rustlings/exercises/smart_pointers/box1.rs

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// box1.rs
//
// At compile time, Rust needs to know how much space a type takes up. This becomes problematic
// for recursive types, where a value can have as part of itself another value of the same type.
// To get around the issue, we can use a `Box` - a smart pointer used to store data on the heap,
// which also allows us to wrap a recursive type.
//
// The recursive type we're implementing in this exercise is the `cons list` - a data structure
// frequently found in functional programming languages. Each item in a cons list contains two
// elements: the value of the current item and the next item. The last item is a value called `Nil`.
//
// Step 1: use a `Box` in the enum definition to make the code compile
// Step 2: create both empty and non-empty cons lists by replacing `todo!()`
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//
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// Note: the tests should not be changed
//
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// Execute `rustlings hint box1` or use the `hint` watch subcommand for a hint.
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#[derive(PartialEq, Debug)]
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pub enum List {
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Cons(i32, Box<List>),
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Nil,
}
fn main() {
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println!("This is an empty cons list: {:?}", create_empty_list());
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println!(
"This is a non-empty cons list: {:?}",
create_non_empty_list()
);
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}
pub fn create_empty_list() -> List {
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List::Nil
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}
pub fn create_non_empty_list() -> List {
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List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))))
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}
#[cfg(test)]
mod tests {
use super::*;
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#[test]
fn test_create_empty_list() {
assert_eq!(List::Nil, create_empty_list())
}
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#[test]
fn test_create_non_empty_list() {
assert_ne!(create_empty_list(), create_non_empty_list())
}
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}