Macros are a powerful part of a Rust programmer's toolkit, and macros by example are a relatively simple way to access this power. Let's write one!
What is a macro? Wikipedia describes it thus:
A macro (short for "macroinstruction", from Greek μακρός 'long') in computer science is a rule or pattern that specifies how a certain input sequence (often a sequence of characters) should be mapped to a replacement output sequence (also often a sequence of characters) according to a defined procedure. The mapping process that instantiates (transforms) a macro use into a specific sequence is known as macro expansion.
Illuminating! But to be more concrete, macros are a special syntax which allows you to generate code at compile time. Macros can be used for compile-time calculation, but more often they're just another way to abstract your code. For example, you've probably already used println!() and vec![]. These each take an arbitrary number of arguments, so you can't express them as simple functions. On the other hand, they always expand to some amount of absolutely standard Rust code. If you're interested, you can use the cargo expand subcommand to view the results of macro expansion in your code.
For further information about macros in Rust, The Rust Book has a good chapter on them.
You can produce a Vec of arbitrary length inline by using the vec![] macro. However, Rust doesn't come with a way to produce a HashMap inline. Rectify this by writing a hashmap!() macro.
For example, a user of your library might write hashmap!('a' => 3, 'b' => 11, 'z' => 32). This should expand to the following code:
{
let mut hm = HashMap::new();
hm.insert('a', 3);
hm.insert('b', 11);
hm.insert('z', 32);
hm
}Note that the maplit crate provides a macro which perfectly solves this exercise. Please implement your own solution instead of using this crate; please make an attempt on your own before viewing its source.
Note that this exercise requires Rust 1.36 or later.
Refer to the exercism help page for Rust installation and learning resources.
Execute the tests with:
$ cargo testAll but the first test have been ignored. After you get the first test to
pass, open the tests source file which is located in the tests directory
and remove the #[ignore] flag from the next test and get the tests to pass
again. Each separate test is a function with #[test] flag above it.
Continue, until you pass every test.
If you wish to run all ignored tests without editing the tests source file, use:
$ cargo test -- --ignoredTo run a specific test, for example some_test, you can use:
$ cargo test some_testIf the specific test is ignored use:
$ cargo test some_test -- --ignoredTo learn more about Rust tests refer to the online test documentation
Make sure to read the Modules chapter if you haven't already, it will help you with organizing your files.
After you have solved the exercise, please consider using the additional utilities, described in the installation guide, to further refine your final solution.
To format your solution, inside the solution directory use
cargo fmtTo see, if your solution contains some common ineffective use cases, inside the solution directory use
cargo clippy --all-targetsGenerally you should submit all files in which you implemented your solution (src/lib.rs in most cases). If you are using any external crates, please consider submitting the Cargo.toml file. This will make the review process faster and clearer.
The exercism/rust repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!
If you want to know more about Exercism, take a look at the contribution guide.
Peter Goodspeed-Niklaus
It's possible to submit an incomplete solution so you can see how others have completed the exercise.