How to use builder pattern to construct a struct which dynamically implements interfaces

Question

I'm trying to use builder patterns (borrowed from Java) to allow structs to implement interfaces. For example, I would ideally like this code pattern:

package main

import "fmt"

type Oner interface {
    One() int
}

type Twoer interface {
    Two() int
}

func main() {
    s := NewObject().
        WithOne(1).
        Build()

    _, ok := s.(Oner)
    fmt.Println(ok) // Prints true

    _, ok = s.(Twoer)
    fmt.Println(ok) // Prints false

    t := NewObject().
        WithOne(1).
        WithTwo(2).
        Build()

    _, ok = t.(Oner)
    fmt.Println(ok) // Prints true

    _, ok = t.(Twoer)
    fmt.Println(ok) // Prints true
}

As you could see, the definition of the builder determines what interfaces s and t implement.

How would one write the function definition of the builder NewObject() so the Build() method returns a struct which can (possibly) implement a Oner and Twoer?

---

Edit:

Here's some clarification on how it's going to be used. I'm constructing a library barring certain structs from being passed into functions if they violate the type safety. For example:

type Oner interface {
    One() int
}

type OneAndTwoer interface {
    Oner

    Two() int
}

type Library interface {
    DoSomethingWithOner(Oner)
    DoSomethingWithOneAndTwoer(Twoer)
}

Though we can define a function which always constructs a OneAndTwoer, my constraints are whenever we construct a OneAndTwoer, this takes a lot longer time than just constructing a Oner

func NewOneAndTwoer() OneAndTwoer {
    // Do some really really complicated logic which takes a lot of time
}

func NewOner() Oner {
    // Do simple logic
}

You could imagine how if we have a Threer, Fourer, etc, this becomes extremely unwieldly, and we have to construct constructors for all possible permutations of attributes.

This is where builder patterns come in handy. Assuming the calculations for One, Two, etc are independent of each other, we can pick and choose which interface we want to create.

Answer 1

Here is a way to do it, though it feels very clunky.

package main

import (
  "fmt"
)

type FieldOner interface {
    FieldOne() int
}

type FieldTwoer interface {
    FieldTwo() int
}

Set up structs One and Two implementing FieldOner and FieldTwoer respectively.

type One struct {
    one int
}

func (f One) FieldOne() int {
    return f.one
}

type Two struct {
    two int
}

func (f Two) FieldTwo() int {
    return f.two
}

Create the FieldBuilder which can store both values and whether it has been given each value, plus WithFieldOne and WithFieldTwo.

type FieldBuilder struct {
    one int
    has_one bool
    two int
    has_two bool
}

func NewObject() FieldBuilder {
    return FieldBuilder{ has_one: false, has_two: false }
}

func (f FieldBuilder) WithFieldOne(one int) FieldBuilder {
    f.one = one
    f.has_one = true
    return f
}

func (f FieldBuilder) WithFieldTwo(two int) FieldBuilder {
    f.two = two
    f.has_two = true
    return f
}

Build might return One, Two, or a combination of One and Two. Since it can return multiple things which have nothing in common between them (a red flag) it returns an interface{}.

func (f FieldBuilder) Build() interface{} {
    switch {
    case f.has_one && f.has_two:
        return struct {
            One
            Two
        }{
            One{one: f.one}, Two{two: f.two},
        }
    case f.has_one:
        return One{ one: f.one }
    case f.has_two:
        return Two{ two: f.two }
    }
    panic("Should never be here")
}

Because Build returns an interface{} it's necessary to typecast the result in order to actually use it possibly defeating the whole point of the exercise.

func main() {
    s := NewObject().
        WithFieldOne(1).
        Build()

    s1, ok := s.(FieldOner)
    fmt.Println(s1.FieldOne())

    _, ok = s.(FieldTwoer)
    fmt.Println(ok) // Prints false

    t := NewObject().
        WithFieldOne(1).
        WithFieldTwo(2).
        Build()

    t1, ok := t.(FieldOner)
    fmt.Println(t1.FieldOne())

    t2, ok := t.(FieldTwoer)
    fmt.Println(t2.FieldTwo())
}

This does not scale particularly well. Two interfaces require three cases. Three will require six. Four will require ten. Five will need fifteen...