vayn · May 18, 2019 15:56 · vayn · May 18, 2019
diff --git a/parser.rs b/parser.rs
 #![allow(dead_code)]

 #[derive(Clone, Debug, PartialEq, Eq)]
 struct Element {
    name: String,
    attributes: Vec<(String, String)>,
    children: Vec<Element>,
 }

 type ParserResult<'a, Output> = Result<(&'a str, Output), &'a str>;

 trait Parser<'a, Output> {
    fn parse(&self, input: &'a str) -> ParserResult<'a, Output>;

    fn map<F, NewOutput>(self, map_fn: F) -> BoxedParser<'a, NewOutput>
    where
        Self: Sized + 'a,
        Output: 'a,
        NewOutput: 'a,
        F: Fn(Output) -> NewOutput + 'a,
    {
        BoxedParser::new(map(self, map_fn))
    }

    fn pred<F>(self, pred_fn: F) -> BoxedParser<'a, Output>
    where
        Self: Sized + 'a,
        Output: 'a,
        F: Fn(&Output) -> bool + 'a,
    {
        BoxedParser::new(pred(self, pred_fn))
    }

    fn and_then<F, NextParser, NewOutput>(self, f: F) -> BoxedParser<'a, NewOutput>
    where
        Self: Sized + 'a,
        Output: 'a,
        NewOutput: 'a,
        NextParser: Parser<'a, NewOutput> + 'a,
        F: Fn(Output) -> NextParser + 'a,
    {
        BoxedParser::new(and_then(self, f))
    }
 }

 impl<'a, F, Output> Parser<'a, Output> for F
 where
    F: Fn(&'a str) -> ParserResult<Output>,
 {
    fn parse(&self, input: &'a str) -> ParserResult<'a, Output> {
        self(input)
    }
 }

 /// enum List<A> {
 ///     Cons(A, List<A>),
 ///     Nil,
 /// }
 /// 
 /// https://bodil.lol/parser-combinators/#to-infinity-and-beyond
 /// 
 /// To which rustc will, very sensibly, object that your recursive type List<A>
 /// has an infinite size, because inside every List::<A>::Cons is another
 /// List<A>, and that means it's List<A>s all the way down into infinity. As far
 /// as rustc is concerned, we're asking for an infinite list, and we're asking it
 /// to be able to allocate an infinite list.
 /// 
 /// In many languages, an infinite list isn't a problem in principle for the type
 /// system, and it's actually not for Rust either. The problem is that in Rust,
 /// as mentioned, we need to be able to allocate it, or, rather, we need to be
 /// able to determine the size of a type up front when we construct it, and when
 /// the type is infinite, that means the size must be infinite too.
 /// 
 /// The solution is to employ a bit of indirection. Instead of our List::Cons
 /// being an element of A and another list of A, instead we make it an element of
 /// A and a pointer to a list of A. We know the size of a pointer, and it's the
 /// same no matter what it points to, and so our List::Cons now has a fixed and
 /// predictable size no matter the size of the list. And the way to turn an owned
 /// thing into a pointer to an owned thing on the heap, in Rust, is to Box it.
 /// 
 /// enum List<A> {
 ///     Cons(A, Box<List<A>>),
 ///     Nil,
 /// }
 ///
 /// Another interesting feature of Box is that the type inside it can be
 /// abstract. This means that instead of our by now incredibly complicated
 /// parser function types, we can let the type checker deal with a very succinct
 /// Box<dyn Parser<'a, A>> instead.
 /// 
 /// That sounds great. What's the downside? Well, we might be losing a cycle or
 /// two to having to follow that pointer, and it could be that the compiler
 /// loses some opportunities to optimise our parser. But recall Knuth's
 /// admonition about premature optimisation: it's going to be fine. You can
 /// afford those cycles.
 /// 
 /// So let's proceed to implement Parser for a boxed parser function in addition
 /// to the bare functions we've been using so far.
 struct BoxedParser<'a, Output> {
    parser: Box<dyn Parser<'a, Output> + 'a>,
 }

 impl<'a, Output> BoxedParser<'a, Output> {
    fn new<P>(parser: P) -> Self
    where
        P: Parser<'a, Output> + 'a,
    {
        BoxedParser {
            parser: Box::new(parser),
        }
    }
 }

 impl<'a, Output> Parser<'a, Output> for BoxedParser<'a, Output> {
    fn parse(&self, input: &'a str) -> ParserResult<'a, Output> {
        self.parser.parse(input)
    }
 }

 /*
 fn match_literal(expected: &'static str)
    -> impl Fn(&str) -> Result<(&str, ()), &str>
 {
    move |input| match input.get(0..expected.len()) {
        Some(next) if next == expected => {
            Ok((&input[expected.len()..], ()))
        }
        _ => Err(input),
    }
 }
 */
 fn match_literal<'a>(expected: &'static str) -> impl Parser<'a, ()> {
    move |input: &'a str| match input.get(0..expected.len()) {
        Some(next) if next == expected => Ok((&input[expected.len()..], ())),
        _ => Err(input),
    }
 }

 #[test]
 fn literal_parser() {
    let parse_joe = match_literal("Hello Joe!");
    assert_eq!(Ok(("", ())), parse_joe.parse("Hello Joe!"));
    assert_eq!(
        Ok((" Hello Robert!", ())),
        parse_joe.parse("Hello Joe! Hello Robert!")
    );
    assert_eq!(Err("Hello Mike!"), parse_joe.parse("Hello Mike!"))
 }

 // fn identifier(input: &str) -> Result<(&str, String), &str> {
 fn identifier(input: &str) -> ParserResult<String> {
    let mut matched = String::new();
    let mut chars = input.chars();

    match chars.next() {
        Some(next) if next.is_alphabetic() => matched.push(next),
        _ => return Err(input),
    }

    while let Some(next) = chars.next() {
        if next.is_alphanumeric() || next == '-' {
            matched.push(next);
        } else {
            break;
        }
    }

    let next_index = matched.len();
    Ok((&input[next_index..], matched))
 }

 #[test]
 fn identifier_parser() {
    assert_eq!(
        Ok(("", "i-am-an-identifier".to_string())),
        identifier("i-am-an-identifier")
    );
    assert_eq!(
        Ok((" entirely an identifier", "not".to_string())),
        identifier("not entirely an identifier")
    );
    assert_eq!(
        Err("!not at all an identifier"),
        identifier("!not at all an identifier")
    );
 }

 /*
 fn pair<P1, P2, R1, R2>(parser1: P1, parser2: P2)
    -> impl Fn(&str) -> Result<(&str, (R1, R2)), &str>
 where
    P1: Fn(&str) -> Result<(&str, R1), &str>,
    P2: Fn(&str) -> Result<(&str, R2), &str>,
 {
    move |input| match parser1(input) {
        Ok((next_input, result1)) => match parser2(next_input) {
            Ok((final_input, result2)) => Ok((final_input, (result1, result2))),
            Err(err) => Err(err),
        },
        Err(err) => Err(err),
    }
 }

 fn pair<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2)
    -> impl Parser<'a, (R1, R2)>
 where
    P1: Parser<'a, R1>,
    P2: Parser<'a, R2>,
 {
    move |input| match parser1.parse(input) {
        Ok((next_input, result1)) => match parser2.parse(next_input) {
            Ok((final_input, result2)) => Ok((final_input, (result1, result2))),
            Err(err) => Err(err),
        }
        Err(err) => Err(err),
    }
 }
 */
 fn pair<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2) -> impl Parser<'a, (R1, R2)>
 where
    P1: Parser<'a, R1>,
    P2: Parser<'a, R2>,
 {
    move |input| {
        parser1.parse(input).and_then(|(next_input, result1)| {
            parser2
                .parse(next_input)
                .map(|(final_input, result2)| (final_input, (result1, result2)))
        })
    }
 }
 /// It looks very neat, but there's a problem: parser2.map() consumes parser2 to
 /// create the wrapped parser, and the function is a Fn, not a FnOnce, so it's
 /// not allowed to consume parser2, just take a reference to it. Rust Problems,
 /// in other words.
 /// 
 /// fn pair<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2)
 ///     -> impl Parser<'a, (R1, R2)>
 /// where
 ///     P1: Parser<'a, R1> + 'a,
 ///     P2: Parser<'a, R2> + 'a,
 ///     R1: 'a + Clone,
 ///     R2: 'a,
 /// {
 ///     parser1.and_then(move |result1| parser2.map(move |result2| {
 ///         (result1.clone(), result2)
 ///     }))
 /// }

 #[test]
 fn pair_combinator() {
    let tag_opener = pair(match_literal("<"), identifier);
    assert_eq!(
        Ok(("/>", ((), "my-first-element".to_string()))),
        tag_opener.parse("<my-first-element/>")
    );
    assert_eq!(Err("oops"), tag_opener.parse("oops"));
    assert_eq!(Err("!oops"), tag_opener.parse("<!oops"));
 }

 /*
 fn map<P, F, A, B>(parser: P, map_fn: F)
    -> impl Fn(&str) -> Result<(&str, B), &str>
 where
    P: Fn(&str) -> Result<(&str, A), &str>,
    F: Fn(A) -> B,
 {
    move |input| match parser(input) {
        Ok((next_input, result)) => Ok((next_input, map_fn(result))),
        Err(err) => Err(err),
    }
 }

 fn map<P, F, A, B>(parser: P, map_fn: F)
    -> impl Fn(&str) -> Result<(&str, B), &str>
 where
    P: Fn(&str) -> Result<(&str, A), &str>,
    F: Fn(A) -> B,
 {
    move |input|
        parser(input)
            .map(|(next_input, result)| (next_input, map_fn(result)))
 }
 */
 fn map<'a, P, F, A, B>(parser: P, map_fn: F) -> impl Parser<'a, B>
 where
    P: Parser<'a, A>,
    F: Fn(A) -> B,
 {
    move |input| {
        parser
            .parse(input)
            .map(|(next_input, result)| (next_input, map_fn(result)))
    }
 }

 fn left<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2) -> impl Parser<'a, R1>
 where
    P1: Parser<'a, R1> + 'a,
    P2: Parser<'a, R2> + 'a,
    R1: 'a,
    R2: 'a,
 {
    // map(pair(parser1, parser2), |(left, _right)| left)
    pair(parser1, parser2).map(|(left, _right)| left)
 }

 fn right<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2) -> impl Parser<'a, R2>
 where
    P1: Parser<'a, R1> + 'a,
    P2: Parser<'a, R2> + 'a,
    R1: 'a,
    R2: 'a,
 {
    // map(pair(parser1, parser2), |(_left, right)| right)
    pair(parser1, parser2).map(|(_left, right)| right)
 }

 #[test]
 fn right_combinator() {
    let tag_opener = right(match_literal("<"), identifier);
    assert_eq!(
        Ok(("/>", "my-first-element".to_string())),
        tag_opener.parse("<my-first-element/>")
    );
    assert_eq!(Err("oops"), tag_opener.parse("oops"));
    assert_eq!(Err("!oops"), tag_opener.parse("<!oops"));
 }

 fn zero_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<A>>
 where
    P: Parser<'a, A>,
 {
    move |mut input| {
        let mut result = Vec::new();

        while let Ok((next_input, next_item)) = parser.parse(input) {
            input = next_input;
            result.push(next_item);
        }

        Ok((input, result))
    }
 }

 #[test]
 fn zero_or_more_combinator() {
    let parser = zero_or_more(match_literal("ha"));
    assert_eq!(Ok(("", vec![(), (), ()])), parser.parse("hahaha"));
    assert_eq!(Ok(("ahah", vec![])), parser.parse("ahah"));
    assert_eq!(Ok(("", vec![])), parser.parse(""));
 }

 fn one_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<A>>
 where
    P: Parser<'a, A>,
 {
    move |mut input| {
        let mut result = Vec::new();

        if let Ok((next_input, first_item)) = parser.parse(input) {
            input = next_input;
            result.push(first_item);
        } else {
            return Err(input);
        }

        while let Ok((next_input, next_item)) = parser.parse(input) {
            input = next_input;
            result.push(next_item);
        }

        Ok((input, result))
    }
 }
 /// Ownership problem https://bodil.lol/parser-combinators/#one-or-more Here, we
 /// run into Rust Problems, and I don't even mean the problem of not having a
 /// cons method for Vec, but I know every Lisp programmer reading that bit of
 /// code was thinking it. No, it's worse than that: it's ownership.
 ///
 /// We own that parser, so we can't go passing it as an argument twice, the
 /// compiler will start shouting at you that you're trying to move an already
 /// moved value. So can we make our combinators take references instead? No, it
 /// turns out, not without running into another whole set of borrow checker
 /// troubles - and we're not going to even go there right now. And because these
 /// parsers are functions, they don't do anything so straightforward as to
 /// implement Clone, which would have saved the day very tidily, so we're stuck
 /// with a constraint that we can't repeat our parsers easily in combinators.
 ///
 /// fn one_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<A>>
 /// where
 ///     P: Parser<'a, A>,
 /// {
 ///     map(pair(parser, zero_or_more(parser)), |(head, mut tail)| {
 ///         tail.insert(0, head);
 ///         tail
 ///     })
 /// }

 #[test]
 fn one_or_more_combinator() {
    let parser = one_or_more(match_literal("ha"));
    assert_eq!(Ok(("", vec![(), (), ()])), parser.parse("hahaha"));
    assert_eq!(Err("ahah"), parser.parse("ahah"));
    assert_eq!(Err(""), parser.parse(""));
 }

 fn any_char(input: &str) -> ParserResult<char> {
    match input.chars().next() {
        Some(next) => Ok((&input[next.len_utf8()..], next)),
        _ => Err(input),
    }
 }

 fn pred<'a, P, A, F>(parser: P, predicate: F) -> impl Parser<'a, A>
 where
    P: Parser<'a, A>,
    F: Fn(&A) -> bool,
 {
    move |input| {
        if let Ok((next_input, value)) = parser.parse(input) {
            if predicate(&value) {
                return Ok((next_input, value));
            }
        }
        Err(input)
    }
 }

 #[test]
 fn predicate_combinator() {
    let parser = pred(any_char, |c| *c == 'o');
    assert_eq!(Ok(("mg", 'o')), parser.parse("omg"));
    assert_eq!(Err("lol"), parser.parse("lol"));
 }

 fn whitespace_char<'a>() -> impl Parser<'a, char> {
    // pred(any_char, |c| c.is_whitespace())
    any_char.pred(|c| c.is_whitespace())
 }

 fn space1<'a>() -> impl Parser<'a, Vec<char>> {
    one_or_more(whitespace_char())
 }

 fn space0<'a>() -> impl Parser<'a, Vec<char>> {
    zero_or_more(whitespace_char())
 }

 fn quoted_string<'a>() -> impl Parser<'a, String> {
    right(
        match_literal("\""),
        left(
            zero_or_more(any_char.pred(|c| *c != '"')),
            match_literal("\""),
        ),
    )
    .map(|chars| chars.into_iter().collect())
 }

 #[test]
 fn quoted_string_parser() {
    assert_eq!(
        Ok(("", "Hello Joe!".to_string())),
        quoted_string().parse("\"Hello Joe!\"")
    );
 }

 fn attribute_pair<'a>() -> impl Parser<'a, (String, String)> {
    pair(identifier, right(match_literal("="), quoted_string()))
 }

 fn attributes<'a>() -> impl Parser<'a, Vec<(String, String)>> {
    zero_or_more(right(space1(), attribute_pair()))
 }

 #[test]
 fn attribute_parser() {
    assert_eq!(
        Ok((
            "",
            vec![
                ("one".to_string(), "1".to_string()),
                ("two".to_string(), "2".to_string())
            ]
        )),
        attributes().parse(" one=\"1\" two=\"2\"")
    );
 }

 fn element_start<'a>() -> impl Parser<'a, (String, Vec<(String, String)>)> {
    right(match_literal("<"), pair(identifier, attributes()))
 }

 fn single_element<'a>() -> impl Parser<'a, Element> {
    // map(
    //     left(element_start(), match_literal("/>")),
    //     |(name, attributes)| Element {
    //         name,
    //         attributes,
    //         children: vec![],
    //     },
    // )
    left(element_start(), match_literal("/>")).map(|(name, attributes)| Element {
        name,
        attributes,
        children: vec![],
    })
 }

 #[test]
 fn single_element_parser() {
    assert_eq!(
        Ok((
            "",
            Element {
                name: "div".to_string(),
                attributes: vec![("class".to_string(), "float".to_string())],
                children: vec![]
            }
        )),
        single_element().parse("<div class=\"float\"/>")
    );
 }

 fn open_element<'a>() -> impl Parser<'a, Element> {
    left(element_start(), match_literal(">")).map(|(name, attributes)| Element {
        name,
        attributes,
        children: vec![],
    })
 }

 fn either<'a, P1, P2, A>(parser1: P1, parser2: P2) -> impl Parser<'a, A>
 where
    P1: Parser<'a, A>,
    P2: Parser<'a, A>,
 {
    move |input| match parser1.parse(input) {
        ok @ Ok(_) => ok,
        Err(_) => parser2.parse(input),
    }
 }

 fn element<'a>() -> impl Parser<'a, Element> {
    whitespace_wrap(either(single_element(), parent_element()))
 }

 fn close_element<'a>(expected_name: String) -> impl Parser<'a, String> {
    right(match_literal("</"), left(identifier, match_literal(">")))
        .pred(move |name| name == &expected_name)
 }

 fn and_then<'a, P, F, A, B, NextP>(parser: P, f: F) -> impl Parser<'a, B>
 where
    P: Parser<'a, A>,
    NextP: Parser<'a, B>,
    F: Fn(A) -> NextP,
 {
    move |input| match parser.parse(input) {
        Ok((next_input, result)) => f(result).parse(next_input),
        Err(err) => Err(err),
    }
 }

 fn parent_element<'a>() -> impl Parser<'a, Element> {
    open_element().and_then(|el| {
        left(zero_or_more(element()), close_element(el.name.clone())).map(move |children| {
            let mut el = el.clone();
            el.children = children;
            el
        })
    })
 }

 fn whitespace_wrap<'a, P, A>(parser: P) -> impl Parser<'a, A>
 where
    P: Parser<'a, A> + 'a,
    A: 'a,
 {
    right(space0(), left(parser, space0()))
 }

 #[test]
 fn xml_parser() {
    let doc = r#"
        <top label="Top">
            <semi-bottom label="Bottom"/>
            <middle>
                <bottom label="Another bottom"/>
            </middle>
        </top>"#;
    let parsed_doc = Element {
        name: "top".to_string(),
        attributes: vec![("label".to_string(), "Top".to_string())],
        children: vec![
            Element {
                name: "semi-bottom".to_string(),
                attributes: vec![("label".to_string(), "Bottom".to_string())],
                children: vec![],
            },
            Element {
                name: "middle".to_string(),
                attributes: vec![],
                children: vec![Element {
                    name: "bottom".to_string(),
                    attributes: vec![("label".to_string(), "Another bottom".to_string())],
                    children: vec![],
                }],
            },
        ],
    };
    assert_eq!(Ok(("", parsed_doc)), element().parse(doc));
 }

 #[test]
 fn mismatched_closing_tag() {
    let doc = r#"
        <top>
            <bottom/>
        </middle>"#;
    assert_eq!(Err("</middle>"), element().parse(doc));
 }
	#![allow(dead_code)]

	#[derive(Clone, Debug, PartialEq, Eq)]
	struct Element {
	name: String,
	attributes: Vec<(String, String)>,
	children: Vec<Element>,
	}

	type ParserResult<'a, Output> = Result<(&'a str, Output), &'a str>;

	trait Parser<'a, Output> {
	fn parse(&self, input: &'a str) -> ParserResult<'a, Output>;

	fn map<F, NewOutput>(self, map_fn: F) -> BoxedParser<'a, NewOutput>
	where
	Self: Sized + 'a,
	Output: 'a,
	NewOutput: 'a,
	F: Fn(Output) -> NewOutput + 'a,
	{
	BoxedParser::new(map(self, map_fn))
	}

	fn pred<F>(self, pred_fn: F) -> BoxedParser<'a, Output>
	where
	Self: Sized + 'a,
	Output: 'a,
	F: Fn(&Output) -> bool + 'a,
	{
	BoxedParser::new(pred(self, pred_fn))
	}

	fn and_then<F, NextParser, NewOutput>(self, f: F) -> BoxedParser<'a, NewOutput>
	where
	Self: Sized + 'a,
	Output: 'a,
	NewOutput: 'a,
	NextParser: Parser<'a, NewOutput> + 'a,
	F: Fn(Output) -> NextParser + 'a,
	{
	BoxedParser::new(and_then(self, f))
	}
	}

	impl<'a, F, Output> Parser<'a, Output> for F
	where
	F: Fn(&'a str) -> ParserResult<Output>,
	{
	fn parse(&self, input: &'a str) -> ParserResult<'a, Output> {
	self(input)
	}
	}

	/// enum List<A> {
	/// Cons(A, List<A>),
	/// Nil,
	/// }
	///
	/// https://bodil.lol/parser-combinators/#to-infinity-and-beyond
	///
	/// To which rustc will, very sensibly, object that your recursive type List<A>
	/// has an infinite size, because inside every List::<A>::Cons is another
	/// List<A>, and that means it's List<A>s all the way down into infinity. As far
	/// as rustc is concerned, we're asking for an infinite list, and we're asking it
	/// to be able to allocate an infinite list.
	///
	/// In many languages, an infinite list isn't a problem in principle for the type
	/// system, and it's actually not for Rust either. The problem is that in Rust,
	/// as mentioned, we need to be able to allocate it, or, rather, we need to be
	/// able to determine the size of a type up front when we construct it, and when
	/// the type is infinite, that means the size must be infinite too.
	///
	/// The solution is to employ a bit of indirection. Instead of our List::Cons
	/// being an element of A and another list of A, instead we make it an element of
	/// A and a pointer to a list of A. We know the size of a pointer, and it's the
	/// same no matter what it points to, and so our List::Cons now has a fixed and
	/// predictable size no matter the size of the list. And the way to turn an owned
	/// thing into a pointer to an owned thing on the heap, in Rust, is to Box it.
	///
	/// enum List<A> {
	/// Cons(A, Box<List<A>>),
	/// Nil,
	/// }
	///
	/// Another interesting feature of Box is that the type inside it can be
	/// abstract. This means that instead of our by now incredibly complicated
	/// parser function types, we can let the type checker deal with a very succinct
	/// Box<dyn Parser<'a, A>> instead.
	///
	/// That sounds great. What's the downside? Well, we might be losing a cycle or
	/// two to having to follow that pointer, and it could be that the compiler
	/// loses some opportunities to optimise our parser. But recall Knuth's
	/// admonition about premature optimisation: it's going to be fine. You can
	/// afford those cycles.
	///
	/// So let's proceed to implement Parser for a boxed parser function in addition
	/// to the bare functions we've been using so far.
	struct BoxedParser<'a, Output> {
	parser: Box<dyn Parser<'a, Output> + 'a>,
	}

	impl<'a, Output> BoxedParser<'a, Output> {
	fn new<P>(parser: P) -> Self
	where
	P: Parser<'a, Output> + 'a,
	{
	BoxedParser {
	parser: Box::new(parser),
	}
	}
	}

	impl<'a, Output> Parser<'a, Output> for BoxedParser<'a, Output> {
	fn parse(&self, input: &'a str) -> ParserResult<'a, Output> {
	self.parser.parse(input)
	}
	}

	/*
	fn match_literal(expected: &'static str)
	-> impl Fn(&str) -> Result<(&str, ()), &str>
	{
	move \|input\| match input.get(0..expected.len()) {
	Some(next) if next == expected => {
	Ok((&input[expected.len()..], ()))
	}
	_ => Err(input),
	}
	}
	*/
	fn match_literal<'a>(expected: &'static str) -> impl Parser<'a, ()> {
	move \|input: &'a str\| match input.get(0..expected.len()) {
	Some(next) if next == expected => Ok((&input[expected.len()..], ())),
	_ => Err(input),
	}
	}

	#[test]
	fn literal_parser() {
	let parse_joe = match_literal("Hello Joe!");
	assert_eq!(Ok(("", ())), parse_joe.parse("Hello Joe!"));
	assert_eq!(
	Ok((" Hello Robert!", ())),
	parse_joe.parse("Hello Joe! Hello Robert!")
	);
	assert_eq!(Err("Hello Mike!"), parse_joe.parse("Hello Mike!"))
	}

	// fn identifier(input: &str) -> Result<(&str, String), &str> {
	fn identifier(input: &str) -> ParserResult<String> {
	let mut matched = String::new();
	let mut chars = input.chars();

	match chars.next() {
	Some(next) if next.is_alphabetic() => matched.push(next),
	_ => return Err(input),
	}

	while let Some(next) = chars.next() {
	if next.is_alphanumeric() \|\| next == '-' {
	matched.push(next);
	} else {
	break;
	}
	}

	let next_index = matched.len();
	Ok((&input[next_index..], matched))
	}

	#[test]
	fn identifier_parser() {
	assert_eq!(
	Ok(("", "i-am-an-identifier".to_string())),
	identifier("i-am-an-identifier")
	);
	assert_eq!(
	Ok((" entirely an identifier", "not".to_string())),
	identifier("not entirely an identifier")
	);
	assert_eq!(
	Err("!not at all an identifier"),
	identifier("!not at all an identifier")
	);
	}

	/*
	fn pair<P1, P2, R1, R2>(parser1: P1, parser2: P2)
	-> impl Fn(&str) -> Result<(&str, (R1, R2)), &str>
	where
	P1: Fn(&str) -> Result<(&str, R1), &str>,
	P2: Fn(&str) -> Result<(&str, R2), &str>,
	{
	move \|input\| match parser1(input) {
	Ok((next_input, result1)) => match parser2(next_input) {
	Ok((final_input, result2)) => Ok((final_input, (result1, result2))),
	Err(err) => Err(err),
	},
	Err(err) => Err(err),
	}
	}

	fn pair<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2)
	-> impl Parser<'a, (R1, R2)>
	where
	P1: Parser<'a, R1>,
	P2: Parser<'a, R2>,
	{
	move \|input\| match parser1.parse(input) {
	Ok((next_input, result1)) => match parser2.parse(next_input) {
	Ok((final_input, result2)) => Ok((final_input, (result1, result2))),
	Err(err) => Err(err),
	}
	Err(err) => Err(err),
	}
	}
	*/
	fn pair<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2) -> impl Parser<'a, (R1, R2)>
	where
	P1: Parser<'a, R1>,
	P2: Parser<'a, R2>,
	{
	move \|input\| {
	parser1.parse(input).and_then(\|(next_input, result1)\| {
	parser2
	.parse(next_input)
	.map(\|(final_input, result2)\| (final_input, (result1, result2)))
	})
	}
	}
	/// It looks very neat, but there's a problem: parser2.map() consumes parser2 to
	/// create the wrapped parser, and the function is a Fn, not a FnOnce, so it's
	/// not allowed to consume parser2, just take a reference to it. Rust Problems,
	/// in other words.
	///
	/// fn pair<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2)
	/// -> impl Parser<'a, (R1, R2)>
	/// where
	/// P1: Parser<'a, R1> + 'a,
	/// P2: Parser<'a, R2> + 'a,
	/// R1: 'a + Clone,
	/// R2: 'a,
	/// {
	/// parser1.and_then(move \|result1\| parser2.map(move \|result2\| {
	/// (result1.clone(), result2)
	/// }))
	/// }

	#[test]
	fn pair_combinator() {
	let tag_opener = pair(match_literal("<"), identifier);
	assert_eq!(
	Ok(("/>", ((), "my-first-element".to_string()))),
	tag_opener.parse("<my-first-element/>")
	);
	assert_eq!(Err("oops"), tag_opener.parse("oops"));
	assert_eq!(Err("!oops"), tag_opener.parse("<!oops"));
	}

	/*
	fn map<P, F, A, B>(parser: P, map_fn: F)
	-> impl Fn(&str) -> Result<(&str, B), &str>
	where
	P: Fn(&str) -> Result<(&str, A), &str>,
	F: Fn(A) -> B,
	{
	move \|input\| match parser(input) {
	Ok((next_input, result)) => Ok((next_input, map_fn(result))),
	Err(err) => Err(err),
	}
	}

	fn map<P, F, A, B>(parser: P, map_fn: F)
	-> impl Fn(&str) -> Result<(&str, B), &str>
	where
	P: Fn(&str) -> Result<(&str, A), &str>,
	F: Fn(A) -> B,
	{
	move \|input\|
	parser(input)
	.map(\|(next_input, result)\| (next_input, map_fn(result)))
	}
	*/
	fn map<'a, P, F, A, B>(parser: P, map_fn: F) -> impl Parser<'a, B>
	where
	P: Parser<'a, A>,
	F: Fn(A) -> B,
	{
	move \|input\| {
	parser
	.parse(input)
	.map(\|(next_input, result)\| (next_input, map_fn(result)))
	}
	}

	fn left<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2) -> impl Parser<'a, R1>
	where
	P1: Parser<'a, R1> + 'a,
	P2: Parser<'a, R2> + 'a,
	R1: 'a,
	R2: 'a,
	{
	// map(pair(parser1, parser2), \|(left, _right)\| left)
	pair(parser1, parser2).map(\|(left, _right)\| left)
	}

	fn right<'a, P1, P2, R1, R2>(parser1: P1, parser2: P2) -> impl Parser<'a, R2>
	where
	P1: Parser<'a, R1> + 'a,
	P2: Parser<'a, R2> + 'a,
	R1: 'a,
	R2: 'a,
	{
	// map(pair(parser1, parser2), \|(_left, right)\| right)
	pair(parser1, parser2).map(\|(_left, right)\| right)
	}

	#[test]
	fn right_combinator() {
	let tag_opener = right(match_literal("<"), identifier);
	assert_eq!(
	Ok(("/>", "my-first-element".to_string())),
	tag_opener.parse("<my-first-element/>")
	);
	assert_eq!(Err("oops"), tag_opener.parse("oops"));
	assert_eq!(Err("!oops"), tag_opener.parse("<!oops"));
	}

	fn zero_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<A>>
	where
	P: Parser<'a, A>,
	{
	move \|mut input\| {
	let mut result = Vec::new();

	while let Ok((next_input, next_item)) = parser.parse(input) {
	input = next_input;
	result.push(next_item);
	}

	Ok((input, result))
	}
	}

	#[test]
	fn zero_or_more_combinator() {
	let parser = zero_or_more(match_literal("ha"));
	assert_eq!(Ok(("", vec![(), (), ()])), parser.parse("hahaha"));
	assert_eq!(Ok(("ahah", vec![])), parser.parse("ahah"));
	assert_eq!(Ok(("", vec![])), parser.parse(""));
	}

	fn one_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<A>>
	where
	P: Parser<'a, A>,
	{
	move \|mut input\| {
	let mut result = Vec::new();

	if let Ok((next_input, first_item)) = parser.parse(input) {
	input = next_input;
	result.push(first_item);
	} else {
	return Err(input);
	}

	while let Ok((next_input, next_item)) = parser.parse(input) {
	input = next_input;
	result.push(next_item);
	}

	Ok((input, result))
	}
	}
	/// Ownership problem https://bodil.lol/parser-combinators/#one-or-more Here, we
	/// run into Rust Problems, and I don't even mean the problem of not having a
	/// cons method for Vec, but I know every Lisp programmer reading that bit of
	/// code was thinking it. No, it's worse than that: it's ownership.
	///
	/// We own that parser, so we can't go passing it as an argument twice, the
	/// compiler will start shouting at you that you're trying to move an already
	/// moved value. So can we make our combinators take references instead? No, it
	/// turns out, not without running into another whole set of borrow checker
	/// troubles - and we're not going to even go there right now. And because these
	/// parsers are functions, they don't do anything so straightforward as to
	/// implement Clone, which would have saved the day very tidily, so we're stuck
	/// with a constraint that we can't repeat our parsers easily in combinators.
	///
	/// fn one_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<A>>
	/// where
	/// P: Parser<'a, A>,
	/// {
	/// map(pair(parser, zero_or_more(parser)), \|(head, mut tail)\| {
	/// tail.insert(0, head);
	/// tail
	/// })
	/// }

	#[test]
	fn one_or_more_combinator() {
	let parser = one_or_more(match_literal("ha"));
	assert_eq!(Ok(("", vec![(), (), ()])), parser.parse("hahaha"));
	assert_eq!(Err("ahah"), parser.parse("ahah"));
	assert_eq!(Err(""), parser.parse(""));
	}

	fn any_char(input: &str) -> ParserResult<char> {
	match input.chars().next() {
	Some(next) => Ok((&input[next.len_utf8()..], next)),
	_ => Err(input),
	}
	}

	fn pred<'a, P, A, F>(parser: P, predicate: F) -> impl Parser<'a, A>
	where
	P: Parser<'a, A>,
	F: Fn(&A) -> bool,
	{
	move \|input\| {
	if let Ok((next_input, value)) = parser.parse(input) {
	if predicate(&value) {
	return Ok((next_input, value));
	}
	}
	Err(input)
	}
	}

	#[test]
	fn predicate_combinator() {
	let parser = pred(any_char, \|c\| *c == 'o');
	assert_eq!(Ok(("mg", 'o')), parser.parse("omg"));
	assert_eq!(Err("lol"), parser.parse("lol"));
	}

	fn whitespace_char<'a>() -> impl Parser<'a, char> {
	// pred(any_char, \|c\| c.is_whitespace())
	any_char.pred(\|c\| c.is_whitespace())
	}

	fn space1<'a>() -> impl Parser<'a, Vec<char>> {
	one_or_more(whitespace_char())
	}

	fn space0<'a>() -> impl Parser<'a, Vec<char>> {
	zero_or_more(whitespace_char())
	}

	fn quoted_string<'a>() -> impl Parser<'a, String> {
	right(
	match_literal("\""),
	left(
	zero_or_more(any_char.pred(\|c\| *c != '"')),
	match_literal("\""),
	),
	)
	.map(\|chars\| chars.into_iter().collect())
	}

	#[test]
	fn quoted_string_parser() {
	assert_eq!(
	Ok(("", "Hello Joe!".to_string())),
	quoted_string().parse("\"Hello Joe!\"")
	);
	}

	fn attribute_pair<'a>() -> impl Parser<'a, (String, String)> {
	pair(identifier, right(match_literal("="), quoted_string()))
	}

	fn attributes<'a>() -> impl Parser<'a, Vec<(String, String)>> {
	zero_or_more(right(space1(), attribute_pair()))
	}

	#[test]
	fn attribute_parser() {
	assert_eq!(
	Ok((
	"",
	vec![
	("one".to_string(), "1".to_string()),
	("two".to_string(), "2".to_string())
	]
	)),
	attributes().parse(" one=\"1\" two=\"2\"")
	);
	}

	fn element_start<'a>() -> impl Parser<'a, (String, Vec<(String, String)>)> {
	right(match_literal("<"), pair(identifier, attributes()))
	}

	fn single_element<'a>() -> impl Parser<'a, Element> {
	// map(
	// left(element_start(), match_literal("/>")),
	// \|(name, attributes)\| Element {
	// name,
	// attributes,
	// children: vec![],
	// },
	// )
	left(element_start(), match_literal("/>")).map(\|(name, attributes)\| Element {
	name,
	attributes,
	children: vec![],
	})
	}

	#[test]
	fn single_element_parser() {
	assert_eq!(
	Ok((
	"",
	Element {
	name: "div".to_string(),
	attributes: vec![("class".to_string(), "float".to_string())],
	children: vec![]
	}
	)),
	single_element().parse("<div class=\"float\"/>")
	);
	}

	fn open_element<'a>() -> impl Parser<'a, Element> {
	left(element_start(), match_literal(">")).map(\|(name, attributes)\| Element {
	name,
	attributes,
	children: vec![],
	})
	}

	fn either<'a, P1, P2, A>(parser1: P1, parser2: P2) -> impl Parser<'a, A>
	where
	P1: Parser<'a, A>,
	P2: Parser<'a, A>,
	{
	move \|input\| match parser1.parse(input) {
	ok @ Ok(_) => ok,
	Err(_) => parser2.parse(input),
	}
	}

	fn element<'a>() -> impl Parser<'a, Element> {
	whitespace_wrap(either(single_element(), parent_element()))
	}

	fn close_element<'a>(expected_name: String) -> impl Parser<'a, String> {
	right(match_literal("</"), left(identifier, match_literal(">")))
	.pred(move \|name\| name == &expected_name)
	}

	fn and_then<'a, P, F, A, B, NextP>(parser: P, f: F) -> impl Parser<'a, B>
	where
	P: Parser<'a, A>,
	NextP: Parser<'a, B>,
	F: Fn(A) -> NextP,
	{
	move \|input\| match parser.parse(input) {
	Ok((next_input, result)) => f(result).parse(next_input),
	Err(err) => Err(err),
	}
	}

	fn parent_element<'a>() -> impl Parser<'a, Element> {
	open_element().and_then(\|el\| {
	left(zero_or_more(element()), close_element(el.name.clone())).map(move \|children\| {
	let mut el = el.clone();
	el.children = children;
	el
	})
	})
	}

	fn whitespace_wrap<'a, P, A>(parser: P) -> impl Parser<'a, A>
	where
	P: Parser<'a, A> + 'a,
	A: 'a,
	{
	right(space0(), left(parser, space0()))
	}

	#[test]
	fn xml_parser() {
	let doc = r#"
	<top label="Top">
	<semi-bottom label="Bottom"/>
	<middle>
	<bottom label="Another bottom"/>
	</middle>
	</top>"#;
	let parsed_doc = Element {
	name: "top".to_string(),
	attributes: vec![("label".to_string(), "Top".to_string())],
	children: vec![
	Element {
	name: "semi-bottom".to_string(),
	attributes: vec![("label".to_string(), "Bottom".to_string())],
	children: vec![],
	},
	Element {
	name: "middle".to_string(),
	attributes: vec![],
	children: vec![Element {
	name: "bottom".to_string(),
	attributes: vec![("label".to_string(), "Another bottom".to_string())],
	children: vec![],
	}],
	},
	],
	};
	assert_eq!(Ok(("", parsed_doc)), element().parse(doc));
	}

	#[test]
	fn mismatched_closing_tag() {
	let doc = r#"
	<top>
	<bottom/>
	</middle>"#;
	assert_eq!(Err("</middle>"), element().parse(doc));
	}
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