Initial

demo.lang

@@ -0,0 +1,6 @@
+i := 0;
+
+while i < 10 {
+  i = i + 1;
+  print i;
+}

src/err.rs

@@ -0,0 +1,151 @@
+use crate::Span;
+
+pub type Result<T> = std::result::Result<T, Diagnostic>;
+
+pub fn error<T>() -> Result<T> {
+  Err(Diagnostic::new(""))
+}
+
+#[derive(Clone)]
+pub struct Diagnostic {
+  reason: String,
+  span: Option<Span>,
+  backtrace: Vec<String>,
+}
+
+impl Diagnostic {
+  pub fn new(reason: impl Into<String>) -> Self {
+    Self {
+      reason: reason.into(),
+      span: None,
+      backtrace: vec![],
+    }
+  }
+}
+
+impl std::fmt::Display for Diagnostic {
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    if let Some(span) = self.span.as_ref() {
+      write!(f, "({}:{}) {}\n", span.row, span.column, self.reason)?;
+    } else {
+      write!(f, "(E) {}\n", self.reason)?;
+    }
+    for b in self.backtrace.iter().rev() {
+      write!(f, "--> {}\n", b)?;
+    }
+    Ok(())
+  }
+}
+
+impl std::fmt::Debug for Diagnostic {
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    write!(f, "{self}")
+  }
+}
+
+impl From<std::io::Error> for Diagnostic {
+  fn from(value: std::io::Error) -> Self {
+    Self {
+      reason: format!("{}", value),
+      span: None,
+      backtrace: vec![],
+    }
+  }
+}
+
+impl From<std::num::ParseIntError> for Diagnostic {
+  fn from(value: std::num::ParseIntError) -> Self {
+    use std::num::IntErrorKind::*;
+    match value.kind() {
+      PosOverflow | NegOverflow => {
+        Diagnostic::new("Integer value is too large to represent")
+      },
+      InvalidDigit => Diagnostic::new("Integer value containts invalid digits"),
+      _ => Diagnostic::new("Integer value could not be parsed"),
+    }
+  }
+}
+
+impl From<std::num::ParseFloatError> for Diagnostic {
+  fn from(_value: std::num::ParseFloatError) -> Self {
+    Diagnostic::new("Float value could not be parsed")
+  }
+}
+
+pub trait IntoDiagnostic<T, S: Into<String>> {
+  fn reason(self, s: S) -> Result<T>;
+  fn trace(self, s: S) -> Result<T>;
+}
+
+pub trait WithSpan<T> {
+  fn span(self, span: &Span) -> Result<T>;
+  fn no_span(self) -> Result<T>;
+}
+
+impl<T> WithSpan<T> for Result<T> {
+  fn span(self, span: &Span) -> Result<T> {
+    self.map_err(|mut e| {
+      e.span = e.span.or(Some(span.clone()));
+      e
+    })
+  }
+
+  fn no_span(self) -> Result<T> {
+    self.map_err(|mut e| {
+      e.span = None;
+      e
+    })
+  }
+}
+
+pub trait CoerceDiagnostic<T> {
+  fn coerce(self) -> Result<T>;
+}
+
+impl<T, S: Into<String>> IntoDiagnostic<T, S> for Option<T> {
+  fn reason(self, s: S) -> Result<T> {
+    match self {
+      Some(t) => Ok(t),
+      None => Err(Diagnostic {
+        reason: s.into(),
+        span: None,
+        backtrace: vec![],
+      }),
+    }
+  }
+
+  fn trace(self, s: S) -> Result<T> {
+    match self {
+      Some(t) => Ok(t),
+      None => Err(Diagnostic {
+        reason: "".into(),
+        span: None,
+        backtrace: vec![s.into()],
+      }),
+    }
+  }
+}
+
+impl<T, E: Into<Diagnostic>, S: Into<String>> IntoDiagnostic<T, S>
+  for std::result::Result<T, E>
+{
+  fn reason(self, s: S) -> Result<T> {
+    self.map_err(|e| e.into()).map_err(|mut e| {
+      e.reason = s.into();
+      e
+    })
+  }
+
+  fn trace(self, s: S) -> Result<T> {
+    self.map_err(|e| e.into()).map_err(|mut e| {
+      e.backtrace.push(s.into());
+      e
+    })
+  }
+}
+
+impl<T, E: Into<Diagnostic>> CoerceDiagnostic<T> for std::result::Result<T, E> {
+  fn coerce(self) -> Result<T> {
+    self.map_err(|e| e.into())
+  }
+}

src/lookahead.rs

@@ -0,0 +1,84 @@
+pub struct Window<const N: usize, T, I>
+where
+  I: Iterator<Item = T>,
+{
+  iterator: I,
+  buffer: [Option<T>; N],
+  exhausted: bool,
+  pub finished: bool,
+}
+
+impl<const N: usize, T, I> std::fmt::Debug for Window<N, T, I>
+where
+  I: Iterator<Item = T>,
+  T: std::fmt::Debug,
+{
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    write!(f, "{:?}", self.buffer)
+  }
+}
+
+impl<const N: usize, T, I> Window<N, T, I>
+where
+  I: Iterator<Item = T>,
+{
+  pub fn new(mut it: I) -> Self {
+    assert!(N > 0, "Lookahead buffer cannot be 0 sized");
+    let mut s = Self {
+      buffer: std::array::from_fn(|_| it.next()),
+      iterator: it,
+      exhausted: false,
+      finished: false,
+    };
+    s.normalize();
+    s
+  }
+
+  pub fn inner(&self) -> &I {
+    &self.iterator
+  }
+
+  fn normalize(&mut self) {
+    for item in &mut self.buffer {
+      if self.exhausted {
+        *item = None;
+      } else if let None = item {
+        self.exhausted = true;
+      }
+    }
+  }
+
+  pub fn peek(&self, n: usize) -> &Option<T> {
+    debug_assert!(n < N, "Peeked further than buffer allows");
+    &self.buffer[n]
+  }
+
+  fn _advance(&mut self) {
+    for i in 1..N {
+      self.buffer[i - 1] = self.buffer[i].take();
+    }
+    self.buffer[N - 1] = match self.iterator.next() {
+      Some(i) if !self.exhausted => Some(i),
+      _ => {
+        self.exhausted = true;
+        None
+      },
+    };
+  }
+}
+
+impl<const N: usize, T, I> Iterator for Window<N, T, I>
+where
+  I: Iterator<Item = T>,
+{
+  type Item = T;
+
+  fn next(&mut self) -> Option<Self::Item> {
+    let r = self.buffer[0].take();
+    if let None = r {
+      self.finished = true;
+    }
+    self._advance();
+    r
+  }
+}

src/main.rs

@@ -1,18 +1,57 @@
+mod err;
+mod lookahead;
+mod parse;
 mod token;
+mod treewalk;
+use std::ops::Add;
+
+use lookahead::*;
+use parse::*;
+use token::*;
+use treewalk::Interpreter;
+
+#[derive(Clone, Copy, Debug)]
+pub struct Span {
+  pub row: usize,
+  pub column: usize,
+}
+
+impl Add<Span> for Span {
+  type Output = Span;
+
+  fn add(self, rhs: Span) -> Self::Output {
+    Span {
+      row: usize::min(self.row, rhs.row),
+      column: usize::max(self.column, rhs.column),
+    }
+  }
+}
+
+fn test_tokenization() {
+  let test_str = r#"
+  ( ) [ ] { } . .. , :  
+  ; + - * / -> => += -=
+  *= /= ! != = == <= >=
+  ? ?= < > literal 10 
+  0x10 0b10 10.0 1.0..2.0
+  2.0..=3.0 if else and
+  or xor not nand nor xnor 
+  print break for while true
+  false "\u263b" '\x30'
+  "#;
+  let mut parser = Tokenizer::new(test_str.chars());
+  while let Some(tok) = parser.next() {
+    println!("{tok:?}");
+  }
+}
 
 fn main() {
-  repl();
+  let src = include_str!("../demo.lang");
-}
+  println!("{src}");
-
+  let tokens: Vec<_> = Tokenizer::new(src.chars())
-pub fn repl() {
+    .filter(|t| t.0.is_meaningful())
-  let mut buffer = String::new();
+    .collect();
-  let stdin = std::io::stdin();
+  let parsed = Parser::new(tokens.into_iter()).file().unwrap();
-  loop {
+  let mut interp = Interpreter::new(parsed.into_iter());
-    stdin.read_line(&mut buffer).unwrap();
+  interp.run().unwrap();
-    let tokens = token::tokenize(&buffer);
-    for tok in tokens {
-      println!("{} : {:?}", &buffer[tok.start..tok.end], tok.ttype);
-    }
-    buffer = String::new();
-  }
 }

src/parse.rs

@@ -0,0 +1,453 @@
+use crate::err::*;
+use crate::{Span, Token, TokenKind};
+
+#[derive(Clone)]
+pub enum ExpressionKind {
+  Integer(i64),
+  Real(f64),
+  String(String),
+  Boolean(bool),
+  Identifier(String),
+  Binary {
+    token: TokenKind,
+    left: Box<Expression>,
+    right: Box<Expression>,
+  },
+  Unary {
+    token: TokenKind,
+    child: Box<Expression>,
+  },
+  Parenthesis(Box<Expression>),
+}
+
+#[derive(Clone)]
+pub struct Expression {
+  pub kind: ExpressionKind,
+  pub span: Span,
+}
+
+impl Expression {
+  pub fn new(kind: ExpressionKind, span: Span) -> Self {
+    Self { kind, span }
+  }
+}
+
+impl std::fmt::Debug for ExpressionKind {
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    use ExpressionKind as e;
+    match self {
+      e::Integer(i) => write!(f, "{i}"),
+      e::Binary { token, left, right } => {
+        write!(f, "({left:?} {token:?} {right:?})")
+      },
+      e::Parenthesis(inner) => write!(f, "{inner:?}"),
+      e::Unary { token, child } => {
+        write!(f, "({token:?} {child:?})")
+      },
+      e::Real(fp) => write!(f, "{fp}"),
+      e::String(s) => write!(f, r#""{s}""#),
+      e::Identifier(i) => write!(f, "{i}"),
+      e::Boolean(b) => write!(f, "{b}"),
+    }
+  }
+}
+
+impl std::fmt::Debug for Expression {
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    write!(f, "{:?}", self.kind)
+  }
+}
+
+#[derive(Debug, Clone)]
+pub enum StatementKind {
+  Mutable {
+    name: String,
+    type_: Option<String>,
+    value: Option<Expression>,
+  },
+  Immutable {
+    name: String,
+    type_: Option<String>,
+    value: Expression,
+  },
+  Assignment {
+    name: String,
+    value: Expression,
+  },
+  If {
+    predicate: Expression,
+    block: Vec<Statement>,
+    else_: Option<Box<Statement>>,
+  },
+  While {
+    predicate: Expression,
+    block: Vec<Statement>,
+  },
+  Print(Expression),
+  Expression(Expression),
+  Block(Vec<Statement>),
+}
+
+#[derive(Debug, Clone)]
+pub struct Statement {
+  pub kind: StatementKind,
+  pub span: Span,
+}
+
+pub type Precedence = usize;
+
+fn binary_prec(tok: &TokenKind) -> Result<(Precedence, bool)> {
+  use TokenKind::*;
+  Ok(match tok {
+    Star | Slash | Percent => (10, false),
+    Plus | Minus => (9, false),
+    And | Nand => (8, false),
+    Xor | Xnor => (7, false),
+    Or | Nor => (6, false),
+    DoubleEqual | BangEqual | Less | LessEqual | Greater | GreaterEqual => {
+      (5, false)
+    },
+    //Colon => Some((5, false)),
+    _ => {
+      return error()
+        .reason(format!("{:?} is not a valid binary operator", tok));
+    },
+  })
+}
+
+fn unary_prefix_prec(tok: &TokenKind) -> Result<Precedence> {
+  use TokenKind::*;
+  Ok(match tok {
+    Minus | Not => 11,
+    Break => 3,
+    _ => {
+      return error()
+        .reason(format!("{tok:?} is not a valid prefix unary operator"));
+    },
+  })
+}
+
+fn unary_postfix_prec(tok: &TokenKind) -> Result<Precedence> {
+  use TokenKind::*;
+  Ok(match tok {
+    Question => 12,
+    Bang => 12,
+    _ => {
+      return error()
+        .reason(format!("{tok:?} is not a valid postfix unary operator"));
+    },
+  })
+}
+
+const PARSER_LOOKAHEAD: usize = 3;
+
+type TokenIter<I> = crate::Window<PARSER_LOOKAHEAD, Token, I>;
+
+pub struct Parser<I: Iterator<Item = Token>> {
+  iter: TokenIter<I>,
+}
+
+impl<I: Iterator<Item = Token>> Parser<I> {
+  pub fn new(iter: I) -> Self {
+    Self {
+      iter: TokenIter::new(iter),
+    }
+  }
+
+  fn skip(&mut self, n: usize) {
+    for _ in 0..n {
+      let _ = self.next();
+    }
+  }
+
+  fn next(&mut self) -> Result<Token> {
+    self.iter.next().reason("Unexpected end of file")
+  }
+
+  fn peek(&self, n: usize) -> Result<Token> {
+    self.iter.peek(n).clone().reason("Unexpected end of file")
+  }
+
+  fn eat(&mut self, expect: TokenKind) -> Result<Token> {
+    match self.look(expect) {
+      Ok(t) => {
+        self.skip(1);
+        Ok(t)
+      },
+      Err(e) => Err(e),
+    }
+  }
+
+  fn look(&mut self, expect: TokenKind) -> Result<Token> {
+    let next = self.peek(0)?;
+    if next.0 == expect {
+      Ok(next)
+    } else {
+      error()
+        .reason(format!("Expected {expect:?}, found {:?}", next.0))
+        .span(&next.1)
+    }
+  }
+
+  pub fn file(&mut self) -> Result<Vec<Statement>> {
+    use TokenKind as t;
+    let mut statements = vec![];
+    loop {
+      // Trim extra ;
+      while self.eat(t::Semicolon).is_ok() {}
+      if self.eat(t::EOF).is_ok() {
+        return Ok(statements);
+      }
+      match self.statement() {
+        Ok(s) => statements.push(s),
+        Err(e) => {
+          return Err(e);
+        },
+      }
+    }
+  }
+
+  pub fn statement(&mut self) -> Result<Statement> {
+    use StatementKind as s;
+    use TokenKind as t;
+    let next = self.peek(0);
+    let next2 = self.peek(1);
+    let statement = match (next, next2) {
+      // (im)mutable declaration
+      (Ok(Token(t::Identifier(name), span)), Ok(Token(t::Colon, _))) => {
+        self.skip(2);
+        let type_ = match self.eat(t::Identifier("".into())) {
+          Ok(Token(t::Identifier(s), _)) => Some(s),
+          _ => None,
+        };
+        match self.eat(t::Equal).or_else(|_| self.eat(t::Colon)) {
+          Ok(Token(t::Colon, _)) => Statement {
+            kind: s::Immutable {
+              name,
+              type_,
+              value: self
+                .expression(0)
+                .trace("while parsing immutable declaration")?,
+            },
+            span,
+          },
+          Ok(Token(t::Equal, _)) => Statement {
+            kind: s::Mutable {
+              name,
+              type_,
+              value: Some(
+                self
+                  .expression(0)
+                  .trace("while parsing mutable declaration")?,
+              ),
+            },
+            span,
+          },
+          _ => return error().reason("Expected expression here"),
+        }
+      },
+      (Ok(Token(t::Identifier(name), span)), Ok(Token(t::Equal, _))) => {
+        self.skip(2);
+        let value = self
+          .expression(0)
+          .trace("while parsing assignment expression")?;
+        Statement {
+          kind: s::Assignment { name, value },
+          span,
+        }
+      },
+      // If
+      (Ok(Token(t::If, span)), _) => {
+        self.skip(1);
+        let predicate = self
+          .expression(0)
+          .reason("Expected predicate after 'if' keyword")
+          .span(&span)?;
+        let block = self.block().trace("while parsing if statement")?;
+        return Ok(Statement {
+          span,
+          kind: s::If {
+            predicate,
+            block: block.0,
+            else_: None,
+          },
+        });
+      },
+      // While
+      (Ok(Token(t::While, span)), _) => {
+        self.skip(1);
+        let predicate = self
+          .expression(0)
+          .reason("Expected predicate after 'while' keyword")
+          .span(&span)?;
+        let block = self.block().trace("while parsing while statement")?;
+        return Ok(Statement {
+          span,
+          kind: s::While {
+            predicate,
+            block: block.0,
+          },
+        });
+      },
+      // (DEBUG) print
+      (Ok(Token(t::Print, span)), _) => {
+        self.skip(1);
+        let expr = self.expression(0).trace("while parsing print statement")?;
+        Statement {
+          span: span + expr.span,
+          kind: s::Print(expr),
+        }
+      },
+      // Block
+      (Ok(Token(t::LeftBrace, _)), _) => {
+        // Skip check for semicolon
+        let (block, span) =
+          self.block().trace("while parsing block statement")?;
+        return Ok(Statement {
+          kind: s::Block(block),
+          span,
+        });
+      },
+      // Expression
+      _ => {
+        let expr = self
+          .expression(0)
+          .trace("while parsing expression statement")?;
+        Statement {
+          span: expr.span,
+          kind: s::Expression(expr),
+        }
+      },
+    };
+    // Check for semicolon
+    if self.eat(t::Semicolon).is_ok() {
+      Ok(statement)
+    } else {
+      error().reason("Expected ;")
+    }
+  }
+
+  pub fn expression(
+    &mut self,
+    mut precedence: Precedence,
+  ) -> Result<Expression> {
+    use ExpressionKind as e;
+    use TokenKind as t;
+    let next = self.peek(0)?;
+    // Unary prefix expression
+    let mut current = if let Ok(p) = unary_prefix_prec(&next.0) {
+      let operator = self.next().expect("unreachable");
+      let child = self
+        .expression(p)
+        .trace(format!("while parsing unary {:?}", operator.0))
+        .span(&operator.1)?;
+      let span = child.span + operator.1;
+      Expression::new(
+        e::Unary {
+          token: operator.0,
+          child: child.into(),
+        },
+        span,
+      )
+    }
+    // Terminal or paren
+    else {
+      self.primary()?
+    };
+    // Precedence climbing loop
+    while let Ok(next) = self.peek(0) {
+      // Binary infix
+      if let Ok((new_precedence, left_assoc)) = binary_prec(&next.0) {
+        if (!left_assoc && new_precedence <= precedence)
+          || (new_precedence < precedence)
+        {
+          return Ok(current);
+        }
+        let operator = self.next().expect("unreachable");
+        let rhs = self
+          .expression(new_precedence)
+          .trace(format!("while parsing binary {:?}", operator.0))
+          .span(&operator.1)?;
+        let span = next.1 + rhs.span;
+        current = Expression::new(
+          e::Binary {
+            token: operator.0,
+            left: current.into(),
+            right: rhs.into(),
+          },
+          span,
+        );
+      }
+      // Unary postfix
+      else if let Ok(new_precedence) = unary_postfix_prec(&next.0) {
+        let operator = self.next().expect("unreachable");
+        let span = next.1 + operator.1;
+        precedence = new_precedence;
+        current = Expression::new(
+          e::Unary {
+            token: operator.0,
+            child: current.into(),
+          },
+          span,
+        );
+      } else {
+        break;
+      }
+    }
+    Ok(current)
+  }
+
+  fn primary(&mut self) -> Result<Expression> {
+    use ExpressionKind as e;
+    use TokenKind as t;
+    let next = self.peek(0)?;
+    let span = next.1;
+    let kind = match next.0 {
+      t::IntegerLiteral(i) => e::Integer(i),
+      t::FloatLiteral(f) => e::Real(f),
+      t::StringLiteral(s) => e::String(s),
+      t::True => e::Boolean(true),
+      t::Identifier(i) => e::Identifier(i),
+      t::LeftParen => {
+        self.eat(t::LeftParen).expect("unreachable");
+        let expr = self
+          .expression(0)
+          .trace("while parsing parenthesized expression")?;
+        self
+          .look(t::RightParen)
+          .reason("Unclosed '('")
+          .span(&expr.span)?;
+        e::Parenthesis(expr.into())
+      },
+      _ => {
+        return error()
+          .span(&span)
+          .reason(format!("Expected primary, found {:?}", next.0));
+      },
+    };
+    self.skip(1);
+    Ok(Expression { kind, span })
+  }
+
+  fn block(&mut self) -> Result<(Vec<Statement>, Span)> {
+    use TokenKind as t;
+    let mut span = self.eat(t::LeftBrace).reason("Expected block")?.1;
+    let mut statements = vec![];
+    loop {
+      let next = self.peek(0)?;
+      span = span + next.1;
+      match self.eat(t::RightBrace) {
+        Ok(t) => {
+          span = span + t.1;
+          break;
+        },
+        _ => {
+          let statement = self.statement()?;
+          span = span + statement.span;
+          statements.push(statement);
+        },
+      };
+    }
+    Ok((statements, span))
+  }
+}

src/token.rs

@@ -1,22 +1,38 @@
+use crate::err::*;
+use crate::Span;
+
 #[derive(Debug, Clone)]
-pub enum TokenType {
+pub enum TokenKind {
-  // Symbols
   LeftParen,
   RightParen,
-  LeftSquare,
-  RightSquare,
   LeftBrace,
   RightBrace,
+  LeftSquare,
+  RightSquare,
+
   Comma,
+  Colon,
+  Semicolon,
+
   Dot,
+  DotDot,
   Plus,
   Minus,
-  Star,
   Slash,
-  Semicolon,
+  Star,
+  Percent,
+  Arrow,
+  FatArrow,
+  PlusEqual,
+  MinusEqual,
+  SlashEqual,
+  StarEqual,
+  PercentEqual,
 
   Bang,
   BangEqual,
+  Question,
+  QuestionEqual,
   Equal,
   DoubleEqual,
   Greater,
@@ -24,187 +40,463 @@ pub enum TokenType {
   Less,
   LessEqual,
 
-  // Literals
+  Pipe,
-  String,
+  Ampersand,
-  Character,
+  Carrot,
-  Number(f64),
+  Hash,
+
+  DotDotEqual,
+
+  Identifier(String),
+  StringLiteral(String),
+  CharLiteral(char),
+  IntegerLiteral(i64),
+  FloatLiteral(f64),
 
-  // Words
-  Ident,
-  And,
-  Or,
-  Self_,
-  Struct,
-  True,
-  False,
-  Fn,
   If,
   Else,
-  Nil,
+  And,
+  Or,
+  Xor,
+  Not,
+  Nand,
+  Nor,
+  Xnor,
   Print,
+  Break,
   Return,
-  Super,
+  Continue,
-  Let,
-  While,
   For,
+  While,
+  True,
+  False,
+  Struct,
+  Enum,
+  Union,
 
-  // Special
+  Whitespace(String),
-  Unrecognized,
+  SmallComment(String),
-  TooLong,
+  BigComment(String),
+
+  Idk,
+  EOF,
 }
 
-/// Type, index
+impl TokenKind {
-#[derive(Debug, Clone)]
+  pub fn is_meaningful(&self) -> bool {
-pub struct Token {
+    match self {
-  pub ttype: TokenType,
+      Self::Whitespace(_)
-  pub start: usize,
+      | Self::SmallComment(_)
-  pub end: usize,
+      | Self::BigComment(_)
+      | Self::Idk => false,
+      _ => true,
+    }
+  }
 }
 
-pub fn tokenize(input: &str) -> Vec<Token> {
+impl PartialEq for TokenKind {
-  let input_str = input;
+  fn eq(&self, other: &Self) -> bool {
-  let mut input = input.char_indices().peekable();
+    std::mem::discriminant(self) == std::mem::discriminant(other)
-  let mut tokens = vec![];
+  }
-  'outer: loop {
+}
-    // Find next non-whitespace line
+
-    let (start, c) = 'ws: loop {
+impl Eq for TokenKind {
-      match input.next() {
+}
-        // Stop at end of input
+
-        None => break 'outer,
+#[derive(Clone, Debug)]
-        Some((index, character)) if !character.is_whitespace() => {
+pub struct Token(pub TokenKind, pub Span);
-          break 'ws (index, character)
+
+fn t(tk: TokenKind, sp: Span) -> Result<Token> {
+  Ok(Token(tk, sp))
+}
+
+const TOKENIZER_LOOKAHEAD: usize = 2;
+
+type CharIter<I> = crate::Window<TOKENIZER_LOOKAHEAD, char, I>;
+
+pub struct Tokenizer<I: Iterator<Item = char>> {
+  iter: CharIter<I>,
+  column: usize,
+  row: usize,
+  finished: bool,
+}
+
+impl<I: Iterator<Item = char>> Tokenizer<I> {
+  pub fn new(iter: I) -> Self {
+    Self {
+      iter: CharIter::new(iter),
+      column: 1,
+      row: 1,
+      finished: false,
+    }
+  }
+
+  pub fn span(&self) -> Span {
+    Span {
+      column: self.column,
+      row: self.row,
+    }
+  }
+
+  fn next_char(&mut self) -> Option<char> {
+    match self.iter.next() {
+      Some(c) if c == '\n' => {
+        self.row += 1;
+        self.column = 1;
+        Some(c)
       },
-        _ => {},
+      Some(c) => {
+        self.column += 1;
+        Some(c)
+      },
+      _ => None,
     }
+  }
+
+  fn delimited(&mut self, terminator: char) -> Option<String> {
+    let mut buffer = String::new();
+    let mut escape = false;
+    loop {
+      let c = match self.next_char() {
+        Some(c) if c == terminator && !escape => {
+          break;
+        },
+        Some(c) => {
+          if c == '\\' {
+            escape = !escape;
+          } else {
+            escape = false;
+          }
+          c
+        },
+        None => return None,
       };
-    let mut end = start + 1;
+      buffer.push(c)
-    let mut advance = || {};
+    }
-    let ttype = match c {
+    Some(buffer)
+  }
+
+  fn peek(&mut self, n: usize) -> Option<char> {
+    self.iter.peek(n).clone()
+  }
+
+  fn _next(&mut self) -> Result<Token> {
+    use TokenKind::*;
+    let position = Span {
+      row: self.row,
+      column: self.column,
+    };
+    let current = match self.next_char() {
+      Some(c) => c,
+      None => return t(EOF, position),
+    };
+    // Parse whitespace
+    if current.is_whitespace() {
+      let mut buffer = String::from(current);
+      while let Some(c) = self.peek(0) {
+        if !c.is_whitespace() {
+          break;
+        }
+        _ = self.next_char();
+        buffer.push(c.clone());
+      }
+      return t(Whitespace(buffer), position);
+    }
+    // Parse multiline comments
+    if let ('/', Some('*')) = (current, self.peek(0)) {
+      let _ = self.next_char();
+      let mut comment_level = 1;
+      let mut buffer = String::new();
+      while let Some(current) = self.next_char() {
+        // Ignore /* */ inside strings
+        if '\"' == current {
+          if let Some(inner_string) = self.delimited('\"') {
+            buffer.push('\"');
+            buffer.push_str(&inner_string);
+            buffer.push('\"');
+            continue;
+          }
+        }
+        if let ('/', Some('*')) = (current, self.peek(0)) {
+          comment_level += 1;
+        } else if let ('*', Some('/')) = (current, self.peek(0)) {
+          comment_level -= 1;
+        }
+        if comment_level == 0 {
+          let _ = self.next_char();
+          break;
+        }
+        buffer.push(current);
+      }
+      return t(BigComment(buffer), position);
+    }
+    // Parse single line comments
+    if let ('/', Some('/')) = (current, self.peek(0)) {
+      let _ = self.next_char();
+      let mut buffer = String::new();
+      while let Some(c) = self.next_char() {
+        if c == '\n' {
+          break;
+        }
+        buffer.push(c);
+      }
+      return t(SmallComment(buffer), position);
+    }
+    let next = self.peek(0);
+    let next_next = self.peek(1);
     // Match single character tokens
-      '(' => TokenType::LeftParen,
-      ')' => TokenType::RightParen,
-      '[' => TokenType::LeftSquare,
-      ']' => TokenType::RightSquare,
-      '{' => TokenType::LeftBrace,
-      '}' => TokenType::RightBrace,
-      ',' => TokenType::Comma,
-      '.' => TokenType::Dot,
-      '+' => TokenType::Plus,
-      '-' => TokenType::Minus,
-      '*' => TokenType::Star,
-      '/' => TokenType::Slash,
-      ';' => TokenType::Semicolon,
-      // Match multicharacter tokens
-      '!' => match input.peek() {
-        Some((_, '=')) => {
-          input.next();
-          end += 1;
-          TokenType::BangEqual
-        },
-        _ => TokenType::Bang,
-      },
-      '=' => match input.peek() {
-        Some((_, '=')) => {
-          input.next();
-          end += 1;
-          TokenType::DoubleEqual
-        },
-        _ => TokenType::Equal,
-      },
-      '<' => match input.peek() {
-        Some((_, '=')) => {
-          input.next();
-          end += 1;
-          TokenType::GreaterEqual
-        },
-        _ => TokenType::Greater,
-      },
-      '>' => match input.peek() {
-        Some((_, '=')) => {
-          input.next();
-          end += 1;
-          TokenType::LessEqual
-        },
-        _ => TokenType::Less,
-      },
-      // Match keywords, identifiers, and literals
-      c if c.is_alphanumeric() => 'case: {
-        // Scan full word
-        while let Some((new_end, next)) = input.peek() {
-          if next.is_alphanumeric() || *next == '_' {
-            let _ = input.next();
-          } else {
-            end = *new_end;
-            break;
-          }
-        }
-        let word = &input_str[start..end];
-        // Attempt to parse hex literal
-        if let Some(s) =
-          word.strip_prefix("0x").or_else(|| word.strip_prefix("0X"))
     {
-          if let Ok(n) = u64::from_str_radix(s, 16) {
+      let not_next = move |c| Some(c) != next;
-            break 'case TokenType::Number(n as f64);
+      let kind = match current {
-          } else {
+        '(' => LeftParen,
-            break 'case TokenType::Unrecognized;
+        ')' => RightParen,
-          }
+        '{' => LeftBrace,
-        }
+        '}' => RightBrace,
-        // Attempt to parse binary literal
+        '[' => LeftSquare,
-        if let Some(s) =
+        ']' => RightSquare,
-          word.strip_prefix("0b").or_else(|| word.strip_prefix("0B"))
+        ',' => Comma,
-        {
+        ':' => Colon,
-          if let Ok(n) = u64::from_str_radix(s, 2) {
+        ';' => Semicolon,
-            break 'case TokenType::Number(n as f64);
+        '|' => Pipe,
-          } else {
+        '&' => Ampersand,
-            break 'case TokenType::Unrecognized;
+        '^' => Carrot,
-          }
+        '#' => Hash,
-        }
+        '.' if not_next('.') => Dot,
-        // Attempt to parse decimal literal
+        '+' if not_next('=') => Plus,
-        if let Ok(f) = word.parse::<f64>() {
+        '-' if not_next('=') && not_next('>') => Minus,
-          break 'case TokenType::Number(f);
+        '*' if not_next('=') => Star,
-        }
+        '/' if not_next('=') => Slash,
-        // Parse keyword or ident
+        '%' if not_next('=') => Percent,
-        match word {
+        '!' if not_next('=') => Bang,
-          "and" => TokenType::And,
+        '?' if not_next('=') => Question,
-          "or" => TokenType::Or,
+        '=' if not_next('=') && not_next('>') => Equal,
-          "self" => TokenType::Self_,
+        '<' if not_next('=') => Less,
-          "struct" => TokenType::Struct,
+        '>' if not_next('=') => Greater,
-          "true" => TokenType::True,
+        _ => Idk,
-          "false" => TokenType::False,
+      };
-          "fn" => TokenType::Fn,
+      if kind != Idk {
-          "if" => TokenType::If,
+        return t(kind, position);
-          "else" => TokenType::Else,
-          "nil" => TokenType::Nil,
-          "print" => TokenType::Print,
-          "return" => TokenType::Return,
-          "super" => TokenType::Super,
-          "let" => TokenType::Let,
-          "while" => TokenType::While,
-          "for" => TokenType::For,
-          _ => TokenType::Ident,
-        }
-      },
-      // Parse string
-      '"' => {
-        while let Some((new_end, next)) = input.next() {
-          match next {
-            '"' => {
-              end = new_end + 1;
-              break;
-            },
-            // Skip escapes and deal with them later
-            '\\' => {
-              let _ = input.next();
-            },
-            _ => {},
-          }
-        }
-        TokenType::String
-      },
-      // Parse character
-      _ => TokenType::Unrecognized,
       };
-    tokens.push(Token { ttype, start, end });
     }
-  tokens
+    // Match two character tokens
+    if let Some(next) = next {
+      let not_next_next = move |c| Some(c) != next_next;
+      let kind = match (current, next) {
+        ('.', '.') if not_next_next('=') => DotDot,
+        ('+', '=') => PlusEqual,
+        ('-', '=') => MinusEqual,
+        ('*', '=') => StarEqual,
+        ('/', '=') => SlashEqual,
+        ('%', '=') => PercentEqual,
+        ('=', '=') => DoubleEqual,
+        ('?', '=') => QuestionEqual,
+        ('!', '=') => BangEqual,
+        ('<', '=') => LessEqual,
+        ('>', '=') => GreaterEqual,
+        ('-', '>') => Arrow,
+        ('=', '>') => FatArrow,
+        _ => Idk,
+      };
+      if kind != Idk {
+        let _ = self.next();
+        return t(kind, position);
+      }
+    }
+    // Match three character tokens
+    if let (Some(next), Some(next_next)) = (next, next_next) {
+      let kind = match (current, next, next_next) {
+        ('.', '.', '=') => DotDotEqual,
+        _ => Idk,
+      };
+      if kind != Idk {
+        let _ = self.next();
+        let _ = self.next();
+        return t(kind, position);
+      }
+    }
+    let mut buffer = String::new();
+    // Match character
+    if current == '\'' {
+      let buffer = self
+        .delimited('\'')
+        .reason("Single quote (') was opened, but never closed")
+        .span(&position)?;
+      let baked = bake_string(&buffer)?;
+      if baked.len() != 1 {
+        return error()
+          .reason("Single quote (') contains more than one character")
+          .span(&position);
+      }
+      let kind = CharLiteral(
+        baked
+          .chars()
+          .next()
+          .reason("Single quote (') contains no characters")
+          .span(&position)?,
+      );
+      return t(kind, position);
+    }
+    // Match string
+    if current == '"' {
+      let buffer = self
+        .delimited('\"')
+        .reason("Double quote (\") was opened, but never closed")
+        .span(&position)?;
+      let kind = StringLiteral(bake_string(&buffer)?);
+      return t(kind, position);
+    }
+    buffer.push(current);
+    // Match number
+    if current.is_ascii_digit() {
+      // Only one dot per number
+      let mut encountered_dot = false;
+      while let Some(c) = self.peek(0) {
+        if c == '.' && !encountered_dot {
+          if let Some('.') = self.peek(1) {
+            break;
+          }
+          encountered_dot = true;
+        } else if !(c == '_' || c == 'x' || c.is_ascii_hexdigit()) {
+          break;
+        }
+        buffer.push(c);
+        let _ = self.next_char();
+      }
+      return t(parse_number(&buffer).span(&position)?, position);
+    }
+    // Match keyword or identifier
+    while let Some(c) = self.peek(0) {
+      if c.is_alphanumeric() || c == '_' {
+        let _ = self.next_char();
+      } else {
+        break;
+      }
+      buffer.push(c);
+    }
+    // Match keywords
+    {
+      let kind = match buffer.as_str() {
+        "if" => If,
+        "else" => Else,
+        "and" => And,
+        "or" => Or,
+        "xor" => Xor,
+        "nand" => Nand,
+        "nor" => Nor,
+        "xnor" => Xnor,
+        "for" => For,
+        "while" => While,
+        "print" => Print,
+        "break" => Break,
+        "return" => Return,
+        "continue" => Continue,
+        "not" => Not,
+        "true" => True,
+        "false" => False,
+        "struct" => Struct,
+        "enum" => Enum,
+        "union" => Union,
+        _ => Identifier(buffer),
+      };
+      return t(kind, position);
+    }
+  }
+}
+
+impl<I: Iterator<Item = char>> Iterator for Tokenizer<I> {
+  type Item = Token;
+
+  fn next(&mut self) -> Option<Self::Item> {
+    loop {
+      match self._next() {
+        Ok(Token(TokenKind::EOF, span)) => {
+          if self.finished {
+            return None;
+          } else {
+            self.finished = true;
+            return Some(Token(TokenKind::EOF, span));
+          }
+        },
+        Ok(r) => return Some(r),
+        _ => {},
+      };
+    }
+  }
+}
+
+fn parse_number(num: &str) -> Result<TokenKind> {
+  use TokenKind::*;
+  let num = num.replace('_', "");
+  // Floating point (only decimal)
+  if num.contains('.') {
+    num.parse::<f64>().map(|f| FloatLiteral(f)).coerce()
+  }
+  // Hex integer
+  else if let Some(hex) = num.strip_prefix("0x") {
+    i64::from_str_radix(hex, 16)
+      .map(|i| IntegerLiteral(i))
+      .coerce()
+  }
+  // Octal integer
+  else if let Some(oct) = num.strip_prefix("0o") {
+    i64::from_str_radix(oct, 8)
+      .map(|i| IntegerLiteral(i))
+      .coerce()
+  }
+  // Binary integer
+  else if let Some(bin) = num.strip_prefix("0b") {
+    i64::from_str_radix(bin, 2)
+      .map(|i| IntegerLiteral(i))
+      .coerce()
+  }
+  // Decimal integer
+  else {
+    num.parse::<i64>().map(|i| IntegerLiteral(i)).coerce()
+  }
+}
+
+fn bake_string(s: &str) -> Result<String> {
+  let mut baked = String::with_capacity(s.len());
+  let mut it = s.chars();
+  loop {
+    match it.next() {
+      Some('\\') => baked.push(match it.next() {
+        Some('n') => '\n',   // New line
+        Some('r') => '\r',   // Carriage return
+        Some('t') => '\t',   // Tab
+        Some('b') => '\x08', // Backspace
+        Some('\\') => '\\',  // Backslash
+        Some('\0') => '\0',  // Null
+        Some('"') => '\"',   // Double quote
+        Some('\'') => '\'',  // Single quote
+        Some('x') => {
+          // Ascii escapes
+          let mut a = || {
+            let a = u32::from_str_radix(&it.next()?.to_string(), 16).ok()?;
+            let b = u32::from_str_radix(&it.next()?.to_string(), 16).ok()?;
+            let num = (a << 4) | b;
+            char::from_u32(num)
+          };
+          a().reason(format!("Found invalid ASCII (\\aXX) escape sequence"))?
+        },
+        Some('u') => {
+          // Unicode escapes
+          let mut a = || {
+            let a = u32::from_str_radix(&it.next()?.to_string(), 16).ok()?;
+            let b = u32::from_str_radix(&it.next()?.to_string(), 16).ok()?;
+            let c = u32::from_str_radix(&it.next()?.to_string(), 16).ok()?;
+            let d = u32::from_str_radix(&it.next()?.to_string(), 16).ok()?;
+            let num = (a << 12) | (b << 8) | (c << 4) | d;
+            char::from_u32(num)
+          };
+          a().reason("Found invalid Unicode (\\uXXXX) escape sequence")?
+        },
+        _ => return Err(Diagnostic::new("Found invalid escape sequence")),
+      }),
+      // Unremarkable character
+      Some(c) => baked.push(c),
+      None => break,
+    }
+  }
+  Ok(baked)
 }

src/treewalk.rs

@@ -0,0 +1,280 @@
+use crate::err::*;
+use std::collections::HashMap;
+
+use crate::{
+  Expression, ExpressionKind, Statement, StatementKind, Token, TokenKind,
+};
+
+#[derive(Debug, Clone)]
+enum Value {
+  Integer(i64),
+  Real(f64),
+  String(String),
+  Boolean(bool),
+  Undefined,
+}
+
+#[derive(Debug, Clone)]
+struct Scope {
+  outer: Option<Box<Scope>>,
+  declarations: HashMap<String, Value>,
+}
+
+impl Scope {
+  fn new() -> Self {
+    Self {
+      outer: None,
+      declarations: Default::default(),
+    }
+  }
+
+  fn enscope(&mut self) -> &mut Self {
+    *self = Self {
+      outer: Some(Box::new(self.clone())),
+      declarations: HashMap::new(),
+    };
+    self
+  }
+
+  fn descope(&mut self) -> &mut Self {
+    if let Some(outer) = &self.outer {
+      *self = *outer.clone();
+    }
+    self
+  }
+
+  fn declare(&mut self, key: String) -> Result<()> {
+    if self.declarations.contains_key(&key) {
+      return error()
+        .reason(format!("Re-declaration of '{key}' in same scope"));
+    }
+    self.declarations.insert(key, Value::Undefined);
+    Ok(())
+  }
+
+  fn assign(&mut self, key: String, value: Value) -> Result<()> {
+    if !self.declarations.contains_key(&key) {
+      if let Some(outer) = &mut self.outer {
+        return outer.assign(key, value);
+      }
+      return error()
+        .reason(format!("Assignemnt to '{key}' before declaration"));
+    }
+    self.declarations.insert(key, value);
+    Ok(())
+  }
+
+  fn access(&self, key: String) -> Result<Value> {
+    match self.declarations.get(&key) {
+      Some(v) => Ok(v.clone()),
+      None => {
+        if let Some(outer) = &self.outer {
+          outer.access(key)
+        } else {
+          error().reason(format!("'{key}' was never declared"))
+        }
+      },
+    }
+  }
+}
+
+pub struct Interpreter<I: Iterator<Item = Statement>> {
+  scope: Scope,
+  iter: I,
+}
+
+impl<I: Iterator<Item = Statement>> Interpreter<I> {
+  pub fn new(iter: I) -> Self {
+    Self {
+      scope: Scope::new(),
+      iter,
+    }
+  }
+
+  fn evaluate_unary(
+    &mut self,
+    token: TokenKind,
+    child: Expression,
+  ) -> Result<Value> {
+    use TokenKind as t;
+    use Value as v;
+    let val = self.evaluate(child)?;
+    Ok(match val {
+      v::Integer(i) => v::Integer(match token {
+        t::Plus => i,
+        t::Minus => -i,
+        _ => {
+          return error()
+            .reason(format!("Unary {token:?} is undefined for integers"));
+        },
+      }),
+      v::Real(r) => v::Real(match token {
+        t::Plus => r,
+        t::Minus => -r,
+        _ => {
+          return error()
+            .reason(format!("Unary {token:?} is undefined for reals"));
+        },
+      }),
+      v::Boolean(b) => v::Boolean(match token {
+        t::Not => !b,
+        _ => {
+          return error()
+            .reason(format!("Unary {token:?} is undefined for booleans"));
+        },
+      }),
+      _ => {
+        return error()
+          .reason(format!("Binary {token:?} is undefined for {val:?}",));
+      },
+    })
+  }
+
+  fn evaluate_binary(
+    &mut self,
+    token: TokenKind,
+    left: Expression,
+    right: Expression,
+  ) -> Result<Value> {
+    use TokenKind as t;
+    use Value::*;
+    let left = self.evaluate(left)?;
+    let right = self.evaluate(right)?;
+    Ok(match (left.clone(), right.clone()) {
+      (Integer(l), Integer(r)) => match token {
+        t::Plus => Integer(l + r),
+        t::Minus => Integer(l - r),
+        t::Star => Integer(l * r),
+        t::Slash => Integer(l / r),
+        t::Percent => Integer(l % r),
+        t::DoubleEqual => Boolean(l == r),
+        t::Less => Boolean(l < r),
+        t::Greater => Boolean(l > r),
+        t::LessEqual => Boolean(l <= r),
+        t::GreaterEqual => Boolean(l >= r),
+        t => {
+          return error()
+            .reason(format!("Binary {t:?} is undefined for integers"));
+        },
+      },
+      (Real(l), Real(r)) => Real(match token {
+        t::Plus => l + r,
+        t::Minus => l - r,
+        t::Star => l * r,
+        t::Slash => l / r,
+        t => {
+          return error()
+            .reason(format!("Binary {t:?} is undefined for reals"));
+        },
+      }),
+      _ => {
+        return error().reason(format!(
+          "Binary {:?} is undefined for {:?} and {:?}",
+          token, left, right
+        ));
+      },
+    })
+  }
+
+  fn evaluate(&mut self, expr: Expression) -> Result<Value> {
+    use ExpressionKind as e;
+    match expr.kind {
+      e::Integer(i) => Ok(Value::Integer(i)),
+      e::Real(r) => Ok(Value::Real(r)),
+      e::String(s) => Ok(Value::String(s)),
+      e::Boolean(b) => Ok(Value::Boolean(b)),
+      e::Identifier(i) => self.scope.access(i),
+      e::Binary { token, left, right } => {
+        self.evaluate_binary(token, *left, *right)
+      },
+      e::Unary { token, child } => self.evaluate_unary(token, *child),
+      e::Parenthesis(e) => self.evaluate(*e),
+    }
+    .span(&expr.span)
+  }
+
+  pub fn execute(&mut self, statement: Statement) -> Result<()> {
+    use StatementKind as s;
+    match statement.kind {
+      s::Mutable { name, value, .. } => {
+        self.scope.declare(name.clone())?;
+        if let Some(value) = value {
+          let value = self.evaluate(value)?;
+          self.scope.assign(name, value)?;
+        }
+      },
+      s::Immutable { name, value, .. } => {
+        self.scope.declare(name.clone())?;
+        let value = self.evaluate(value)?;
+        self.scope.assign(name, value)?;
+      },
+      s::Assignment { name, value } => {
+        let span = value.span;
+        let value = self.evaluate(value).span(&span)?;
+        self.scope.assign(name, value).span(&span)?;
+      },
+      s::Print(e) => {
+        let e = self.evaluate(e)?;
+        println!("{e:?}");
+      },
+      s::Expression(e) => {
+        self.evaluate(e)?;
+      },
+      s::Block(block) => self.block(block)?,
+      s::If {
+        predicate,
+        block,
+        else_,
+      } => {
+        let span = predicate.span;
+        let value = self.evaluate(predicate)?;
+        if let Value::Boolean(b) = value {
+          if b {
+            self.block(block)?;
+          }
+        } else {
+          return error()
+            .reason("Predicate for 'if' statement must be a boolean")
+            .span(&span);
+        }
+      },
+      s::While { predicate, block } => {
+        let span = predicate.span;
+        loop {
+          match self.evaluate(predicate.clone())? {
+            Value::Boolean(true) => self.block(block.clone())?,
+            Value::Boolean(false) => break,
+            _ => {
+              return error()
+                .reason("Predicate for 'while' statement must be a boolean")
+                .span(&span);
+            },
+          }
+        }
+      },
+    }
+    Ok(())
+  }
+
+  fn block(&mut self, block: Vec<Statement>) -> Result<()> {
+    self.scope.enscope();
+    for s in block.into_iter() {
+      let span = s.span;
+      self.execute(s).span(&span)?;
+    }
+    self.scope.descope();
+    Ok(())
+  }
+
+  pub fn run(&mut self) -> Result<()> {
+    loop {
+      let next = match self.iter.next() {
+        Some(n) => n,
+        None => break,
+      };
+      let span = next.span;
+      self.execute(next).span(&span)?;
+    }
+    Ok(())
+  }
+}