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< title > The todo! macro - Easy Rust< / title >
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< h2 > < a class = "header" href = "#the-todo-macro" id = "the-todo-macro" > The todo! macro< / a > < / h2 >
< p > 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:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > 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,
}
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fn get_book(book: & Book) -> Option< String> {} // ⚠️ get_book should take a & Book and return an Option< String>
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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" ),
}
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}
fn main() {
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let book_type = BookType::HardCover;
check_book_type(& book_type); // Okay, let's check this function!
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}
< / code > < / pre > < / pre >
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< p > But Rust is not happy with < code > get_book< / code > and < code > delete_book< / code > . It says:< / p >
< pre > < code class = "language-text" > 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 `()`
< / code > < / pre >
< p > But you don't care about < code > get_book< / code > and < code > delete_book< / code > right now. This is where you can use < code > todo!()< / code > . If you add that to the function, Rust will not complain, and will compile.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > struct Book {}
fn get_book(book: & Book) -> Option< String> {
todo!() // todo means " I will do it later, please be quiet"
}
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fn delete_book(book: Book) -> Result< (), String> {
todo!()
}
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fn main() {}
< / code > < / pre > < / pre >
< p > So now the code compiles and you can see the result of < code > check_book_type< / code > : < code > It's hardcover< / code > .< / p >
< p > But careful, because it only compiles - you can't use the function. If you call a function with < code > todo!()< / code > inside it, it will panic.< / p >
< p > Also, < code > todo!()< / code > functions still need real input and output types. If you just write this, it will not compile:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > struct Book {}
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fn get_book(book: & Book) -> WorldsBestType { // ⚠️
todo!()
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}
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fn main() {}
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< / code > < / pre > < / pre >
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< p > It will say:< / p >
< pre > < code class = "language-text" > 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
< / code > < / pre >
< p > < code > todo!()< / code > is actually the same as another macro: < code > unimplemented!()< / code > . Programmers were using < code > unimplemented!()< / code > a lot but it was long to type, so they created < code > todo!()< / code > which is shorter.< / p >
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