Build a parser combinator step by step

parser-combinator-demo.js

// Error handling helper
const ok = (found, remain) => ({ found, remain });
const error = (msg) => ({ error: msg });

// Primitive parsers

const digit = (input) => {
    const [head, ...rest] = input;
    if (!isNaN(+head) && +head >= 0 && +head <= 9) {
        return ok(head, rest);
    } else {
        return error(`Expected a digit, found '${head}'`);
    }
}

const letter = (input) => {
    const [head, ...rest] = input;
    if (head.match(/[a-zA-Z]/) !== null) {
        return ok(head, rest);
    } else {
        return error(`Expected a letter, found '${head}'`);
    }
};

const char = (expected) => (input) => {
    const [head, ...rest] = input;
    if (head === expected) {
        return ok(head, rest);
    } else {
        return error(`Expected '${expected}', found '${head}'`);
    }
};

// Combinators

const or = (...parsers) => {
    return (input) => {
        for (let parser of parsers) {
            const ret = parser(input);
            if (ret.error) continue;
            return ret;
        }
        return error('Could not parsed the given input');
    };
};

const match = (parsers, fn) => {
    return (input) => {
        let remain = input;
        let results = [];
        for (let parser of parsers) {
            const ret = parser(remain);
            if (ret.error) return ret;
            remain = ret.remain;
            results.push(ret.found);
        }
        return ok(fn(...results), remain);
    }
};

// Combined parsers
const digitOrLetter = or(digit, letter);
const colorCode = match([digitOrLetter, digitOrLetter], (first, second) => `${first}${second}`);

const hexColor = match([char('#'), colorCode, colorCode, colorCode], (_, red, green, blue) => {
    return { red, green, blue };
});

// Try it out
const test = "#f5b2c5";
const result = hexColor(test);
debug(result.found);