sjansen/mecha

package main

import "fmt"

func main() {
    base := NewFrobnicator().
        WithA(3).
        WithB(4)
    f1 := base.
        WithAdd().
        Build()
    f2 := base.
        WithMultiply().
        Build()
    fmt.Printf("f1(1)=%-3d f1(2)=%-3d\n", f1.Frob(1), f1.Frob(2))
    fmt.Printf("f2(1)=%-3d f2(2)=%-3d\n", f2.Frob(1), f2.Frob(2))
}

type Frobnicator struct {
    a, b int
    frob func(int, int) int
}

func (f *Frobnicator) Frob(c int) int {
    return f.frob(
        f.frob(c, f.a),
        f.frob(c, f.b),
    )
}

func NewFrobnicator() Builder {
    return func(f *Frobnicator) *Frobnicator {
        return f
    }
}

func (b Builder) Build() *Frobnicator {
    return b(&Frobnicator{})
}

type Builder func(f *Frobnicator) *Frobnicator

func (b Builder) WithA(x int) Builder {
    return func(f *Frobnicator) *Frobnicator {
        b(f).a = x
        return f
    }
}

func (b Builder) WithB(x int) Builder {
    return func(f *Frobnicator) *Frobnicator {
        b(f).b = x
        return f
    }
}

func (b Builder) WithAdd() Builder {
    return func(f *Frobnicator) *Frobnicator {
        b(f).frob = func(a, b int) int {
            return a + b
        }
        return f
    }
}

func (b Builder) WithMultiply() Builder {
    return func(f *Frobnicator) *Frobnicator {
        b(f).frob = func(a, b int) int {
            return a * b
        }
        return f
    }
}