Ranges and in

Ranges

A range is a run of consecutive integers:

If b is not greater than a (or, for ..=, less than a), the range is empty. Ranges only count upward. For other steps, use std.iter’s range.

A range is mostly used as the source of a @for loop:

macro show(value: int) {
    @emit value
}

@for i in 0..3 {
    show i
}

@for i in 0..=3 {
    show i * 10
}

0 1 2 0 10 20 30

Inside a macro, a range can be stored in a constant and used later. A top-level @for needs its range written in place, because it’s unrolled before anything else in the file is evaluated.

in

value in source is 1 if source contains value, and 0 otherwise. The source can be:

• A range. i in 0..len checks 0 <= i && i < len. Nothing is iterated, so it’s cheap even for a range with a trillion elements.
• A struct value, such as an array or a string. x in arr is true if x equals one of the elements @for would visit: the pub fields that aren’t skip, as described in Struct fields.

macro show(value: int) {
    @emit value
}

show 5 in 0..5
show 5 in 0..=5
show 1000 in 0..1_000_000_000_000
show 'b' in "abc"
show 'z' in "abc"

0 1 1 1 0

in is often used in an @assert:

macro checked_index(idx: int, len: int) {
    @assert idx in 0..len, "index out of bounds"
    @emit idx
}

checked_index 3, 4

3

macro checked_index(idx: int, len: int) {
    @assert idx in 0..len, "index out of bounds"
    @emit idx
}

checked_index 4, 4

index out of bounds

in binds as tightly as <, and a range binds more loosely than anything else, so i + 1 in 0..n + 1 means (i + 1) in 0..(n + 1).

in isn’t for types

x in SomeEnum doesn’t test whether x is a valid variant. in always asks whether a value you have contains something, the same question @for walks through. Asking whether a value belongs to a type is a different question, so it doesn’t share the keyword.