Const equivalents of std functions and const parsing.

Features

This crate provides:

• Const fn equivalents of standard library functions, methods, and operators.

• destructure/if_let_Some/while_let_Some macros 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};
use konst::const_panic::{self, PanicFmt};

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


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

const DIRECTION: Direction = result::unwrap!(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)]
enum Direction {
    Forward,
    Backward,
    Left,
    Right,
}

impl Direction {
    const fn try_parse(input: &str) -> Result<Self, ParseDirectionError> {
        // As of Rust 1.89.0, 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),
        }
    }
}

// `PanicFmt` derives the `PanicFmt` trait for debug-printing in `result::unwrap`.
// To use the `PanicFmt` derive you need to enable the "const_panic_derive" feature.
#[derive(Debug, PartialEq, PanicFmt)]
pub struct ParseDirectionError;

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

Parsing CSV

This example demonstrates how CSV can be parsed into integers.

This example requires the "iter" feature (enabled by default).

use konst::{iter, result, string};


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


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

static PARSED: [u64; 5] = iter::collect_const!(u64 =>
    string::split(CSV, ","),
        map(str::trim_ascii),
        map(|s| result::unwrap!(u64::from_str_radix(s, 10))),
);

Parsing a struct

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

This requires the "iter" and "parsing_proc" features (enabled by default).

use konst::{result, try_};
use konst::parsing::{Parser, ParseError, parser_method};


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


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
    ";
    
    result::unwrap!(parse_struct(&mut Parser::new(input)))
};

#[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<'p>(parser: &mut Parser<'p>) -> Result<Struct<'p>, ParseError<'p>> {
    let mut name = "<none>";
    let mut amount = 0;
    let mut repeating = Shape::Circle;
    let mut colors = [Color::Red; 4];
    
    parser.trim_end();
    if !parser.is_empty() {
        loop {
            // keeps a copy of the parser at this position to parse the field name
            let mut prev_parser = parser.trim_start().copy();

            // skip past the `=` to parse the field value
            try_!(parser.find_skip('=')).trim_start();

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

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

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

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

pub const fn parse_colors<'p, const LEN: usize>(
    parser: &mut Parser<'p>,
) -> Result<[Color; LEN], ParseError<'p>> {
    let mut colors = konst::array::ArrayBuilder::of_copy();

    while !colors.is_full() {
        colors.push(try_!(parse_color(parser.trim_start())));
        
        // returns an error if there aren't enough comma-separated colors
        if let Err(e) = parser.strip_prefix(",") && !colors.is_full() {
            return Err(e);
        }
    }

    Ok(colors.build())
}

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

Cargo features

These are the features of this crate:

• "iter"(enabled by default): Enables all iteration-related items that take/return iterators,

• "cmp"(enabled by default): Enables all comparison-related items, 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 strings, and the primitive::parse_* functions.

• "const_panic_derive"(disabled by default): Enables the "derive" feature of the const_panic public dependency.

• "alloc"(disabled by default): Enables items that use types from the alloc crate.

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.

No-std support

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

Minimum Supported Rust Version

konst requires Rust 1.89.0.

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