Generic Method Parameters in Golang

Question

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

Answer 1

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.