Generic Method Parameters in Golang

I need help with making this work for any type.

I have got a function I need to accept other types that have ID property.

I have tried using interfaces but that did not work for my ID property case. Here is the code:

package main


import (
  "fmt"
  "strconv"
  )

type Mammal struct{
  ID int
  Name string 
}

type Human struct {  
  ID int
  Name string 
  HairColor string
}

func Count(ms []Mammal) *[]string { // How can i get this function to accept any type not just []Mammal
   IDs := make([]string, len(ms))
   for i, m := range ms {
     IDs[i] = strconv.Itoa(int(m.ID))
   }
   return &IDs
}

func main(){
  mammals := []Mammal{
    Mammal{1, "Carnivorious"},
    Mammal{2, "Ominivorious"},
  }
  
  humans := []Human{
    Human{ID:1, Name: "Peter", HairColor: "Black"},
    Human{ID:2, Name: "Paul", HairColor: "Red"},
  } 
  numberOfMammalIDs := Count(mammals)
  numberOfHumanIDs := Count(humans)
  fmt.Println(numberOfMammalIDs)
  fmt.Println(numberOfHumanIDs)
}

I get this

> error prog.go:39: cannot use humans (type []Human) as type []Mammal in argument to Count

See Go Playground for more details here http://play.golang.org/p/xzWgjkzcmH

Use interfaces instead of concrete types, and use embedded interfaces so the common methods do not have to be listed in both types:

type Mammal interface {
	GetID() int
	GetName() string
}

type Human interface {
	Mammal

	GetHairColor() string
}

And here is the implementation of these interfaces based on your code which uses embedded types (structs):

type MammalImpl struct {
	ID   int
	Name string
}

func (m MammalImpl) GetID() int {
	return m.ID
}

func (m MammalImpl) GetName() string {
	return m.Name
}

type HumanImpl struct {
	MammalImpl
	HairColor string
}

func (h HumanImpl) GetHairColor() string {
	return h.HairColor
}

But then of course in your Count() function you can only refer to the method and not the field of the implementation:

IDs[i] = strconv.Itoa(m.GetID())  // Access ID via the method: GetID()

And creating your slices of Mammals and Humans:

mammals := []Mammal{
	MammalImpl{1, "Carnivorious"},
	MammalImpl{2, "Ominivorious"},
}

humans := []Mammal{
	HumanImpl{MammalImpl: MammalImpl{ID: 1, Name: "Peter"}, HairColor: "Black"},
	HumanImpl{MammalImpl: MammalImpl{ID: 2, Name: "Paul"}, HairColor: "Red"},
}

Here is the complete working code on Go Playground.

You can't do precisely what you ask for in Go. Closest way to do things in Go would be something like this as shown in code below:

type Ids interface{
  Id() int
}

func (this Mammal) Id() int{
  return this.ID
} 

func (this Human) Id() int{
  return this.ID
} 


func Count(ms []Ids) *[]string {
...
    IDs[i] = strconv.Itoa(int(m.Id()))
...
}

func main(){
  mammals := []Ids{
    Mammal{1, "Carnivorious"},
    Mammal{2, "Ominivorious"},
  }

  humans := []Ids{
    Human{ID:1, Name: "Peter", HairColor: "Black"},
    Human{ID:2, Name: "Paul", HairColor: "Red"},
  }
  ...
}  

here is the working example