Combinators: and_then
map()
was described as a chainable way to simplify match
statements. However, using map()
on a function that returns an Option<T>
results in the nested Option<Option<T>>
. Chaining multiple calls together can then become confusing. That's where another combinator called and_then()
, known in some languages as flatmap, comes in.
and_then()
calls its function input with the wrapped value and returns the result. If the Option
is None
, then it returns None
instead.
In the following example, cookable_v3()
results in an Option<Food>
. Using map()
instead of and_then()
would have given an Option<Option<Food>>
, which is an invalid type for eat()
.
#![allow(dead_code)]
#[derive(Debug)] enum Food { CordonBleu, Steak, Sushi }
#[derive(Debug)] enum Day { Monday, Tuesday, Wednesday }
// We don't have the ingredients to make Sushi.
fn have_ingredients(food: Food) -> Option<Food> {
match food {
Food::Sushi => None,
_ => Some(food),
}
}
// We have the recipe for everything except Cordon Bleu.
fn have_recipe(food: Food) -> Option<Food> {
match food {
Food::CordonBleu => None,
_ => Some(food),
}
}
// To make a dish, we need both the recipe and the ingredients.
// We can represent the logic with a chain of `match`es:
fn cookable_v1(food: Food) -> Option<Food> {
match have_recipe(food) {
None => None,
Some(food) => have_ingredients(food),
}
}
// This can conveniently be rewritten more compactly with `and_then()`:
fn cookable_v3(food: Food) -> Option<Food> {
have_recipe(food).and_then(have_ingredients)
}
// Otherwise we'd need to `flatten()` an `Option<Option<Food>>`
// to get an `Option<Food>`:
fn cookable_v2(food: Food) -> Option<Food> {
have_recipe(food).map(have_ingredients).flatten()
}
fn eat(food: Food, day: Day) {
match cookable_v3(food) {
Some(food) => println!("Yay! On {:?} we get to eat {:?}.", day, food),
None => println!("Oh no. We don't get to eat on {:?}?", day),
}
}
fn main() {
let (cordon_bleu, steak, sushi) = (Food::CordonBleu, Food::Steak, Food::Sushi);
eat(cordon_bleu, Day::Monday);
eat(steak, Day::Tuesday);
eat(sushi, Day::Wednesday);
}