map-macro

Declarative macros for initializing collections from the rust standard library.

The map! macro allows for statically initializing a hash map. set! is does the same, only for hash sets. Both macros have an equivalent version using a b-tree data structure, btree_map! and btree_set!.

The vec_no_clone is a more flexible version of the vec! macro the standard library provides. It allows you to create vectors with the vec![some_value; count] syntax, without cloning some_value.

This crate has zero dependencies.

Table of Contents

• Maps
  • Explicitly typed values for trait objects
• Sets
• B-tree based maps and sets
• Vectors without cloning

Maps

Some languages provide neat syntactic sugar for creating non-empty maps/dictionaries. For example, in python you can create a non-empty map by running the following code:

hello = {
  "en": "Hello",
  "de": "Hallo",
  "fr": "Bonjour",
  "es": "Hola",
}

In rust, creating a non-empty hash map is not as straight-forward:

use std::collections::HashMap;

let mut hello = HashMap::new();

hello.insert("en", "Hello");
hello.insert("de", "Hallo");
hello.insert("fr", "Bonjour");
hello.insert("es", "Hola");

More less-readable boilerplate code is needed. Even worse, hello must be declared as mutable, even if we do not want it to be mutable after we have added our four entries. The map-macro crate offers a better way of declaring non-empty maps using the map! macro. Creating the same hello map from the example above can be simplified to:

use map_macro::map;

let hello = map! {
  "en" => "Hello",
  "de" => "Hallo",
  "fr" => "Bonjour",
  "es" => "Hola",
};

That's it. Looks nearly as neat as the python version, with the added benefit that hello is not mutable after we created it.

The map! macro is powerful enough to create maps from non-static keys and values as well, you are not limited to literals:

use map_macro::map;

fn hello_in_french() -> &'static str {
  "Bonjour"
}

fn spanish_language_code() -> &'static str {
  "es"
}

let hello = map! {
  "en" => "Hello",
  "de" => "Hallo",
  "fr" => hello_in_french(),
  spanish_language_code() => "Hola",
};

Explicitly typed values for trait objects

As shown in the examples above, rust uses type inference to infer the correct type for the created hash map. Unfortunately, type inference alone can not detect trait objects. This will not work, because rustc is unable to figure out the right type when creating hello:

use std::collections::HashMap;
use std::fmt::Debug;

use map_macro::map;

let hello: HashMap<&str, &dyn Debug> = map! {
  "en" => &"Hello",
  "de" => &"Hallo",
  "fr" => &"Bonjour",
  "es" => &"Hola",
};

The map_e! macro enables you to use trait objects as values through type coercion, making the example above compile successfully:

use std::collections::HashMap;
use std::fmt::Debug;

use map_macro::map_e;

let hello: HashMap<&str, &dyn Debug> = map_e! {
  "en" => &"Hello",
  "de" => &"Hallo",
  "fr" => &"Bonjour",
  "es" => &"Hola",
};

Note that you need to give an explicit type to the binding when you use map_e!, because it relies on knowing what type it should coerce the values to. Also, only values and not keys can be trait objects, because keys must implement the Hash trait, which is not object save.

btree_map_e! is the equivalent to map_e! for creating a b-tree map with trait object values:

use std::collections::BTreeMap;
use std::fmt::Debug;

use map_macro::btree_map_e;

let hello: BTreeMap<&str, &dyn Debug> = btree_map_e! {
  "en" => &"Hello",
  "de" => &"Hallo",
  "fr" => &"Bonjour",
  "es" => &"Hola",
};

Sets

Rust has the same cumbersome creation process for creating sets.

In python you can create a set like this:

x = set([1, 2, 3])

Not as neat as a map, but still quite concise. Dart even comes with syntactic sugar for creating a set:

final x = {1, 2, 3};

In rust, you would have to write:

use std::collections::HashSet;

let mut x = HashSet::new();

x.insert(1);
x.insert(2);
x.insert(3);

The set! macro provided by the map-macro crate lets you write the same code as:

use map_macro::set;

let x = set! { 1, 2, 3 };

Again, nearly as neat as dart!

The set! macro is as powerful as the map! macro:

use map_macro::set;

fn one() -> i32 {
  1
}

let x = set! { one(), 2, 3 };

use std::collections::HashSet;
use map_macro::set;

let x: HashSet<i32> = set! {};

assert_eq!(x.len(), 0);

B-tree based maps and sets

Besides hashtable-based maps and sets, rust's standard library offers maps and sets based on the b-tree data structure. They offer similar functionality to their hashtable-based counterparts. map-macro provides the btree_map! and btree_set! macros to statically initialize the b-tree-based maps and sets. They work exactly like the map! and set! macros:

use map_macro::{btree_map, btree_set};

let hello = btree_map! {
  "en" => "Hello",
  "de" => "Hallo",
  "fr" => "Bonjour",
  "es" => "Hola",
};

assert_eq!(hello["en"], "Hello");

let x = btree_set! { 1, 2, 3 };

assert!(x.contains(&1));

assert!(!x.contains(&4))

Vectors without cloning

When using the vec![some_value; count] syntax, the type of some_value has to implement the Clone trait, because some_value is cloned count - 1 times into all the vector elements, except the first one.

This could either be undesired behavior (you don't want clones of some_value, because its type implements Clone in a way that doesn't fit your needs) or the type you wish to pre-populate your vector with doesn't implement Clone.

For example, this will result in a panic during compile time:

struct UnclonableWrapper(u8);

// panics
let x = vec![UnclonableWrapper(0); 5];

The vec_no_clone! macro takes a different approach. Instead of cloning UnclonableWrapper(0), it treats it as an expression which is called 5 times in this case. So 5 independent UnclonableWrapper objects, each with its own location in memory, are created:

use map_macro::vec_no_clone;

struct UnclonableWrapper(u8);

let x = vec_no_clone![UnclonableWrapper(0); 5];

assert_eq!(x.len(), 5);

Without vec_no_clone! you'd have to write something far less readable and more complex to reason about like this to create the same vector:

struct UnclonableWrapper(u8);

let x: Vec<UnclonableWrapper> = (0..5)
  .map(|_| UnclonableWrapper(0))
  .collect();

assert_eq!(x.len(), 5);

vec_no_clone! is not only useful for types not implementing Clone, but also for types where cloning them is not what you want. The best example would be a reference counted pointer, std::rc::Rc. When you clone an Rc instance, a new smart pointer instance referencing the same location in memory is created. If you'd rather have multiple independent reference counted pointers to different memory locations, you can use vec_no_clone! as well:

use map_macro::vec_no_clone;

use std::cell::RefCell;
use std::rc::Rc;

// simply clones the reference counted pointer for each element that 
// is not the first
let shared_vec = vec![Rc::new(RefCell::new(0)); 2];
{
  let mut first = shared_vec[0].borrow_mut();
  *first += 1;
}

assert_eq!(*shared_vec[0].borrow(), 1);

// the second element is a clone of the reference counted pointer at 
// the first element of the vector, referencing the same address in
// memory, therefore being mutated as well
assert_eq!(*shared_vec[1].borrow(), 1);

// the `vec_no_clone!` macro does not clone the object created by the
// first expression but instead calls the expression for each element 
// in the vector, creating two independent objects, each with their 
// own address in memory
let unshared_vec = vec_no_clone![Rc::new(RefCell::new(0)); 2];

{
  let mut first = unshared_vec[0].borrow_mut();
  *first += 1;
}

assert_eq!(*unshared_vec[0].borrow(), 1);

// the second element is not the same cloned reference counted
// pointer as it would be if it were constructed with the `vec!` macro
// from the standard library like it was above, therefore it is not
// mutated
assert_eq!(*unshared_vec[1].borrow(), 0);

Note that vec_no_clone! treats the value as an expression, so you must provide the initialization as input directly. This, for example, won't work:

use map_macro::vec_no_clone;

struct UnclonableWrapper(u8);

let a = UnclonableWrapper(0);

// a will have moved into the first element of x, raising a compile
// time error for the second element.
let x = vec_no_clone![a; 5];

You can also use the macro with a list of elements, like vec!:

use map_macro::vec_no_clone;

let v1 = vec_no_clone![0, 1, 2, 3];
let v2 = vec![0, 1, 2, 3];

assert_eq!(v1, v2);

let v1: Vec<u8> = vec_no_clone![];
let v2: Vec<u8> = vec![];

assert_eq!(v1, v2);