Functional options - Sharing options between different types

Question

When implementing functional options for different types to enable easier configuration, I want to share some common options between these types.

Considering the following simple example (where name represents a common complex shared type and id and location represent differences in the types):

type A struct{
  Name string
  ID   int
}

type OptionFuncA func(*A)

type B struct{
  Name string
  Location string
}

type OptionFuncB func(*B)

func NewA(options ...OptionFuncA) A {
  obj := A{}

  for _, option := range options {
      option(&obj)
  }

  return obj
}

func NewB(options ...OptionFuncB) B {
  obj := B{}

  for _, option := range options {
      option(&obj)
  }

  return obj
}

In this example I want to add functional options to set the name of types A and B. I can do it like that:

type OptionA struct{}
type OptionB struct{}

func (opt OptionA) WithName(name string) OptionFuncA {
  return func(a *A) {
      a.Name = name
  }
}

func (opt OptionB) WithName(name string) OptionFuncB {
  return func(b *B) {
      b.Name = name
  }
}

func (opt OptionA) WithID(id int) OptionFuncA {
  return func(a *A) {
      a.ID = id
  }
}

func (opt OptionB) WithLocation(location string) OptionFuncB {
  return func(b *B) {
      b.Location = location
  }
}

This allows me to use the code like this:

optA := OptionA{}
a := NewA(optA.WithName("A"), optA.WithID(1))

optB := OptionB{}
b := NewB(optB.WithName("B"), optB.WithLocation("location"))

I want to have type checking on the different options for type A and B, while avoiding duplication of code which is equal for different types. Since functional options require to return the fitting function type OptionFuncA and OptionFuncB for type A and B respectively, I am wondering whether a solution exists.

Alternatively, I want to define a common interface, which different OptionX types have to implement:

// Assumption: Option is the common interface
// with options for multiple types.
var _ Option = OptionA{}
var _ Option = OptionB{}

While the actual code for setting the name in this example obviously can be shared, I cannot find a solution to define a common interface for the functional options of both types. The different required return value for the functional options does not allow it from my pov.

Is it somehow possible to enforce that OptionA and OptionB have to implement some shared options, without loosing type safety for all other available options of a specific type?

Answer 1

If that fits your design, you can create semantic building blocks with embeddable structs. Then unified option could be implemented as a collection of options for those embeddings.

Such approach leverages composition and improves code consistency by establishing a single owner of domain data (e.g. WithName is a single owner of name data as opposed to A and B had their own Name which were not syntactically related).

https://play.golang.org/p/DZMbqXVVurs

package main

import "fmt"

type WithName struct {
    FirstName string
    LastName  string
}

type WithAge struct{ Age int }
type WithID struct{ ID int }
type WithLocation struct{ Location string }

type Option struct {
    WithNameFunc     func(v *WithName)
    WithAgeFunc      func(v *WithAge)
    WithIDFunc       func(v *WithID)
    WithLocationFunc func(v *WithLocation)
}

func Name(first, last string) Option {
    return Option{
        WithNameFunc: func(v *WithName) {
            v.FirstName = first
            v.LastName = last
        },
    }
}

func ID(id int) Option {
    return Option{
        WithIDFunc: func(v *WithID) {
            v.ID = id
        },
    }
}

type A struct {
    WithName
    WithID
}

type B struct {
    WithName
    WithAge
    WithLocation
}

func NewA(options ...Option) A {
    obj := A{}

    for _, option := range options {
        if option.WithNameFunc != nil {
            option.WithNameFunc(&obj.WithName)
        }
        if option.WithIDFunc != nil {
            option.WithIDFunc(&obj.WithID)
        }
    }

    return obj
}

func NewB(options ...Option) B {
    obj := B{}

    for _, option := range options {
        if option.WithNameFunc != nil {
            option.WithNameFunc(&obj.WithName)
        }
        if option.WithAgeFunc != nil {
            option.WithAgeFunc(&obj.WithAge)
        }
        if option.WithLocationFunc != nil {
            option.WithLocationFunc(&obj.WithLocation)
        }
    }

    return obj
}

func main() {
    a :=  NewA(Name("John", "Doe"), ID(123))
    fmt.Println(a.FirstName, a.LastName, a.ID) // John Doe 123
}