Const equivalents of std functions and const parsing.

Features

This crate provides:

• Const fn equivalents of standard library functions and methods.

• destructure macro to allow destructuring types in const without getting "cannot drop in const" errors.

• Compile-time parsing through the Parser type, and parser_method macro.

Examples

Parsing an enum

This example demonstrates how you can parse a simple enum from an environment variable, at compile-time.

use konst::{
    eq_str,
    option,
    result::unwrap_ctx,
};

#[derive(Debug, PartialEq)]
enum Direction {
    Forward,
    Backward,
    Left,
    Right,
}

impl Direction {
    const fn try_parse(input: &str) -> Result<Self, ParseDirectionError> {
        // As of Rust 1.65.0, string patterns don't work in const contexts
        match () {
            _ if eq_str(input, "forward") => Ok(Direction::Forward),
            _ if eq_str(input, "backward") => Ok(Direction::Backward),
            _ if eq_str(input, "left") => Ok(Direction::Left),
            _ if eq_str(input, "right") => Ok(Direction::Right),
            _ => Err(ParseDirectionError),
        }
    }
}

const CHOICE: &str = option::unwrap_or!(option_env!("chosen-direction"), "forward");

const DIRECTION: Direction = unwrap_ctx!(Direction::try_parse(CHOICE));

fn main() {
    match DIRECTION {
        Direction::Forward => assert_eq!(CHOICE, "forward"),
        Direction::Backward => assert_eq!(CHOICE, "backward"),
        Direction::Left => assert_eq!(CHOICE, "left"),
        Direction::Right => assert_eq!(CHOICE, "right"),
    }
}

#[derive(Debug, PartialEq)]
pub struct ParseDirectionError;

use std::fmt::{self, Display};

impl Display for ParseDirectionError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("Failed to parse a Direction")
    }
}

impl ParseDirectionError {
    const fn panic(&self) -> ! {
        panic!("failed to parse a Direction")
    }
}

Parsing CSV

This example demonstrates how CSV can be parsed into integers.

This example requires the "parsing" and "iter" features (both are enabled by default).

use konst::{
    primitive::parse_u64,
    result::unwrap_ctx,
    iter, string,
};

const CSV: &str = "3, 8, 13, 21, 34";

static PARSED: [u64; 5] = iter::collect_const!(u64 =>
    string::split(CSV, ","),
        map(string::trim),
        map(|s| unwrap_ctx!(parse_u64(s))),
);

assert_eq!(PARSED, [3, 8, 13, 21, 34]);

Parsing a struct

This example demonstrates how a key-value pair format can be parsed into a struct.

This requires the "parsing" feature (enabled by default).

use konst::{
    parsing::{Parser, ParseValueResult},
    eq_str,
    for_range, parser_method, try_, unwrap_ctx,
};

const PARSED: Struct = {
    // You can also parse strings from environment variables, or from an `include_str!(....)`
    let input = "\
        colors = red, blue, green, blue
        amount = 1000
        repeating = circle
        name = bob smith
    ";
    
    unwrap_ctx!(parse_struct(Parser::new(input))).0
};

fn main(){
    assert_eq!(
        PARSED,
        Struct{
            name: "bob smith",
            amount: 1000,
            repeating: Shape::Circle,
            colors: [Color::Red, Color::Blue, Color::Green, Color::Blue],
        }
    );
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Struct<'a> {
    pub name: &'a str,
    pub amount: usize,
    pub repeating: Shape,
    pub colors: [Color; 4],
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Shape {
    Circle,
    Square,
    Line,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum Color {
    Red,
    Blue,
    Green,
}

pub const fn parse_struct(mut parser: Parser<'_>) -> ParseValueResult<'_, Struct<'_>> {
    let mut name = "<none>";
    let mut amount = 0;
    let mut repeating = Shape::Circle;
    let mut colors = [Color::Red; 4];
    
    parser = parser.trim_end();
    if !parser.is_empty() {
        loop {
            let mut prev_parser = parser.trim_start();

            parser = try_!(parser.find_skip('='));

            parser_method!{prev_parser, strip_prefix;
                "name" => (name, parser) = try_!(parser.trim_start().split_keep('\n')),
                "amount" => (amount, parser) = try_!(parser.trim_start().parse_usize()),
                "repeating" => (repeating, parser) = try_!(parse_shape(parser.trim_start())),
                "colors" => (colors, parser) = try_!(parse_colors(parser.trim_start())),
                _ => {
                    let err = &"could not parse Struct field name";
                    return Err(prev_parser.into_other_error(err));
                }
            }

            if parser.is_empty() {
                break
            }
            parser = try_!(parser.strip_prefix("\n"));
        }
    }

    Ok((Struct{name, amount, repeating, colors}, parser))
}

pub const fn parse_shape(mut parser: Parser<'_>) -> ParseValueResult<'_, Shape> {
    let shape = parser_method!{parser, strip_prefix;
        "circle" => Shape::Circle,
        "square" => Shape::Square,
        "line" => Shape::Line,
        _ => return Err(parser.into_other_error(&"could not parse Shape"))
    };
    Ok((shape, parser))
}

pub const fn parse_colors<const LEN: usize>(
    mut parser: Parser<'_>,
) -> ParseValueResult<'_, [Color; LEN]> {
    let mut colors = [Color::Red; LEN];

    for_range!{i in 0..LEN =>
        (colors[i], parser) = try_!(parse_color(parser.trim_start()));
        
        match parser.strip_prefix(",") {
            Ok(next) => parser = next,
            Err(_) if i == LEN - 1 => {}
            Err(e) => return Err(e),
        }
    }

    Ok((colors, parser))
}

pub const fn parse_color(mut parser: Parser<'_>) -> ParseValueResult<'_, Color> {
    let color = parser_method!{parser, strip_prefix;
        "red" => Color::Red,
        "blue" => Color::Blue,
        "green" => Color::Green,
        _ => return Err(parser.into_other_error(&"could not parse Color"))
    };
    Ok((color, parser))
}

Cargo features

These are the features of these crates:

• "iter"(enabled by default): Enables all iteration items, including macros/functions that take/return iterators,

• "cmp"(enabled by default): Enables all comparison functions and macros, the string equality and ordering comparison functions don't require this feature.

• "parsing_proc"(enabled by default): Enables the "parsing" feature, compiles the konst_proc_macros dependency, and enables the parser_method macro. You can use this feature instead of "parsing" if the slightly longer compile times aren't a problem.

• "parsing"(enabled by default): Enables the parsing module (for parsing from &str and &[u8]), the primitive::parse_* functions, try_rebind, and rebind_if_ok macros.

• "alloc": Enables items that use types from the alloc crate, including Vec and String.

Rust release related

None of thse features are enabled by default.

• "rust_latest_stable": enables the latest "rust_1_*" feature. Only recommendable if you can update the Rust compiler every stable release.

• "rust_1_83": Enables const functions that take mutable references, array::{from_fn_, map_} macros, and destructure macro.

No-std support

konst is #![no_std], it can be used anywhere Rust can be used.

Minimum Supported Rust Version

konst requires Rust 1.65.0.

Features that require newer versions of Rust, or the nightly compiler, need to be explicitly enabled with crate features.