#else #if #if-statement #when

no-std kiam

Better syntax for if/else chains (when!)

2 releases

0.1.1 Apr 13, 2022
0.1.0 Feb 8, 2021

#168 in No standard library

MIT license

115KB
84 lines

kiam

CI status crates.io documentation (docs.rs) documentation (master) LICENSE

("kiam" is "when" in Esperanto)

This crate introduces when! macro which provides better syntax for if/else if/else chains. The syntax is similar to match.

(idea is borrowed from kotlin)

[dependencies] 
kiam = "0.1"

Usage

Usage is similar to the usage of match, but instead of patterns, branches are guarded by boolean expression:

kiam::when! {
    false => (),
    true => (),
    // ...
}

_ can be used as a default branch (it's also required to use when! in expression possition):

let x = kiam::when! {
    false => 0,
    _ => 1,
};

assert_eq!(x, 1);

You can also use let <pat> = to match a pattern, but in difference with match you'll have to provide an expression for every pattern:

let a = None;
let b = Some(17);
let fallible = || Err(());

let x = kiam::when! {
    let Some(x) = a => x,
    let Ok(x) = fallible() => x,
    let Some(x) = b => (x as u32) + 1,
    _ => 1,
};

assert_eq!(x, 18);

Last notes:

  • You can also compare structure litetals without brackets (you can't do this with if/else if/else chain)
  • You can mixup boolean-braches with pattern matching
  • Only one branch is executed (not to be confused with switch in C-like languages)
let mut x = 0;

kiam::when! {
    let Ok(_) = Err::<(), _>(()) => x = 1,
    true => x = 2,
    true => x = 3,
    let Some(n) = Some(42) => x = n,
};

assert_eq!(x, 2);
#[derive(PartialEq)]
struct Struct { a: i32 }

// This does not compile because of the ambiguity
if Struct { a: 0 } == Struct { a: 0 } {
    // ...
}
#[derive(PartialEq)]
struct Struct { a: i32 }

kiam::when! {
    // This, on the other hand, compiles fine
    Struct { a: 0 } == Struct { a: 0 } => {
        // ...
    },
}

No runtime deps