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_v2()
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) => match have_ingredients(food) { None => None, Some(food) => Some(food), }, } } // This can conveniently be rewritten more compactly with `and_then()`: fn cookable_v2(food: Food) -> Option<Food> { have_recipe(food).and_then(have_ingredients) } fn eat(food: Food, day: Day) { match cookable_v2(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); }
See also:
closures, Option
, and Option::and_then()