//! Lexer for the Lux language

#![allow(dead_code)]

use crate::ast::Span;
use std::fmt;
use std::iter::Peekable;
use std::str::Chars;

/// Part of an interpolated string
#[derive(Debug, Clone, PartialEq)]
pub enum StringPart {
    /// Literal text
    Literal(String),
    /// Expression to be evaluated (stored as source text to be parsed later)
    Expr(String),
}

/// Token types
#[derive(Debug, Clone, PartialEq)]
pub enum TokenKind {
    // Literals
    Int(i64),
    Float(f64),
    String(String),
    /// Interpolated string with embedded expressions: "Hello, {name}!"
    InterpolatedString(Vec<StringPart>),
    Char(char),
    Bool(bool),

    // Identifiers and keywords
    Ident(String),

    // Keywords
    Fn,
    Let,
    If,
    Then,
    Else,
    Match,
    With,
    Effect,
    Handler,
    Run,
    Resume,
    Type,
    True,
    False,
    Import,
    Pub,
    As,
    From,   // from (for migrations)
    Latest, // latest (for @latest version constraint)
    Trait,  // trait (for type classes)
    Impl,   // impl (for trait implementations)
    For,    // for (in impl Trait for Type)

    // Documentation
    DocComment(String), // /// doc comment

    // Behavioral type keywords
    Is,            // is (for behavioral properties)
    Pure,          // pure
    Total,         // total
    Idempotent,    // idempotent
    Deterministic, // deterministic
    Commutative,   // commutative
    Where,         // where (for constraints)
    Assume,        // assume (for unverified properties)

    // Operators
    Plus,       // +
    PlusPlus,   // ++
    Minus,      // -
    Star,       // *
    Slash,      // /
    Percent,    // %
    Eq,         // =
    EqEq,       // ==
    Ne,         // !=
    Lt,         // <
    Le,         // <=
    Gt,         // >
    Ge,         // >=
    And,        // &&
    Or,         // ||
    Not,        // !
    Pipe,       // |
    PipeGt,     // |>
    Arrow,      // =>
    ThinArrow,  // ->
    Dot,        // .
    DotDotDot,  // ...
    Colon,      // :
    ColonColon, // ::
    Comma,      // ,
    Semi,       // ;
    At,         // @

    // Delimiters
    LParen,   // (
    RParen,   // )
    LBrace,   // {
    RBrace,   // }
    LBracket, // [
    RBracket, // ]

    // Special
    Underscore, // _
    Newline,
    Eof,
}

impl fmt::Display for TokenKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            TokenKind::Int(n) => write!(f, "{}", n),
            TokenKind::Float(n) => write!(f, "{}", n),
            TokenKind::String(s) => write!(f, "\"{}\"", s),
            TokenKind::InterpolatedString(parts) => {
                write!(f, "\"")?;
                for part in parts {
                    match part {
                        StringPart::Literal(s) => write!(f, "{}", s)?,
                        StringPart::Expr(e) => write!(f, "{{{}}}", e)?,
                    }
                }
                write!(f, "\"")
            }
            TokenKind::Char(c) => write!(f, "'{}'", c),
            TokenKind::Bool(b) => write!(f, "{}", b),
            TokenKind::Ident(s) => write!(f, "{}", s),
            TokenKind::Fn => write!(f, "fn"),
            TokenKind::Let => write!(f, "let"),
            TokenKind::If => write!(f, "if"),
            TokenKind::Then => write!(f, "then"),
            TokenKind::Else => write!(f, "else"),
            TokenKind::Match => write!(f, "match"),
            TokenKind::With => write!(f, "with"),
            TokenKind::Effect => write!(f, "effect"),
            TokenKind::Handler => write!(f, "handler"),
            TokenKind::Run => write!(f, "run"),
            TokenKind::Resume => write!(f, "resume"),
            TokenKind::Type => write!(f, "type"),
            TokenKind::Import => write!(f, "import"),
            TokenKind::Pub => write!(f, "pub"),
            TokenKind::As => write!(f, "as"),
            TokenKind::From => write!(f, "from"),
            TokenKind::Latest => write!(f, "latest"),
            TokenKind::Trait => write!(f, "trait"),
            TokenKind::Impl => write!(f, "impl"),
            TokenKind::For => write!(f, "for"),
            TokenKind::DocComment(s) => write!(f, "/// {}", s),
            TokenKind::Is => write!(f, "is"),
            TokenKind::Pure => write!(f, "pure"),
            TokenKind::Total => write!(f, "total"),
            TokenKind::Idempotent => write!(f, "idempotent"),
            TokenKind::Deterministic => write!(f, "deterministic"),
            TokenKind::Commutative => write!(f, "commutative"),
            TokenKind::Where => write!(f, "where"),
            TokenKind::Assume => write!(f, "assume"),
            TokenKind::True => write!(f, "true"),
            TokenKind::False => write!(f, "false"),
            TokenKind::Plus => write!(f, "+"),
            TokenKind::PlusPlus => write!(f, "++"),
            TokenKind::Minus => write!(f, "-"),
            TokenKind::Star => write!(f, "*"),
            TokenKind::Slash => write!(f, "/"),
            TokenKind::Percent => write!(f, "%"),
            TokenKind::Eq => write!(f, "="),
            TokenKind::EqEq => write!(f, "=="),
            TokenKind::Ne => write!(f, "!="),
            TokenKind::Lt => write!(f, "<"),
            TokenKind::Le => write!(f, "<="),
            TokenKind::Gt => write!(f, ">"),
            TokenKind::Ge => write!(f, ">="),
            TokenKind::And => write!(f, "&&"),
            TokenKind::Or => write!(f, "||"),
            TokenKind::Not => write!(f, "!"),
            TokenKind::Pipe => write!(f, "|"),
            TokenKind::PipeGt => write!(f, "|>"),
            TokenKind::Arrow => write!(f, "=>"),
            TokenKind::ThinArrow => write!(f, "->"),
            TokenKind::Dot => write!(f, "."),
            TokenKind::DotDotDot => write!(f, "..."),
            TokenKind::Colon => write!(f, ":"),
            TokenKind::ColonColon => write!(f, "::"),
            TokenKind::Comma => write!(f, ","),
            TokenKind::Semi => write!(f, ";"),
            TokenKind::At => write!(f, "@"),
            TokenKind::LParen => write!(f, "("),
            TokenKind::RParen => write!(f, ")"),
            TokenKind::LBrace => write!(f, "{{"),
            TokenKind::RBrace => write!(f, "}}"),
            TokenKind::LBracket => write!(f, "["),
            TokenKind::RBracket => write!(f, "]"),
            TokenKind::Underscore => write!(f, "_"),
            TokenKind::Newline => write!(f, "\\n"),
            TokenKind::Eof => write!(f, "EOF"),
        }
    }
}

/// A token with its source location
#[derive(Debug, Clone)]
pub struct Token {
    pub kind: TokenKind,
    pub span: Span,
}

impl Token {
    pub fn new(kind: TokenKind, span: Span) -> Self {
        Self { kind, span }
    }
}

/// Lexer error
#[derive(Debug, Clone)]
pub struct LexError {
    pub message: String,
    pub span: Span,
}

impl fmt::Display for LexError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "Lexer error at {}-{}: {}",
            self.span.start, self.span.end, self.message
        )
    }
}

/// The lexer
pub struct Lexer<'a> {
    source: &'a str,
    chars: Peekable<Chars<'a>>,
    pos: usize,
}

impl<'a> Lexer<'a> {
    pub fn new(source: &'a str) -> Self {
        Self {
            source,
            chars: source.chars().peekable(),
            pos: 0,
        }
    }

    /// Tokenize the entire source
    pub fn tokenize(mut self) -> Result<Vec<Token>, LexError> {
        let mut tokens = Vec::new();
        loop {
            let token = self.next_token()?;
            let is_eof = token.kind == TokenKind::Eof;
            tokens.push(token);
            if is_eof {
                break;
            }
        }
        Ok(tokens)
    }

    fn next_token(&mut self) -> Result<Token, LexError> {
        self.skip_whitespace_and_comments();

        let start = self.pos;

        let Some(c) = self.advance() else {
            return Ok(Token::new(TokenKind::Eof, Span::new(start, start)));
        };

        let kind = match c {
            // Single-character tokens
            '+' => {
                if self.peek() == Some('+') {
                    self.advance();
                    TokenKind::PlusPlus
                } else {
                    TokenKind::Plus
                }
            }
            '*' => TokenKind::Star,
            '%' => TokenKind::Percent,
            '(' => TokenKind::LParen,
            ')' => TokenKind::RParen,
            '{' => TokenKind::LBrace,
            '}' => TokenKind::RBrace,
            '[' => TokenKind::LBracket,
            ']' => TokenKind::RBracket,
            ',' => TokenKind::Comma,
            ';' => TokenKind::Semi,
            '@' => TokenKind::At,
            '\n' => TokenKind::Newline,

            // Multi-character tokens
            '-' => {
                if self.peek() == Some('>') {
                    self.advance();
                    TokenKind::ThinArrow
                } else {
                    TokenKind::Minus
                }
            }
            '/' => {
                if self.peek() == Some('/') {
                    self.advance(); // consume second '/'
                    // Check if this is a doc comment (///)
                    if self.peek() == Some('/') {
                        self.advance(); // consume third '/'
                        return Ok(self.scan_doc_comment(start));
                    } else {
                        // Regular line comment
                        self.skip_line_comment();
                        return self.next_token();
                    }
                } else {
                    TokenKind::Slash
                }
            }
            '=' => {
                if self.peek() == Some('=') {
                    self.advance();
                    TokenKind::EqEq
                } else if self.peek() == Some('>') {
                    self.advance();
                    TokenKind::Arrow
                } else {
                    TokenKind::Eq
                }
            }
            '!' => {
                if self.peek() == Some('=') {
                    self.advance();
                    TokenKind::Ne
                } else {
                    TokenKind::Not
                }
            }
            '<' => {
                if self.peek() == Some('=') {
                    self.advance();
                    TokenKind::Le
                } else {
                    TokenKind::Lt
                }
            }
            '>' => {
                if self.peek() == Some('=') {
                    self.advance();
                    TokenKind::Ge
                } else {
                    TokenKind::Gt
                }
            }
            '&' => {
                if self.peek() == Some('&') {
                    self.advance();
                    TokenKind::And
                } else {
                    return Err(LexError {
                        message: "Expected '&&'".into(),
                        span: Span::new(start, self.pos),
                    });
                }
            }
            '|' => {
                if self.peek() == Some('|') {
                    self.advance();
                    TokenKind::Or
                } else if self.peek() == Some('>') {
                    self.advance();
                    TokenKind::PipeGt
                } else {
                    TokenKind::Pipe
                }
            }
            '.' => {
                if self.peek() == Some('.') {
                    // Check for ... (need to peek past second dot)
                    // We look at source directly since we can only peek one ahead
                    let next_next = self.source[self.pos..].chars().nth(1);
                    if next_next == Some('.') {
                        self.advance(); // consume second '.'
                        self.advance(); // consume third '.'
                        TokenKind::DotDotDot
                    } else {
                        TokenKind::Dot
                    }
                } else {
                    TokenKind::Dot
                }
            }
            ':' => {
                if self.peek() == Some(':') {
                    self.advance();
                    TokenKind::ColonColon
                } else {
                    TokenKind::Colon
                }
            }
            '_' => {
                if self.peek().map_or(false, |c| c.is_alphanumeric()) {
                    // It's an identifier starting with _
                    self.scan_ident_rest(start)
                } else {
                    TokenKind::Underscore
                }
            }

            // String literals
            '"' => self.scan_string(start)?,

            // Char literals
            '\'' => self.scan_char(start)?,

            // Numbers
            c if c.is_ascii_digit() => self.scan_number(c, start)?,

            // Identifiers and keywords
            c if c.is_alphabetic() || c == '_' => self.scan_ident_rest(start),

            _ => {
                return Err(LexError {
                    message: format!("Unexpected character: '{}'", c),
                    span: Span::new(start, self.pos),
                });
            }
        };

        Ok(Token::new(kind, Span::new(start, self.pos)))
    }

    fn advance(&mut self) -> Option<char> {
        let c = self.chars.next()?;
        self.pos += c.len_utf8();
        Some(c)
    }

    fn peek(&mut self) -> Option<char> {
        self.chars.peek().copied()
    }

    fn skip_whitespace_and_comments(&mut self) {
        while let Some(c) = self.peek() {
            if c == ' ' || c == '\t' || c == '\r' {
                self.advance();
            } else if c == '/' {
                // Check for comment
                let mut chars = self.chars.clone();
                chars.next(); // consume '/'
                if chars.peek() == Some(&'/') {
                    self.skip_line_comment();
                } else {
                    break;
                }
            } else {
                break;
            }
        }
    }

    fn skip_line_comment(&mut self) {
        while let Some(c) = self.peek() {
            if c == '\n' {
                break;
            }
            self.advance();
        }
    }

    fn scan_doc_comment(&mut self, start: usize) -> Token {
        // Skip leading whitespace after ///
        while self.peek() == Some(' ') || self.peek() == Some('\t') {
            self.advance();
        }

        // Collect the rest of the line
        let mut content = String::new();
        while let Some(c) = self.peek() {
            if c == '\n' {
                break;
            }
            content.push(c);
            self.advance();
        }

        Token::new(
            TokenKind::DocComment(content.trim_end().to_string()),
            Span::new(start, self.pos),
        )
    }

    fn scan_string(&mut self, _start: usize) -> Result<TokenKind, LexError> {
        let mut parts: Vec<StringPart> = Vec::new();
        let mut current_literal = String::new();

        loop {
            match self.advance() {
                Some('"') => break,
                Some('\\') => {
                    // Check for escaped brace
                    match self.peek() {
                        Some('{') => {
                            self.advance();
                            current_literal.push('{');
                        }
                        Some('}') => {
                            self.advance();
                            current_literal.push('}');
                        }
                        _ => {
                            let escape_start = self.pos;
                            let escaped = match self.advance() {
                                Some('n') => '\n',
                                Some('r') => '\r',
                                Some('t') => '\t',
                                Some('\\') => '\\',
                                Some('"') => '"',
                                Some('0') => '\0',
                                Some('\'') => '\'',
                                Some('{') => '{',
                                Some('}') => '}',
                                Some('x') => {
                                    // Hex escape \xNN
                                    let h1 = self.advance().and_then(|c| c.to_digit(16));
                                    let h2 = self.advance().and_then(|c| c.to_digit(16));
                                    match (h1, h2) {
                                        (Some(d1), Some(d2)) => {
                                            let byte = (d1 * 16 + d2) as u8;
                                            byte as char
                                        }
                                        _ => {
                                            return Err(LexError {
                                                message: "Invalid hex escape sequence: expected \\xNN".into(),
                                                span: Span::new(escape_start - 1, self.pos),
                                            });
                                        }
                                    }
                                }
                                Some('u') => {
                                    // Unicode escape \u{NNNN} or \uNNNN
                                    if self.peek() == Some('{') {
                                        self.advance(); // consume '{'
                                        let mut hex = String::new();
                                        while let Some(c) = self.peek() {
                                            if c == '}' {
                                                self.advance();
                                                break;
                                            }
                                            if c.is_ascii_hexdigit() {
                                                hex.push(c);
                                                self.advance();
                                            } else {
                                                return Err(LexError {
                                                    message: format!("Invalid unicode escape: expected hex digit, got '{}'", c),
                                                    span: Span::new(escape_start - 1, self.pos),
                                                });
                                            }
                                        }
                                        match u32::from_str_radix(&hex, 16).ok().and_then(char::from_u32) {
                                            Some(c) => c,
                                            None => {
                                                return Err(LexError {
                                                    message: format!("Invalid unicode escape: \\u{{{}}}", hex),
                                                    span: Span::new(escape_start - 1, self.pos),
                                                });
                                            }
                                        }
                                    } else {
                                        // \uNNNN format (4 hex digits)
                                        let mut hex = String::new();
                                        for _ in 0..4 {
                                            match self.advance() {
                                                Some(c) if c.is_ascii_hexdigit() => hex.push(c),
                                                _ => {
                                                    return Err(LexError {
                                                        message: "Invalid unicode escape: expected 4 hex digits".into(),
                                                        span: Span::new(escape_start - 1, self.pos),
                                                    });
                                                }
                                            }
                                        }
                                        match u32::from_str_radix(&hex, 16).ok().and_then(char::from_u32) {
                                            Some(c) => c,
                                            None => {
                                                return Err(LexError {
                                                    message: format!("Invalid unicode escape: \\u{}", hex),
                                                    span: Span::new(escape_start - 1, self.pos),
                                                });
                                            }
                                        }
                                    }
                                }
                                Some(c) => {
                                    return Err(LexError {
                                        message: format!("Invalid escape sequence: \\{}", c),
                                        span: Span::new(escape_start - 1, self.pos),
                                    });
                                }
                                None => {
                                    return Err(LexError {
                                        message: "Unterminated string".into(),
                                        span: Span::new(_start, self.pos),
                                    });
                                }
                            };
                            current_literal.push(escaped);
                        }
                    }
                }
                Some('{') => {
                    // Start of interpolation
                    if !current_literal.is_empty() {
                        parts.push(StringPart::Literal(std::mem::take(&mut current_literal)));
                    }

                    // Scan the expression until matching '}'
                    let mut expr_text = String::new();
                    let mut brace_depth = 1;

                    loop {
                        match self.advance() {
                            Some('{') => {
                                brace_depth += 1;
                                expr_text.push('{');
                            }
                            Some('}') => {
                                brace_depth -= 1;
                                if brace_depth == 0 {
                                    break;
                                }
                                expr_text.push('}');
                            }
                            Some(c) => expr_text.push(c),
                            None => {
                                return Err(LexError {
                                    message: "Unterminated interpolation in string".into(),
                                    span: Span::new(_start, self.pos),
                                });
                            }
                        }
                    }

                    parts.push(StringPart::Expr(expr_text));
                }
                Some(c) => current_literal.push(c),
                None => {
                    return Err(LexError {
                        message: "Unterminated string".into(),
                        span: Span::new(_start, self.pos),
                    });
                }
            }
        }

        // If we have no interpolations, return a simple string
        if parts.is_empty() {
            return Ok(TokenKind::String(current_literal));
        }

        // Add any remaining literal
        if !current_literal.is_empty() {
            parts.push(StringPart::Literal(current_literal));
        }

        Ok(TokenKind::InterpolatedString(parts))
    }

    fn scan_char(&mut self, start: usize) -> Result<TokenKind, LexError> {
        let c = match self.advance() {
            Some('\\') => match self.advance() {
                Some('n') => '\n',
                Some('r') => '\r',
                Some('t') => '\t',
                Some('\\') => '\\',
                Some('\'') => '\'',
                Some(c) => c,
                None => {
                    return Err(LexError {
                        message: "Unterminated character literal".into(),
                        span: Span::new(start, self.pos),
                    });
                }
            },
            Some(c) => c,
            None => {
                return Err(LexError {
                    message: "Unterminated character literal".into(),
                    span: Span::new(start, self.pos),
                });
            }
        };

        if self.advance() != Some('\'') {
            return Err(LexError {
                message: "Expected closing quote for character literal".into(),
                span: Span::new(start, self.pos),
            });
        }

        Ok(TokenKind::Char(c))
    }

    fn scan_number(&mut self, first: char, start: usize) -> Result<TokenKind, LexError> {
        let mut num_str = String::new();
        num_str.push(first);

        while let Some(c) = self.peek() {
            if c.is_ascii_digit() || c == '_' {
                if c != '_' {
                    num_str.push(c);
                }
                self.advance();
            } else {
                break;
            }
        }

        // Check for float
        if self.peek() == Some('.') {
            // Look ahead to make sure it's not a method call
            let mut chars = self.chars.clone();
            chars.next(); // consume '.'
            if chars.peek().map_or(false, |c| c.is_ascii_digit()) {
                self.advance(); // consume '.'
                num_str.push('.');
                while let Some(c) = self.peek() {
                    if c.is_ascii_digit() || c == '_' {
                        if c != '_' {
                            num_str.push(c);
                        }
                        self.advance();
                    } else {
                        break;
                    }
                }
                let f: f64 = num_str.parse().map_err(|_| LexError {
                    message: "Invalid float literal".into(),
                    span: Span::new(start, self.pos),
                })?;
                return Ok(TokenKind::Float(f));
            }
        }

        let n: i64 = num_str.parse().map_err(|_| LexError {
            message: "Invalid integer literal".into(),
            span: Span::new(start, self.pos),
        })?;
        Ok(TokenKind::Int(n))
    }

    fn scan_ident_rest(&mut self, start: usize) -> TokenKind {
        while let Some(c) = self.peek() {
            if c.is_alphanumeric() || c == '_' {
                self.advance();
            } else {
                break;
            }
        }

        let ident = &self.source[start..self.pos];
        match ident {
            "fn" => TokenKind::Fn,
            "let" => TokenKind::Let,
            "if" => TokenKind::If,
            "then" => TokenKind::Then,
            "else" => TokenKind::Else,
            "match" => TokenKind::Match,
            "with" => TokenKind::With,
            "effect" => TokenKind::Effect,
            "handler" => TokenKind::Handler,
            "run" => TokenKind::Run,
            "resume" => TokenKind::Resume,
            "type" => TokenKind::Type,
            "import" => TokenKind::Import,
            "pub" => TokenKind::Pub,
            "as" => TokenKind::As,
            "from" => TokenKind::From,
            "latest" => TokenKind::Latest,
            "trait" => TokenKind::Trait,
            "impl" => TokenKind::Impl,
            "for" => TokenKind::For,
            "is" => TokenKind::Is,
            "pure" => TokenKind::Pure,
            "total" => TokenKind::Total,
            "idempotent" => TokenKind::Idempotent,
            "deterministic" => TokenKind::Deterministic,
            "commutative" => TokenKind::Commutative,
            "where" => TokenKind::Where,
            "assume" => TokenKind::Assume,
            "true" => TokenKind::Bool(true),
            "false" => TokenKind::Bool(false),
            _ => TokenKind::Ident(ident.to_string()),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn lex(source: &str) -> Vec<TokenKind> {
        Lexer::new(source)
            .tokenize()
            .unwrap()
            .into_iter()
            .map(|t| t.kind)
            .filter(|k| !matches!(k, TokenKind::Newline))
            .collect()
    }

    #[test]
    fn test_basic_tokens() {
        assert_eq!(
            lex("fn let if else"),
            vec![
                TokenKind::Fn,
                TokenKind::Let,
                TokenKind::If,
                TokenKind::Else,
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_operators() {
        assert_eq!(
            lex("+ - * / == != |>"),
            vec![
                TokenKind::Plus,
                TokenKind::Minus,
                TokenKind::Star,
                TokenKind::Slash,
                TokenKind::EqEq,
                TokenKind::Ne,
                TokenKind::PipeGt,
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_numbers() {
        assert_eq!(
            lex("42 3.14"),
            vec![TokenKind::Int(42), TokenKind::Float(3.14), TokenKind::Eof]
        );
    }

    #[test]
    fn test_strings() {
        assert_eq!(
            lex("\"hello\" \"world\""),
            vec![
                TokenKind::String("hello".into()),
                TokenKind::String("world".into()),
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_string_interpolation_simple() {
        assert_eq!(
            lex("\"Hello, {name}!\""),
            vec![
                TokenKind::InterpolatedString(vec![
                    StringPart::Literal("Hello, ".into()),
                    StringPart::Expr("name".into()),
                    StringPart::Literal("!".into()),
                ]),
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_string_interpolation_multiple() {
        assert_eq!(
            lex("\"{x} + {y} = {x + y}\""),
            vec![
                TokenKind::InterpolatedString(vec![
                    StringPart::Expr("x".into()),
                    StringPart::Literal(" + ".into()),
                    StringPart::Expr("y".into()),
                    StringPart::Literal(" = ".into()),
                    StringPart::Expr("x + y".into()),
                ]),
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_string_interpolation_escaped_braces() {
        assert_eq!(
            lex("\"literal \\{braces\\}\""),
            vec![
                TokenKind::String("literal {braces}".into()),
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_string_no_interpolation() {
        // Plain strings without interpolation should remain as String tokens
        assert_eq!(
            lex("\"no interpolation here\""),
            vec![
                TokenKind::String("no interpolation here".into()),
                TokenKind::Eof
            ]
        );
    }

    #[test]
    fn test_function() {
        assert_eq!(
            lex("fn add(a: Int, b: Int): Int = a + b"),
            vec![
                TokenKind::Fn,
                TokenKind::Ident("add".into()),
                TokenKind::LParen,
                TokenKind::Ident("a".into()),
                TokenKind::Colon,
                TokenKind::Ident("Int".into()),
                TokenKind::Comma,
                TokenKind::Ident("b".into()),
                TokenKind::Colon,
                TokenKind::Ident("Int".into()),
                TokenKind::RParen,
                TokenKind::Colon,
                TokenKind::Ident("Int".into()),
                TokenKind::Eq,
                TokenKind::Ident("a".into()),
                TokenKind::Plus,
                TokenKind::Ident("b".into()),
                TokenKind::Eof
            ]
        );
    }
}