Crate konst

Const equivalents of std functions and const parsing.

Features

This crate provides:

• Const fn equivalents of standard library functions and methods.

• 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_proc" 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(there’s currently none). Only recommendable if you can update the Rust compiler every stable release.

• "rust_1_83": enables the "mut_refs" feature, newer array macros, and destructure macro (this feature isn’t enabled by "rust_latest_stable" because Rust 1.83.0 isn’t stable as of this konst release)

• "mut_refs"(disabled by default): Enables const functions that take mutable references. Use this whenever mutable references in const contexts are stabilized. Also enables the "rust_latest_stable" feature.

• "nightly_mut_refs"(disabled by default): Enables the "mut_refs" feature. Requires Rust nightly.

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.

Re-exports

• pub use crate::parsing::Parser;
  parsing
• pub use crate::string::cmp_str;
• pub use crate::string::eq_str;
• pub use crate::result::unwrap_ctx;
• pub use crate::string::cmp_option_str;
  cmp
• pub use crate::string::eq_option_str;
  cmp
• pub use ::const_panic;

Modules

• alloc_typealloc
  Generic constants for types from the alloc crate, including String and Vec.
• array
  Const equivalents of array functions.
• chr
  Const equivalents of char functions.
• cmpcmp
  Comparisong-related items.
• ffi
  const equivalents of core::ffi functions
• iteriter
  Const equivalent of iterators with a specific next function signature.
• manually_drop
  Const fn equivalents of ManuallyDrop<T> methods.
• maybe_uninit
  Const fn equivalents of MaybeUninit<T> methods.
• nonzero
  const fn equivalents of NonZero* methods.
• option
  const equivalents of Option methods.
• other
  const fn equivalents of methods from miscelaneous standard library types.
• parsingparsing
  Parsing using const fn methods.
• polymorphism
  Miscelaneous items used for emulating polymorphism without trait methods.
• primitive
  const fn equivalents of primitive type methods.
• ptr
  Const equivalents of raw pointer and NonNull methods.
• range
  const fn equivalents of range methods.
• result
  const equivalents of Result methods.
• slice
  const fn equivalents of slice methods.
• string
  const fn equivalents of str methods.

Macros

• assertc_eqcmp
  For asserting that two values are equal.
• assertc_necmp
  For asserting that two values are unequal.
• coerce_to_cmpcmp
  Coerces reference to a type that has a const_eq or const_cmp method.
• const_cmpcmp
  Compares two values for ordering.
• const_cmp_forcmp
  Compares two standard library types for ordering, that can’t be compared with const_cmp.
• const_eqcmp
  Compares two values for equality.
• const_eq_forcmp
  Compares two standard library types for equality, that can’t be compared with const_eq.
• destructurerust_1_83
  Destructures a struct/tuple/array into all of its elements/fields.
• for_range
  For loop over a range
• impl_cmpcmp
  For implementing const comparison semi-manually.
• konst
  Emulates the inline const feature, eg: const{ foo() },
• maxcmp
  Const equivalent of std::cmp::max
• max_bycmp
  Const equivalent of std::cmp::max_by
• max_by_keycmp
  Const equivalent of std::cmp::max_by_key
• mincmp
  Const equivalent of std::cmp::min
• min_bycmp
  Const equivalent of std::cmp::min_by
• min_by_keycmp
  Const equivalent of std::cmp::min_by_key
• parse_withparsing
  Parses a type that impls HasParser with the passed in Parser.
• parser_methodparsing_proc
  Calls a Parser method with many alternative string literals.
• rebind_if_okparsing
  Like an if let Ok, but also reassigns variables with the value in the Ok variant.
• try_
  ?-like macro, which allows optionally mapping errors.
• try_equalcmp
  Evaluates to $ord if it is Ordering::Equal, otherwise returns it from the enclosing function.
• try_opt
  ?-like macro for Options.
• try_rebindparsing
  Like the ? operator, but also reassigns variables with the value in the Ok variant.