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.

Problem Statement

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);

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.


Peter Goodspeed-Niklaus
Last updated 22 October 2021
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