, Dogs etc and for your Model. Then you implement them as you want and it is pretty straight forward to do ToModelList().

Answer 2

you can make *CatModel and *DogModel both implement type PetModel {} interface, and just return []Pet in function signature.
func (cats []*Cat) []PetModel {
...
return []*CatModel {...}
}
func (dogs []*Dog) []PetModel {
...
return []*DogModel {...}
}
BTW: return a pointer of a slice in golang is useless.

Answer 3

If you strip away redundant assignments, and unnecessary pointers-to-slices, you'll find you have little code left, and duplicating it for each of your model types doesn't look so bad.
func CatsToCatModels(cats []*Cat) []*CatModel {
var result []*CatModel
for _, cat := range cats {
result = append(result, cat.ToModel())
}
return result
}
Unless this code is used in a lot of places I'd also consider just inlining it, since it's trivial code and only 4 lines when inlined.
Yes, you can replace all the types with interface{} and make the code generic, but I don't think it's a good tradeoff here.