use crate::lexer::*;
use crate::spans::*;
use chumsky::{input::ValueInput, prelude::*, recursive::Recursive};
use std::fmt;
use struct_scalpel::Dissectible;

// #[derive(Clone, Debug, PartialEq)]
// pub struct WhenClause {
//     pub cond: Spanned<Ast>,
//     pub body: Spanned<Ast>,
// }

// impl fmt::Display for WhenClause {
//     fn fmt(self: &WhenClause, f: &mut fmt::Formatter) -> fmt::Result {
//         write!(f, "cond: {}, body: {}", self.cond.0, self.body.0)
//     }
// }

// #[derive(Clone, Debug, PartialEq)]
// pub struct MatchClause {
//     pub patt: Spanned<Pattern>,
//     pub guard: Option<Spanned<Ast>>,
//     pub body: Spanned<Ast>,
// }

// impl fmt::Display for MatchClause {
//     fn fmt(self: &MatchClause, f: &mut fmt::Formatter) -> fmt::Result {
//         write!(
//             f,
//             "pattern: {}, guard: {:?} body: {}",
//             self.patt.0, self.guard, self.body.0
//         )
//     }
// }

#[derive(Clone, Debug, PartialEq, Eq)]
pub enum StringPart {
    Data(String),
    Word(String),
    Inline(String),
}

impl fmt::Display for StringPart {
    fn fmt(self: &StringPart, f: &mut fmt::Formatter) -> fmt::Result {
        let rep = match self {
            StringPart::Word(s) => format!("{{{s}}}"),
            StringPart::Data(s) => s.to_string(),
            StringPart::Inline(s) => s.to_string(),
        };
        write!(f, "{}", rep)
    }
}

pub struct LFn {
    name: &'static str,
    clauses: Vec<Spanned<Ast>>,
    doc: Option<&'static str>,
}

#[derive(Clone, Debug, PartialEq, Dissectible)]
pub enum Ast {
    // a special Error node
    // may come in handy?
    Error,

    // expression nodes
    Placeholder,
    Nil,
    Boolean(bool),
    Number(f64),
    Keyword(&'static str),
    Word(&'static str),
    String(&'static str),
    Interpolated(Vec<Spanned<StringPart>>),
    Block(Vec<Spanned<Self>>),
    If(Box<Spanned<Self>>, Box<Spanned<Self>>, Box<Spanned<Self>>),
    Tuple(Vec<Spanned<Self>>),
    Arguments(Vec<Spanned<Self>>),
    List(Vec<Spanned<Self>>),
    Dict(Vec<Spanned<Self>>),
    Let(Box<Spanned<Self>>, Box<Spanned<Self>>),
    LBox(&'static str, Box<Spanned<Self>>),
    Synthetic(Box<Spanned<Self>>, Box<Spanned<Self>>, Vec<Spanned<Self>>),
    When(Vec<Spanned<Self>>),
    WhenClause(Box<Spanned<Self>>, Box<Spanned<Self>>),
    Match(Box<Spanned<Self>>, Vec<Spanned<Self>>),
    MatchClause(
        Box<Spanned<Self>>,
        Box<Option<Spanned<Self>>>,
        Box<Spanned<Self>>,
    ),
    Fn(&'static str, Box<Spanned<Ast>>, Option<&'static str>),
    FnBody(Vec<Spanned<Ast>>),
    FnDeclaration(&'static str),
    Panic(Box<Spanned<Self>>),
    Do(Vec<Spanned<Self>>),
    Repeat(Box<Spanned<Self>>, Box<Spanned<Self>>),
    Splat(&'static str),
    Pair(&'static str, Box<Spanned<Self>>),
    Loop(Box<Spanned<Self>>, Vec<Spanned<Self>>),
    Recur(Vec<Spanned<Self>>),

    // pattern nodes
    NilPattern,
    BooleanPattern(bool),
    NumberPattern(f64),
    StringPattern(&'static str),
    InterpolatedPattern(Vec<Spanned<StringPart>>, StringMatcher),
    KeywordPattern(&'static str),
    WordPattern(&'static str),
    AsPattern(&'static str, &'static str),
    Splattern(Box<Spanned<Self>>),
    PlaceholderPattern,
    TuplePattern(Vec<Spanned<Self>>),
    ListPattern(Vec<Spanned<Self>>),
    PairPattern(&'static str, Box<Spanned<Self>>),
    DictPattern(Vec<Spanned<Self>>),
}

impl fmt::Display for Ast {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use Ast::*;
        match self {
            Error => write!(f, "Error"),
            Nil => write!(f, "nil"),
            String(s) => write!(f, "String: \"{}\"", s),
            Interpolated(strs) => {
                write!(
                    f,
                    "Interpolated: \"{}\"",
                    strs.iter()
                        .map(|(s, _)| s.to_string())
                        .collect::<Vec<_>>()
                        .join("")
                )
            }
            Boolean(b) => write!(f, "Boolean: {}", b),
            Number(n) => write!(f, "Number: {}", n),
            Keyword(k) => write!(f, "Keyword: :{}", k),
            Word(w) => write!(f, "Word: {}", w),
            Block(b) => write!(
                f,
                "Block: <{}>",
                b.iter()
                    .map(|(line, _)| line.to_string())
                    .collect::<Vec<_>>()
                    .join("\n")
            ),
            If(cond, then_branch, else_branch) => write!(
                f,
                "If: {} Then: {} Else: {}",
                cond.0, then_branch.0, else_branch.0
            ),
            Let(pattern, expression) => {
                write!(f, "Let: {} = {}", pattern.0, expression.0)
            }
            Dict(entries) => write!(
                f,
                "#{{{}}}",
                entries
                    .iter()
                    .map(|pair| pair.0.to_string())
                    .collect::<Vec<_>>()
                    .join(", ")
            ),
            List(l) => write!(
                f,
                "List: [{}]",
                l.iter()
                    .map(|(line, _)| line.to_string())
                    .collect::<Vec<_>>()
                    .join("\n")
            ),
            Tuple(t) | Ast::Arguments(t) => write!(
                f,
                "Tuple: ({})",
                t.iter()
                    .map(|(line, _)| line.to_string())
                    .collect::<Vec<_>>()
                    .join("\n")
            ),
            Synthetic(root, first, rest) => write!(
                f,
                "Synth: [{}, {}, {}]",
                root.0,
                first.0,
                rest.iter()
                    .map(|(term, _)| term.to_string())
                    .collect::<Vec<_>>()
                    .join("\n")
            ),
            When(clauses) => write!(
                f,
                "When: [{}]",
                clauses
                    .iter()
                    .map(|clause| clause.0.to_string())
                    .collect::<Vec<_>>()
                    .join("\n")
            ),
            Placeholder => todo!(),
            LBox(_name, _rhs) => todo!(),
            Match(value, clauses) => {
                write!(
                    f,
                    "match: {} with {}",
                    &value.0.to_string(),
                    clauses
                        .iter()
                        .map(|clause| clause.0.to_string())
                        .collect::<Vec<_>>()
                        .join("\n")
                )
            }
            FnBody(clauses) => {
                write!(
                    f,
                    "{}",
                    clauses
                        .iter()
                        .map(|clause| clause.0.to_string())
                        .collect::<Vec<_>>()
                        .join("\n")
                )
            }
            Fn(name, body, ..) => {
                write!(f, "fn: {name}\n{}", body.0)
            }
            FnDeclaration(_name) => todo!(),
            Panic(_expr) => todo!(),
            Do(terms) => {
                write!(
                    f,
                    "do: {}",
                    terms
                        .iter()
                        .map(|(term, _)| term.to_string())
                        .collect::<Vec<_>>()
                        .join(" > ")
                )
            }
            Repeat(_times, _body) => todo!(),
            Splat(word) => {
                write!(f, "splat: {}", word)
            }
            Pair(k, v) => {
                write!(f, "pair: {} {}", k, v.0)
            }
            Loop(init, body) => {
                write!(
                    f,
                    "loop: {} with {}",
                    init.0,
                    body.iter()
                        .map(|clause| clause.0.to_string())
                        .collect::<Vec<_>>()
                        .join("\n")
                )
            }
            Recur(args) => {
                write!(
                    f,
                    "recur: {}",
                    args.iter()
                        .map(|(arg, _)| arg.to_string())
                        .collect::<Vec<_>>()
                        .join(", ")
                )
            }
            MatchClause(pattern, guard, body) => {
                write!(
                    f,
                    "match clause: {} if {:?} -> {}",
                    pattern.0, guard, body.0
                )
            }
            WhenClause(cond, body) => {
                write!(f, "when clause: {} -> {}", cond.0, body.0)
            }

            NilPattern => write!(f, "nil"),
            BooleanPattern(b) => write!(f, "{}", b),
            NumberPattern(n) => write!(f, "{}", n),
            StringPattern(s) => write!(f, "{}", s),
            KeywordPattern(k) => write!(f, ":{}", k),
            WordPattern(w) => write!(f, "{}", w),
            AsPattern(w, t) => write!(f, "{} as :{}", w, t),
            Splattern(p) => write!(f, "...{}", p.0),
            PlaceholderPattern => write!(f, "_"),
            TuplePattern(t) => write!(
                f,
                "({})",
                t.iter()
                    .map(|x| x.0.to_string())
                    .collect::<Vec<_>>()
                    .join(", ")
            ),
            ListPattern(l) => write!(
                f,
                "({})",
                l.iter()
                    .map(|x| x.0.to_string())
                    .collect::<Vec<_>>()
                    .join(", ")
            ),
            DictPattern(entries) => write!(
                f,
                "#{{{}}}",
                entries
                    .iter()
                    .map(|(pair, _)| pair.to_string())
                    .collect::<Vec<_>>()
                    .join(", ")
            ),
            PairPattern(key, value) => write!(f, ":{} {}", key, value.0),
            InterpolatedPattern(strprts, _) => write!(
                f,
                "interpolated: \"{}\"",
                strprts
                    .iter()
                    .map(|part| part.0.to_string())
                    .collect::<Vec<_>>()
                    .join("")
            ),
        }
    }
}

pub struct StringMatcher(pub Box<dyn Fn(String) -> Option<Vec<(String, String)>>>);

impl PartialEq for StringMatcher {
    fn eq(&self, _other: &StringMatcher) -> bool {
        true
    }
}

impl Clone for StringMatcher {
    fn clone(&self) -> StringMatcher {
        unreachable!()
    }
}

impl fmt::Display for StringMatcher {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "string matcher")
    }
}

impl fmt::Debug for StringMatcher {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "string matcher")
    }
}

// #[derive(Clone, Debug, PartialEq)]
// pub enum Pattern {
//     Nil,
//     Boolean(bool),
//     Number(f64),
//     String(&'static str),
//     Interpolated(Vec<Spanned<StringPart>>, StringMatcher),
//     Keyword(&'static str),
//     Word(&'static str),
//     As(&'static str, &'static str),
//     Splattern(Box<Spanned<Self>>),
//     Placeholder,
//     Tuple(Vec<Spanned<Self>>),
//     List(Vec<Spanned<Self>>),
//     Pair(&'static str, Box<Spanned<Self>>),
//     Dict(Vec<Spanned<Self>>),
// }

// impl fmt::Display for Pattern {
//     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//         match self {
//             Pattern::Nil => write!(f, "nil"),
//             Pattern::Boolean(b) => write!(f, "{}", b),
//             Pattern::Number(n) => write!(f, "{}", n),
//             Pattern::String(s) => write!(f, "{}", s),
//             Pattern::Keyword(k) => write!(f, ":{}", k),
//             Pattern::Word(w) => write!(f, "{}", w),
//             Pattern::As(w, t) => write!(f, "{} as {}", w, t),
//             Pattern::Splattern(p) => write!(f, "...{}", p.0),
//             Pattern::Placeholder => write!(f, "_"),
//             Pattern::Tuple(t) => write!(
//                 f,
//                 "({})",
//                 t.iter()
//                     .map(|x| x.0.to_string())
//                     .collect::<Vec<_>>()
//                     .join(", ")
//             ),
//             Pattern::List(l) => write!(
//                 f,
//                 "({})",
//                 l.iter()
//                     .map(|x| x.0.to_string())
//                     .collect::<Vec<_>>()
//                     .join(", ")
//             ),
//             Pattern::Dict(entries) => write!(
//                 f,
//                 "#{{{}}}",
//                 entries
//                     .iter()
//                     .map(|(pair, _)| pair.to_string())
//                     .collect::<Vec<_>>()
//                     .join(", ")
//             ),
//             Pattern::Pair(key, value) => write!(f, ":{} {}", key, value.0),
//             Pattern::Interpolated(strprts, _) => write!(
//                 f,
//                 "interpolated: \"{}\"",
//                 strprts
//                     .iter()
//                     .map(|part| part.0.to_string())
//                     .collect::<Vec<_>>()
//                     .join("")
//             ),
//         }
//     }
// }

fn is_word_char(c: char) -> bool {
    if c.is_ascii_alphanumeric() {
        return true;
    };
    matches!(c, '_' | '/' | '?' | '!')
}

fn parse_string(s: &'static str, span: SimpleSpan) -> Result<Vec<Spanned<StringPart>>, String> {
    let mut parts = vec![];
    let mut current_part = String::new();
    let mut start = span.start;
    let mut is_word = false;

    let mut chars = s.char_indices();
    while let Some((i, char)) = chars.next() {
        match char {
            '{' => {
                if is_word {
                    return Err("interpolations must only contain words".to_string());
                };
                match chars.next() {
                    None => return Err("unclosed brace".to_string()),
                    Some((_, '{')) => current_part.push('{'),
                    Some((i, c)) => {
                        if !current_part.is_empty() {
                            parts.push((
                                StringPart::Data(current_part),
                                SimpleSpan::new(start, start + i),
                            ));
                        };
                        current_part = String::new();
                        start = i;
                        is_word = true;
                        if c.is_ascii_lowercase() {
                            current_part.push(c);
                        } else {
                            return Err("interpolations must only contain words".to_string());
                        }
                    }
                }
            }
            '}' => {
                if is_word {
                    parts.push((
                        StringPart::Word(current_part.clone()),
                        SimpleSpan::new(start, start + i),
                    ));
                    current_part = String::new();
                    start = i;
                    is_word = false;
                } else {
                    match chars.next() {
                        None => return Err("unclosed brace".to_string()),
                        Some((_, c)) => current_part.push(c),
                    }
                }
            }
            _ => {
                if is_word {
                    if is_word_char(char) {
                        current_part.push(char)
                    } else {
                        return Err("interpolations must only contain words".to_string());
                    }
                } else {
                    current_part.push(char)
                }
            }
        }
    }

    if current_part == s {
        parts.push((
            StringPart::Inline(current_part),
            SimpleSpan::new(start, span.end),
        ))
    }

    Ok(parts)
}

pub fn compile_string_pattern(parts: Vec<Spanned<StringPart>>) -> StringMatcher {
    StringMatcher(Box::new(move |scrutinee| {
        let mut last_match = 0;
        let mut parts_iter = parts.iter();
        let mut matches = vec![];
        while let Some((part, _)) = parts_iter.next() {
            match part {
                StringPart::Data(string) => match scrutinee.find(string.as_str()) {
                    Some(i) => {
                        // if i = 0, we're at the beginning
                        if i == 0 && last_match == 0 {
                            last_match = i + string.len();
                            continue;
                        }
                        // in theory, we only hit this branch if the first part is Data
                        unreachable!("internal Ludus error: bad string pattern")
                    }
                    None => return None,
                },
                StringPart::Word(word) => {
                    let to_test = scrutinee.get(last_match..scrutinee.len()).unwrap();
                    match parts_iter.next() {
                        None => matches.push((word.clone(), to_test.to_string())),
                        Some(part) => {
                            let (StringPart::Data(part), _) = part else {
                                unreachable!("internal Ludus error: bad string pattern")
                            };
                            match to_test.find(part) {
                                None => return None,
                                Some(i) => {
                                    matches.push((
                                        word.clone(),
                                        to_test.get(last_match..i).unwrap().to_string(),
                                    ));
                                    last_match = i + part.len();
                                    continue;
                                }
                            }
                        }
                    }
                }
                _ => unreachable!("internal Ludus error"),
            }
        }
        Some(matches)
    }))
}

pub fn parser<I>(
) -> impl Parser<'static, I, Spanned<Ast>, extra::Err<Rich<'static, Token, Span>>> + Clone
where
    I: ValueInput<'static, Token = Token, Span = Span>,
{
    use Ast::*;

    let mut expr = Recursive::declare();

    let mut pattern = Recursive::declare();

    let mut simple = Recursive::declare();

    let mut nonbinding = Recursive::declare();

    let separators = recursive(|separators| {
        just(Token::Punctuation(","))
            .or(just(Token::Punctuation("\n")))
            .then(separators.clone().repeated())
    });

    let terminators = recursive(|terminators| {
        just(Token::Punctuation(";"))
            .or(just(Token::Punctuation("\n")))
            .then(terminators.clone().repeated())
    });

    let placeholder_pattern =
        select! {Token::Punctuation("_") => PlaceholderPattern}.map_with(|p, e| (p, e.span()));

    let word_pattern = select! { Token::Word(w) => WordPattern(w) }.map_with(|w, e| (w, e.span()));

    let atom_pattern = select! {
        Token::Nil => NilPattern,
        Token::Boolean(b) => BooleanPattern(b),
        Token::Number(n) => NumberPattern(n),
        Token::Keyword(k) => KeywordPattern(k),
    }
    .map_with(|a, e| (a, e.span()));

    let string_pattern = select! {Token::String(s) => s}.try_map_with(|s, e| {
        let parsed = parse_string(s, e.span());
        match parsed {
            Ok(parts) => match parts[0] {
                (StringPart::Inline(_), _) => Ok((StringPattern(s), e.span())),
                _ => Ok((
                    InterpolatedPattern(parts.clone(), compile_string_pattern(parts)),
                    e.span(),
                )),
            },
            Err(msg) => Err(Rich::custom(e.span(), msg)),
        }
    });

    let bare_splat = just(Token::Punctuation("...")).map_with(|_, e| {
        (
            Splattern(Box::new((PlaceholderPattern, e.span()))),
            e.span(),
        )
    });

    let splattable = word_pattern.or(placeholder_pattern);

    let patt_splat = just(Token::Punctuation("..."))
        .ignore_then(splattable)
        .map_with(|x, e| (Splattern(Box::new(x)), e.span()));

    let splattern = patt_splat.or(bare_splat);

    let tuple_pattern = pattern
        .clone()
        .or(splattern.clone())
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(just(Token::Punctuation("(")), just(Token::Punctuation(")")))
        .map_with(|tuple, e| (TuplePattern(tuple), e.span()))
        .labelled("tuple pattern");

    let list_pattern = pattern
        .clone()
        .or(splattern.clone())
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(just(Token::Punctuation("[")), just(Token::Punctuation("]")))
        .map_with(|list, e| (ListPattern(list), e.span()));

    let pair_pattern = select! {Token::Keyword(k) => k}
        .then(pattern.clone())
        .map_with(|(key, patt), e| (PairPattern(key, Box::new(patt)), e.span()));

    let shorthand_pattern = select! {Token::Word(w) => w}.map_with(|w, e| {
        (
            PairPattern(w, Box::new((WordPattern(w), e.span()))),
            e.span(),
        )
    });

    let dict_pattern = pair_pattern
        .or(shorthand_pattern)
        .or(splattern.clone())
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(
            just(Token::Punctuation("#{")),
            just(Token::Punctuation("}")),
        )
        .map_with(|dict, e| (DictPattern(dict), e.span()));

    let keyword = select! {Token::Keyword(k) => Keyword(k),}.map_with(|k, e| (k, e.span()));

    let as_pattern = select! {Token::Word(w) => w}
        .then_ignore(just(Token::Reserved("as")))
        .then(select! {Token::Keyword(k) => k})
        .map_with(|(w, t), e| (AsPattern(w, t), e.span()));

    pattern.define(
        atom_pattern
            .or(string_pattern)
            .or(as_pattern)
            .or(word_pattern)
            .or(placeholder_pattern)
            .or(tuple_pattern.clone())
            .or(list_pattern)
            .or(dict_pattern)
            .labelled("pattern"),
    );

    let placeholder =
        select! {Token::Punctuation("_") => Placeholder}.map_with(|p, e| (p, e.span()));

    let word = select! { Token::Word(w) => Word(w) }
        .map_with(|w, e| (w, e.span()))
        .labelled("word");

    let value = select! {
        Token::Nil => Nil,
        Token::Boolean(b) => Boolean(b),
        Token::Number(n) => Number(n),
    }
    .map_with(|v, e| (v, e.span()));

    let string = select! {Token::String(s) => s}.try_map_with(|s, e| {
        let parsed = parse_string(s, e.span());
        match parsed {
            Ok(parts) => match parts[0] {
                (StringPart::Inline(_), _) => Ok((String(s), e.span())),
                _ => Ok((Interpolated(parts), e.span())),
            },
            Err(msg) => Err(Rich::custom(e.span(), msg)),
        }
    });

    let tuple = simple
        .clone()
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(just(Token::Punctuation("(")), just(Token::Punctuation(")")))
        .map_with(|tuple, e| (Tuple(tuple), e.span()));

    let args = simple
        .clone()
        .or(placeholder)
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(just(Token::Punctuation("(")), just(Token::Punctuation(")")))
        .map_with(|args, e| (Arguments(args), e.span()));

    let synth_root = word.or(keyword);

    let synth_term = keyword.or(args);

    let synthetic = synth_root
        .then(synth_term.clone())
        .then(synth_term.clone().repeated().collect())
        .map_with(|((root, first), rest), e| {
            (Synthetic(Box::new(root), Box::new(first), rest), e.span())
        });

    let splat = just(Token::Punctuation("..."))
        .ignore_then(word)
        .map_with(|(w, _), e| {
            (
                Splat(if let Word(w) = w { w } else { unreachable!() }),
                e.span(),
            )
        });

    let list = simple
        .clone()
        .or(splat.clone())
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(just(Token::Punctuation("[")), just(Token::Punctuation("]")))
        .map_with(|list, e| (List(list), e.span()));

    let pair = select! {Token::Keyword(k) => k}
        .then(simple.clone())
        .map_with(|(key, value), e| (Pair(key, Box::new(value)), e.span()));

    let shorthand = select! {Token::Word(w) => w}
        .map_with(|w, e| (Pair(w, Box::new((Word(w), e.span()))), e.span()));

    let dict = pair
        .or(shorthand)
        .or(splat.clone())
        .separated_by(separators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(
            just(Token::Punctuation("#{")),
            just(Token::Punctuation("}")),
        )
        .map_with(|dict, e| (Dict(dict), e.span()));

    let recur = just(Token::Reserved("recur"))
        .ignore_then(tuple.clone())
        .map_with(|args, e| {
            let (Tuple(args), _) = args else {
                unreachable!()
            };
            (Recur(args), e.span())
        });

    let block = expr
        .clone()
        .separated_by(terminators.clone())
        .allow_leading()
        .allow_trailing()
        .collect()
        .delimited_by(just(Token::Punctuation("{")), just(Token::Punctuation("}")))
        .map_with(|block, e| (Block(block), e.span()))
        .recover_with(via_parser(nested_delimiters(
            Token::Punctuation("{"),
            Token::Punctuation("}"),
            [
                (Token::Punctuation("("), Token::Punctuation(")")),
                (Token::Punctuation("["), Token::Punctuation("]")),
            ],
            |span| (Error, span),
        )));

    let if_ = just(Token::Reserved("if"))
        .ignore_then(simple.clone())
        .then_ignore(terminators.clone().or_not())
        .then_ignore(just(Token::Reserved("then")))
        .then(expr.clone())
        .then_ignore(terminators.clone().or_not())
        .then_ignore(just(Token::Reserved("else")))
        .then(expr.clone())
        .map_with(|((condition, then_branch), else_branch), e| {
            (
                If(
                    Box::new(condition),
                    Box::new(then_branch),
                    Box::new(else_branch),
                ),
                e.span(),
            )
        });

    let when_clause = simple
        .clone()
        .then_ignore(just(Token::Punctuation("->")))
        .then(expr.clone())
        .map_with(|(cond, body), e| (WhenClause(Box::new(cond), Box::new(body)), e.span()));

    let when = just(Token::Reserved("when"))
        .ignore_then(
            when_clause
                .separated_by(terminators.clone())
                .allow_trailing()
                .allow_leading()
                .collect()
                .delimited_by(just(Token::Punctuation("{")), just(Token::Punctuation("}"))),
        )
        .map_with(|clauses, e| (When(clauses), e.span()));

    let guarded_clause = pattern
        .clone()
        .then_ignore(just(Token::Reserved("if")))
        .then(simple.clone())
        .then_ignore(just(Token::Punctuation("->")))
        .then(expr.clone())
        .map_with(|((patt, guard), body), e| {
            (
                MatchClause(Box::new(patt), Box::new(Some(guard)), Box::new(body)),
                e.span(),
            )
        });

    let match_clause = pattern
        .clone()
        .then_ignore(just(Token::Punctuation("->")))
        .then(expr.clone())
        .map_with(|(patt, body), e| {
            (
                MatchClause(Box::new(patt), Box::new(None), Box::new(body)),
                e.span(),
            )
        });

    let r#match = just(Token::Reserved("match"))
        .ignore_then(simple.clone())
        .then_ignore(just(Token::Reserved("with")))
        .then(
            match_clause
                .clone()
                .or(guarded_clause)
                .separated_by(terminators.clone())
                .allow_leading()
                .allow_trailing()
                .collect()
                .delimited_by(just(Token::Punctuation("{")), just(Token::Punctuation("}"))),
        )
        .map_with(|(expr, clauses), e| (Match(Box::new(expr), clauses), e.span()));

    let conditional = when.or(if_).or(r#match);

    let panic = just(Token::Reserved("panic!"))
        .ignore_then(nonbinding.clone())
        .map_with(|expr, e| (Panic(Box::new(expr)), e.span()));

    let do_ = just(Token::Reserved("do"))
        .ignore_then(
            nonbinding
                .clone()
                .separated_by(
                    just(Token::Punctuation(">")).then(just(Token::Punctuation("\n")).repeated()),
                )
                .collect(),
        )
        .map_with(|exprs, e| (Do(exprs), e.span()));

    let repeat = just(Token::Reserved("repeat"))
        .ignore_then(simple.clone())
        .then(block.clone())
        .map_with(|(count, body), e| (Repeat(Box::new(count), Box::new(body)), e.span()));

    let fn_guarded = tuple_pattern
        .clone()
        .then_ignore(just(Token::Reserved("if")))
        .then(simple.clone())
        .then_ignore(just(Token::Punctuation("->")))
        .then(nonbinding.clone())
        .map_with(|((patt, guard), body), e| {
            (
                MatchClause(Box::new(patt), Box::new(Some(guard)), Box::new(body)),
                e.span(),
            )
        })
        .labelled("function clause");

    let fn_unguarded = tuple_pattern
        .clone()
        .then_ignore(just(Token::Punctuation("->")))
        .then(nonbinding.clone())
        .map_with(|(patt, body), e| {
            (
                MatchClause(Box::new(patt), Box::new(None), Box::new(body)),
                e.span(),
            )
        })
        .labelled("function clause");

    let fn_clause = fn_guarded.clone().or(fn_unguarded.clone());

    let lambda = just(Token::Reserved("fn"))
        .ignore_then(fn_unguarded.clone())
        .map_with(|clause, e| {
            (
                Fn("", Box::new((Ast::FnBody(vec![clause]), e.span())), None),
                e.span(),
            )
        });

    let fn_clauses = fn_clause
        .clone()
        .separated_by(terminators.clone())
        .allow_leading()
        .allow_trailing()
        .collect();

    let loop_multiclause = fn_clauses
        .clone()
        .delimited_by(just(Token::Punctuation("{")), just(Token::Punctuation("}")));

    let fn_single_clause = fn_clause.clone().map_with(|c, _| vec![c]);

    let r#loop = just(Token::Reserved("loop"))
        .ignore_then(tuple.clone())
        .then_ignore(just(Token::Reserved("with")))
        .then(loop_multiclause.clone().or(fn_single_clause.clone()))
        .map_with(|(init, body), e| (Loop(Box::new(init), body), e.span()));

    simple.define(
        synthetic
            .or(recur)
            .or(word)
            .or(keyword)
            .or(value)
            .or(tuple.clone())
            .or(list)
            .or(dict)
            .or(string)
            .or(lambda.clone())
            .labelled("simple expression"),
    );

    nonbinding.define(
        simple
            .clone()
            .or(conditional)
            .or(block)
            .or(panic)
            .or(do_)
            .or(repeat)
            .or(r#loop)
            .labelled("nonbinding expression"),
    );

    let let_ = just(Token::Reserved("let"))
        .ignore_then(pattern.clone())
        .then_ignore(just(Token::Punctuation("=")))
        .then(nonbinding.clone())
        .map_with(|(pattern, expression), e| {
            (Let(Box::new(pattern), Box::new(expression)), e.span())
        });

    let box_ = just(Token::Reserved("box"))
        .ignore_then(word)
        .then_ignore(just(Token::Punctuation("=")))
        .then(nonbinding.clone())
        .map_with(|(word, expr), e| {
            let name = if let Word(w) = word.0 {
                w
            } else {
                unreachable!()
            };
            (LBox(name, Box::new(expr)), e.span())
        });

    let fn_decl = just(Token::Reserved("fn"))
        .ignore_then(word)
        .map_with(|(word, _), e| {
            let name = if let Word(w) = word {
                w
            } else {
                unreachable!()
            };
            (FnDeclaration(name), e.span())
        });

    let fn_named = just(Token::Reserved("fn"))
        .ignore_then(word)
        .then(fn_unguarded.clone())
        .map_with(|(word, clause), e| {
            let name = if let Word(word) = word.0 {
                word
            } else {
                unreachable!()
            };
            (
                Fn(name, Box::new((Ast::FnBody(vec![clause]), e.span())), None),
                e.span(),
            )
        });

    let docstr = select! {Token::String(s) => s};

    let fn_multiclause = separators
        .clone()
        .or_not()
        .ignore_then(docstr.or_not())
        .then(fn_clauses.clone())
        .delimited_by(just(Token::Punctuation("{")), just(Token::Punctuation("}")))
        .map_with(|(docstr, clauses), e| (docstr, clauses, e.span()));

    let fn_compound = just(Token::Reserved("fn"))
        .ignore_then(word)
        .then(fn_multiclause)
        .map_with(|(word, (docstr, clauses, _)), e| {
            let name = if let Word(word) = word.0 {
                word
            } else {
                unreachable!()
            };
            (
                Fn(name, Box::new((Ast::FnBody(clauses), e.span())), docstr),
                e.span(),
            )
        });

    let fn_ = fn_named.or(fn_compound).or(fn_decl);

    let binding = let_.or(box_).or(fn_);

    expr.define(binding.or(nonbinding));

    let script = expr
        .separated_by(terminators.clone())
        .allow_trailing()
        .allow_leading()
        .collect()
        .map_with(|exprs, e| (Block(exprs), e.span()));

    script
}