Easy Rust

The todo! macro

Sometimes you want to write code in general to help you imagine your project. For example, imagine a simple project to do something with books. Here's what you think as you write it:

struct Book {} // Okay, first I need a book struct.
               // Nothing in there yet - will add later

enum BookType { // A book can be hardcover or softcover, so add an enum
    HardCover,
    SoftCover,
}

fn get_book(book: &Book) -> Option<String> {} // ⚠️ get_book should take a &Book and return an Option<String>

fn delete_book(book: Book) -> Result<(), String> {} // delete_book should take a Book and return a Result...
                                                    // TODO: impl block and make these functions methods...
fn check_book_type(book_type: &BookType) { // Let's make sure the match statement works
    match book_type {
        BookType::HardCover => println!("It's hardcover"),
        BookType::SoftCover => println!("It's softcover"),
    }
}

fn main() {
    let book_type = BookType::HardCover;
    check_book_type(&book_type); // Okay, let's check this function!
}

But Rust is not happy with get_book and delete_book. It says:

error[E0308]: mismatched types
  --> src\main.rs:32:29
   |
32 | fn get_book(book: &Book) -> Option<String> {}
   |    --------                 ^^^^^^^^^^^^^^ expected enum `std::option::Option`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:   expected enum `std::option::Option<std::string::String>`
           found unit type `()`

error[E0308]: mismatched types
  --> src\main.rs:34:31
   |
34 | fn delete_book(book: Book) -> Result<(), String> {}
   |    -----------                ^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:   expected enum `std::result::Result<(), std::string::String>`
           found unit type `()`

But you don't care about get_book and delete_book right now. This is where you can use todo!(). If you add that to the function, Rust will not complain, and will compile.

struct Book {}

fn get_book(book: &Book) -> Option<String> {
    todo!() // todo means "I will do it later, please be quiet"
}

fn delete_book(book: Book) -> Result<(), String> {
    todo!()
}

fn main() {}

So now the code compiles and you can see the result of check_book_type: It's hardcover.

But careful, because it only compiles - you can't use the function. If you call a function with todo!() inside it, it will panic.

Also, todo!() functions still need real input and output types. If you just write this, it will not compile:

struct Book {}

fn get_book(book: &Book) -> WorldsBestType { // ⚠️
    todo!()
}

fn main() {}

It will say:

error[E0412]: cannot find type `WorldsBestType` in this scope
  --> src\main.rs:32:29
   |
32 | fn get_book(book: &Book) -> WorldsBestType {
   |                             ^^^^^^^^^^^^^^ not found in this scope

todo!() is actually the same as another macro: unimplemented!(). Programmers were using unimplemented!() a lot but it was long to type, so they created todo!() which is shorter.