icu_pattern

icu_pattern is a utility crate of the ICU4X project.

It includes a [Pattern] struct which wraps a paid of [Parser] and [Interpolator] allowing for parsing and interpolation of ICU placeholder patterns, like "{0} days" or "{0}, {1}" with custom elements and string literals.

Placeholders & Elements

The [Parser] is generic over any Placeholder which implements FromStr allowing the consumer to parse placeholder patterns such as "{0}, {1}", "{date}, {time}" or any other.

The [Interpolator] can interpolate the [Pattern] against any iterator over Element.

Examples

In the following example we're going to use a custom Token type, and an Element type which will be either a Token or a string slice.

For the purpose of the example, a higher level interpolate_to_string method is being used.

use icu_pattern::Pattern;
use std::{borrow::Cow, convert::TryInto, fmt::Display};

#[derive(Debug, PartialEq)]
enum ExampleToken {
    Year,
    Month,
    Day,
    Hour,
    Minute,
}

impl Display for ExampleToken {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "[{:?}]", self)
    }
}

#[derive(Debug, PartialEq)]
enum ExampleElement<'s> {
    Token(ExampleToken),
    Literal(Cow<'s, str>),
}

impl Display for ExampleElement<'_> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Token(token) => token.fmt(f),
            Self::Literal(lit) => lit.fmt(f),
        }
    }
}

let pattern: Pattern<usize> =
    "{0}, {1}".try_into().expect("Failed to parse a pattern.");

let replacements = vec![
    vec![
        ExampleElement::Token(ExampleToken::Year),
        ExampleElement::Literal("-".into()),
        ExampleElement::Token(ExampleToken::Month),
        ExampleElement::Literal("-".into()),
        ExampleElement::Token(ExampleToken::Day),
    ],
    vec![
        ExampleElement::Token(ExampleToken::Hour),
        ExampleElement::Literal(":".into()),
        ExampleElement::Token(ExampleToken::Minute),
    ],
];

assert_eq!(
    pattern
        .interpolate_to_string::<ExampleElement, _>(&replacements)
        .expect("Failed to interpolate a pattern."),
    "[Year]-[Month]-[Day], [Hour]:[Minute]"
);

Combinators

In the example above, the replacements will be parsed at compile time and stored on a [Vec], which is a collection type that has an implementation for [ReplacementProvider] trait.

In real use, the consumer may want to use different models of replacement provider, and different element schemas. Because the replacement is an iterator itself, it allows for other, more specialized parsers, to be used to lazily parse particular patterns that are meant to replace the placeholders. This allows for lazy parsing of those specialized patterns to be triggered only if the placeholder pattern encounters a placeholder key that requires given pattern to be used.

More Information

For more information on development, authorship, contributing etc. please visit ICU4X home page.