Functional Options Pattern (FOP) is a Go idiom for giving a function optional parameters with sensible defaults. Go has no keyword-level support for default parameters, so we can do that behavior using a first-class function.
What we want to achieve
I usually write TypeScript, so here’s a quick example for comparison.
type Curry = {
rice: string;
powder: string;
spice?: string
};
function createCurry(
rice: string,
powder: string,
spice?: string
) {
return {
rice,
powder,
spice,
};
}
const curry1 = createCurry("あきたこまち", "ジャワカレー");
console.log({ curry1 });
const curry2 = createCurry("コシヒカリ", "バーモントカレー", "クミン");
console.log({ curry2 });Example
If we translate the same example into Go, we can use FOP like this.
type Curry struct {
rice string
powder string
spice *string
}
type Option func(*Curry)
func NewCurry(rice string, powder string, opts ...Option) *Curry {
curry := &Curry{
rice: rice,
powder: powder,
}
for _, opt := range opts {
opt(curry)
}
return curry
}
func WithSpice(spice string) func(*Curry) {
return func(c *Curry) {
s := spice
c.spice = &s
}
}
func main() {
curry1 := NewCurry("あきたこまち", "ジャワカレー")
fmt.Printf("curry1: %v", curry1)
curry2 := NewCurry("コシヒカリ", "バーモントカレー", WithSpice("クミン"))
fmt.Printf("curry2: %v", curry2)
}