Python-style collection comprehensions in Rust

The structure is consistent with Python's list and dictionary comprehensions.

• If the loop body (everything before the first for) contains two expressions separated by :, it will be treated as a hash map comprehension.

• If the loop body ends with a ;, it will be a statement comprehension (it won't evaluate to a collection).

• Otherwise it's a vector comprehension.

Comprehensions consist of a body followed by a for ... in ... expression, followed by any combination of for ... in ... or if ... expressions.

The for ... in ... and if ... expressions are nested left-to-right, so

c!(do_something(x, y); for x in 0..10 if x % 2 == 1 for y in 'a'..='z');

is equivalent to:

for x in 0..10 {
    if x % 2 == 1 {
        for y in 'a'..='z' {
            do_something(x, y);
        }
    }
}

Examples

Vectors

• A simple vector comprehension:

let v = c![x * (1 + x) for x in 1..=10];
println!("{:?}", v);

[2, 6, 12, 20, 30, 42, 56, 72, 90, 110]

• Multiple iterators and conditionals:

let v = c![x * y for x in 1..=10 if x % 2 != 0 for y in -2..=2 if x > y];
println!("{:?}", v);

[-2, -1, 0, -6, -3, 0, 3, 6, -10, -5, 0, 5, 10, -14, -7, 0, 7, 14, -18, -9, 0, 9, 18]

Hash Maps

• A simple hash map comprehension:

let m = c!{x: x * (1 + x) for x in 1..=10};
println!("{:?}", m);

{2: 6, 5: 30, 8: 72, 4: 20, 3: 12, 6: 42, 1: 2, 7: 56, 9: 90, 10: 110}

• Complex hash map comprehension:

let m = c!{format!("[{}|{}]", x, y): y.to_string().repeat(x) for x in 1..=3 for y in 'A'..='C'};
println!("{:?}", m);

{"[3|C]": "CCC", "[3|B]": "BBB", "[3|A]": "AAA", "[2|B]": "BB", "[1|C]": "C", "[1|A]": "A", "[1|B]": "B", "[2|C]": "CC", "[2|A]": "AA"}

Statements

• A simple statement comprehension:

let mut n = 1;
c!(n *= x; for x in 1..=10);
println!("{}", n);

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