Add Value type and replace dyn-nodes with call-exprs 🏗

- In addition to syntax trees there are now `Value`s, which syntax trees can be evaluated into (e.g. the tree is `5+5` and the value is `10`)
- Parsing is completely pure, function calls are not parsed into nodes, but into simple call expressions, which are resolved later
- Functions aren't dynamic nodes anymore, but simply functions which receive their arguments as a table and the layouting context
- Functions may return any `Value`
- Layouting is powered by functions which return the new `Commands` value, which informs the layouting engine what to do
- When a function returns a non-`Commands` value, the layouter simply dumps the value into the document in monospace

7 files changed, 227 insertions, 917 deletions

diff --git a/src/syntax/decoration.rs b/src/syntax/decoration.rs
index a686596c..fe961be3 100644
--- a/src/syntax/decoration.rs
+++ b/src/syntax/decoration.rs
@@ -9,18 +9,18 @@ use super::span::SpanVec;
 pub type Decorations = SpanVec<Decoration>;
 
 /// Decorations for semantic syntax highlighting.
-#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 #[cfg_attr(feature = "serialize", derive(Serialize))]
 #[cfg_attr(feature = "serialize", serde(rename_all = "camelCase"))]
 pub enum Decoration {
-    /// A valid, successfully resolved function name.
-    ResolvedFunc,
-    /// An invalid, unresolved function name.
-    UnresolvedFunc,
-    /// The key part of a key-value entry in a table.
-    TableKey,
     /// Text in italics.
     Italic,
     /// Text in bold.
     Bold,
+    /// A valid, successfully resolved name.
+    Resolved,
+    /// An invalid, unresolved name.
+    Unresolved,
+    /// The key part of a key-value entry in a table.
+    TableKey,
 }
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
deleted file mode 100644
index e92a75b0..00000000
--- a/src/syntax/expr.rs
+++ /dev/null
@@ -1,639 +0,0 @@
-//! Expressions in function headers.
-
-use std::fmt::{self, Debug, Formatter};
-use std::str::FromStr;
-use std::u8;
-
-use fontdock::{FontStyle, FontWeight, FontWidth};
-
-use crate::layout::{Dir, SpecAlign};
-use crate::length::{Length, ScaleLength};
-use crate::paper::Paper;
-use crate::table::{BorrowedKey, Table};
-use crate::Feedback;
-use super::parsing::FuncCall;
-use super::span::{Span, Spanned};
-use super::tokens::is_identifier;
-use super::tree::SyntaxTree;
-
-/// An expression.
-#[derive(Clone, PartialEq)]
-pub enum Expr {
-    /// An identifier: `ident`.
-    Ident(Ident),
-    /// A string: `"string"`.
-    Str(String),
-    /// A boolean: `true, false`.
-    Bool(bool),
-    /// A number: `1.2, 200%`.
-    Number(f64),
-    /// A length: `2cm, 5.2in`.
-    Length(Length),
-    /// A color value with alpha channel: `#f79143ff`.
-    Color(RgbaColor),
-    /// A syntax tree containing typesetting content.
-    Tree(SyntaxTree),
-    /// A table: `(false, 12cm, greeting="hi")`.
-    Table(TableExpr),
-    /// An operation that negates the contained expression.
-    Neg(Box<Spanned<Expr>>),
-    /// An operation that adds the contained expressions.
-    Add(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
-    /// An operation that subtracts the contained expressions.
-    Sub(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
-    /// An operation that multiplies the contained expressions.
-    Mul(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
-    /// An operation that divides the contained expressions.
-    Div(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
-    /// A function call: `cmyk(37.7, 0, 3.9, 1.1)`.
-    Call(FuncCall),
-}
-
-impl Expr {
-    /// A natural-language name of the type of this expression, e.g.
-    /// "identifier".
-    pub fn name(&self) -> &'static str {
-        use Expr::*;
-        match self {
-            Ident(_) => "identifier",
-            Str(_) => "string",
-            Bool(_) => "bool",
-            Number(_) => "number",
-            Length(_) => "length",
-            Color(_) => "color",
-            Tree(_) => "syntax tree",
-            Table(_) => "table",
-            Neg(_) => "negation",
-            Add(_, _) => "addition",
-            Sub(_, _) => "subtraction",
-            Mul(_, _) => "multiplication",
-            Div(_, _) => "division",
-            Call(_) => "function call",
-        }
-    }
-}
-
-impl Debug for Expr {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        use Expr::*;
-        match self {
-            Ident(i) => i.fmt(f),
-            Str(s) => s.fmt(f),
-            Bool(b) => b.fmt(f),
-            Number(n) => n.fmt(f),
-            Length(s) => s.fmt(f),
-            Color(c) => c.fmt(f),
-            Tree(t) => t.fmt(f),
-            Table(t) => t.fmt(f),
-            Neg(e) => write!(f, "-{:?}", e),
-            Add(a, b) => write!(f, "({:?} + {:?})", a, b),
-            Sub(a, b) => write!(f, "({:?} - {:?})", a, b),
-            Mul(a, b) => write!(f, "({:?} * {:?})", a, b),
-            Div(a, b) => write!(f, "({:?} / {:?})", a, b),
-            Call(c) => c.fmt(f),
-        }
-    }
-}
-
-/// An identifier as defined by unicode with a few extra permissible characters.
-#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
-pub struct Ident(pub String);
-
-impl Ident {
-    /// Create a new identifier from a string checking that it is a valid.
-    pub fn new(ident: impl AsRef<str> + Into<String>) -> Option<Self> {
-        if is_identifier(ident.as_ref()) {
-            Some(Self(ident.into()))
-        } else {
-            None
-        }
-    }
-
-    /// Return a reference to the underlying string.
-    pub fn as_str(&self) -> &str {
-        self.0.as_str()
-    }
-}
-
-impl Debug for Ident {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "`{}`", self.0)
-    }
-}
-
-/// An 8-bit RGBA color.
-///
-/// # Example
-/// ```typst
-/// [page: background=#423abaff]
-///                   ^^^^^^^^
-/// ```
-#[derive(Copy, Clone, Eq, PartialEq, Hash)]
-pub struct RgbaColor {
-    /// Red channel.
-    pub r: u8,
-    /// Green channel.
-    pub g: u8,
-    /// Blue channel.
-    pub b: u8,
-    /// Alpha channel.
-    pub a: u8,
-    /// This is true if this value was provided as a fail-over by the parser
-    /// because the user-defined value was invalid. This color may be
-    /// overwritten if this property is true.
-    pub healed: bool,
-}
-
-impl RgbaColor {
-    /// Constructs a new color.
-    pub fn new(r: u8, g: u8, b: u8, a: u8) -> Self {
-        Self { r, g, b, a, healed: false }
-    }
-
-    /// Constructs a new color with the healed property set to true.
-    pub fn new_healed(r: u8, g: u8, b: u8, a: u8) -> Self {
-        Self { r, g, b, a, healed: true }
-    }
-}
-
-impl FromStr for RgbaColor {
-    type Err = ParseColorError;
-
-    /// Constructs a new color from a hex string like `7a03c2`. Do not specify a
-    /// leading `#`.
-    fn from_str(hex_str: &str) -> Result<Self, Self::Err> {
-        if !hex_str.is_ascii() {
-            return Err(ParseColorError);
-        }
-
-        let len = hex_str.len();
-        let long  = len == 6 || len == 8;
-        let short = len == 3 || len == 4;
-        let alpha = len == 4 || len == 8;
-
-        if !long && !short {
-            return Err(ParseColorError);
-        }
-
-        let mut values: [u8; 4] = [255; 4];
-
-        for elem in if alpha { 0..4 } else { 0..3 } {
-            let item_len = if long { 2 } else { 1 };
-            let pos = elem * item_len;
-
-            let item = &hex_str[pos..(pos+item_len)];
-            values[elem] = u8::from_str_radix(item, 16)
-                .map_err(|_| ParseColorError)?;
-
-            if short {
-                // Duplicate number for shorthand notation, i.e. `a` -> `aa`
-                values[elem] += values[elem] * 16;
-            }
-        }
-
-        Ok(Self::new(values[0], values[1], values[2], values[3]))
-    }
-}
-
-impl Debug for RgbaColor {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        if f.alternate() {
-            write!(
-                f, "rgba({:02}, {:02}, {:02}, {:02})",
-                self.r, self.g, self.b, self.a,
-            )?;
-        } else {
-            write!(
-                f, "#{:02x}{:02x}{:02x}{:02x}",
-                self.r, self.g, self.b, self.a,
-            )?;
-        }
-        if self.healed {
-            f.write_str(" [healed]")?;
-        }
-        Ok(())
-    }
-}
-
-/// The error when parsing an `RgbaColor` fails.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub struct ParseColorError;
-
-impl std::error::Error for ParseColorError {}
-
-impl fmt::Display for ParseColorError {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.pad("invalid color")
-    }
-}
-
-/// A table expression.
-///
-/// # Example
-/// ```typst
-/// (false, 12cm, greeting="hi")
-/// ```
-pub type TableExpr = Table<TableExprEntry>;
-
-impl TableExpr {
-    /// Retrieve and remove the matching value with the lowest number key,
-    /// skipping and ignoring all non-matching entries with lower keys.
-    pub fn take<T: TryFromExpr>(&mut self) -> Option<T> {
-        for (&key, entry) in self.nums() {
-            let expr = entry.val.as_ref();
-            if let Some(val) = T::try_from_expr(expr, &mut Feedback::new()) {
-                self.remove(key);
-                return Some(val);
-            }
-        }
-        None
-    }
-
-    /// Retrieve and remove the matching value with the lowest number key,
-    /// removing and generating errors for all non-matching entries with lower
-    /// keys.
-    pub fn expect<T: TryFromExpr>(&mut self, f: &mut Feedback) -> Option<T> {
-        while let Some((num, _)) = self.first() {
-            let entry = self.remove(num).unwrap();
-            if let Some(val) = T::try_from_expr(entry.val.as_ref(), f) {
-                return Some(val);
-            }
-        }
-        None
-    }
-
-    /// Retrieve and remove a matching value associated with the given key if
-    /// there is any.
-    ///
-    /// Generates an error if the key exists but the value does not match.
-    pub fn take_with_key<'a, K, T>(&mut self, key: K, f: &mut Feedback) -> Option<T>
-    where
-        K: Into<BorrowedKey<'a>>,
-        T: TryFromExpr,
-    {
-        self.remove(key).and_then(|entry| {
-            let expr = entry.val.as_ref();
-            T::try_from_expr(expr, f)
-        })
-    }
-
-    /// Retrieve and remove all matching pairs with number keys, skipping and
-    /// ignoring non-matching entries.
-    ///
-    /// The pairs are returned in order of increasing keys.
-    pub fn take_all_num<'a, T>(&'a mut self) -> impl Iterator<Item = (u64, T)> + 'a
-    where
-        T: TryFromExpr,
-    {
-        let mut skip = 0;
-        std::iter::from_fn(move || {
-            for (&key, entry) in self.nums().skip(skip) {
-                let expr = entry.val.as_ref();
-                if let Some(val) = T::try_from_expr(expr, &mut Feedback::new()) {
-                    self.remove(key);
-                    return Some((key, val));
-                }
-                skip += 1;
-            }
-
-            None
-        })
-    }
-
-
-    /// Retrieve and remove all matching values with number keys, skipping and
-    /// ignoring non-matching entries.
-    ///
-    /// The values are returned in order of increasing keys.
-    pub fn take_all_num_vals<'a, T: 'a>(&'a mut self) -> impl Iterator<Item = T> + 'a
-    where
-        T: TryFromExpr,
-    {
-        self.take_all_num::<T>().map(|(_, v)| v)
-    }
-
-    /// Retrieve and remove all matching pairs with string keys, skipping and
-    /// ignoring non-matching entries.
-    ///
-    /// The pairs are returned in order of increasing keys.
-    pub fn take_all_str<'a, T>(&'a mut self) -> impl Iterator<Item = (String, T)> + 'a
-    where
-        T: TryFromExpr,
-    {
-        let mut skip = 0;
-        std::iter::from_fn(move || {
-            for (key, entry) in self.strs().skip(skip) {
-                let expr = entry.val.as_ref();
-                if let Some(val) = T::try_from_expr(expr, &mut Feedback::new()) {
-                    let key = key.clone();
-                    self.remove(&key);
-                    return Some((key, val));
-                }
-                skip += 1;
-            }
-
-            None
-        })
-    }
-
-    /// Generated `"unexpected argument"` errors for all remaining entries.
-    pub fn unexpected(&self, f: &mut Feedback) {
-        for entry in self.values() {
-            let span = Span::merge(entry.key, entry.val.span);
-            error!(@f, span, "unexpected argument");
-        }
-    }
-}
-
-/// An entry in a table expression.
-///
-/// Contains the key's span and the value.
-#[derive(Clone, PartialEq)]
-pub struct TableExprEntry {
-    pub key: Span,
-    pub val: Spanned<Expr>,
-}
-
-impl TableExprEntry {
-    /// Create a new entry.
-    pub fn new(key: Span, val: Spanned<Expr>) -> Self {
-        Self { key, val }
-    }
-
-    /// Create an entry for a positional argument with the same span for key and
-    /// value.
-    pub fn val(val: Spanned<Expr>) -> Self {
-        Self { key: Span::ZERO, val }
-    }
-}
-
-impl Debug for TableExprEntry {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        if f.alternate() {
-            f.write_str("key")?;
-            self.key.fmt(f)?;
-            f.write_str(" ")?;
-        }
-        self.val.fmt(f)
-    }
-}
-
-/// A trait for converting expressions into specific types.
-pub trait TryFromExpr: Sized {
-    // This trait takes references because we don't want to move the expression
-    // out of its origin in case this returns `None`. This solution is not
-    // perfect because we need to do some cloning in the impls for this trait,
-    // but I haven't got a better solution, for now.
-
-    /// Try to convert an expression into this type.
-    ///
-    /// Returns `None` and generates an appropriate error if the expression is
-    /// not valid for this type.
-    fn try_from_expr(expr: Spanned<&Expr>, f: &mut Feedback) -> Option<Self>;
-}
-
-macro_rules! impl_match {
-    ($type:ty, $name:expr, $($p:pat => $r:expr),* $(,)?) => {
-        impl TryFromExpr for $type {
-            fn try_from_expr(expr: Spanned<&Expr>, f: &mut Feedback) -> Option<Self> {
-                #[allow(unreachable_patterns)]
-                match expr.v {
-                    $($p => Some($r)),*,
-                    other => {
-                        error!(
-                            @f, expr.span,
-                            "expected {}, found {}", $name, other.name()
-                        );
-                        None
-                    }
-                }
-            }
-        }
-    };
-}
-
-macro_rules! impl_ident {
-    ($type:ty, $name:expr, $parse:expr) => {
-        impl TryFromExpr for $type {
-            fn try_from_expr(expr: Spanned<&Expr>, f: &mut Feedback) -> Option<Self> {
-                if let Expr::Ident(ident) = expr.v {
-                    let val = $parse(ident.as_str());
-                    if val.is_none() {
-                        error!(@f, expr.span, "invalid {}", $name);
-                    }
-                    val
-                } else {
-                    error!(
-                        @f, expr.span,
-                        "expected {}, found {}", $name, expr.v.name()
-                    );
-                    None
-                }
-            }
-        }
-    };
-}
-
-impl<T: TryFromExpr> TryFromExpr for Spanned<T> {
-    fn try_from_expr(expr: Spanned<&Expr>, f: &mut Feedback) -> Option<Self> {
-        let span = expr.span;
-        T::try_from_expr(expr, f).map(|v| Spanned { v, span })
-    }
-}
-
-impl_match!(Expr, "expression", e => e.clone());
-impl_match!(Ident, "identifier", Expr::Ident(i) => i.clone());
-impl_match!(String, "string", Expr::Str(s) => s.clone());
-impl_match!(bool, "bool", Expr::Bool(b) => b.clone());
-impl_match!(f64, "number", Expr::Number(n) => n.clone());
-impl_match!(Length, "length", Expr::Length(l) => l.clone());
-impl_match!(SyntaxTree, "tree", Expr::Tree(t) => t.clone());
-impl_match!(TableExpr, "table", Expr::Table(t) => t.clone());
-impl_match!(ScaleLength, "number or length",
-    &Expr::Length(length) => ScaleLength::Absolute(length),
-    &Expr::Number(scale) => ScaleLength::Scaled(scale),
-);
-
-/// A value type that matches identifiers and strings and implements
-/// `Into<String>`.
-pub struct StringLike(pub String);
-
-impl From<StringLike> for String {
-    fn from(like: StringLike) -> String {
-        like.0
-    }
-}
-
-impl_match!(StringLike, "identifier or string",
-    Expr::Ident(Ident(s)) => StringLike(s.clone()),
-    Expr::Str(s) => StringLike(s.clone()),
-);
-
-impl_ident!(Dir, "direction", |s| match s {
-    "ltr" => Some(Self::LTR),
-    "rtl" => Some(Self::RTL),
-    "ttb" => Some(Self::TTB),
-    "btt" => Some(Self::BTT),
-    _ => None,
-});
-
-impl_ident!(SpecAlign, "alignment", |s| match s {
-    "left" => Some(Self::Left),
-    "right" => Some(Self::Right),
-    "top" => Some(Self::Top),
-    "bottom" => Some(Self::Bottom),
-    "center" => Some(Self::Center),
-    _ => None,
-});
-
-impl_ident!(FontStyle, "font style", FontStyle::from_name);
-impl_ident!(Paper, "paper", Paper::from_name);
-
-impl TryFromExpr for FontWeight {
-    fn try_from_expr(expr: Spanned<&Expr>, f: &mut Feedback) -> Option<Self> {
-        match expr.v {
-            &Expr::Number(weight) => {
-                const MIN: u16 = 100;
-                const MAX: u16 = 900;
-
-                Some(Self(if weight < MIN as f64 {
-                    error!(@f, expr.span, "the minimum font weight is {}", MIN);
-                    MIN
-                } else if weight > MAX as f64 {
-                    error!(@f, expr.span, "the maximum font weight is {}", MAX);
-                    MAX
-                } else {
-                    weight.round() as u16
-                }))
-            }
-            Expr::Ident(ident) => {
-                let weight = Self::from_name(ident.as_str());
-                if weight.is_none() {
-                    error!(@f, expr.span, "invalid font weight");
-                }
-                weight
-            }
-            other => {
-                error!(
-                    @f, expr.span,
-                    "expected font weight (name or number), found {}",
-                    other.name(),
-                );
-                None
-            }
-        }
-    }
-}
-
-impl TryFromExpr for FontWidth {
-    fn try_from_expr(expr: Spanned<&Expr>, f: &mut Feedback) -> Option<Self> {
-        match expr.v {
-            &Expr::Number(width) => {
-                const MIN: u16 = 1;
-                const MAX: u16 = 9;
-
-                Self::new(if width < MIN as f64 {
-                    error!(@f, expr.span, "the minimum font width is {}", MIN);
-                    MIN
-                } else if width > MAX as f64 {
-                    error!(@f, expr.span, "the maximum font width is {}", MAX);
-                    MAX
-                } else {
-                    width.round() as u16
-                })
-            }
-            Expr::Ident(ident) => {
-                let width = Self::from_name(ident.as_str());
-                if width.is_none() {
-                    error!(@f, expr.span, "invalid font width");
-                }
-                width
-            }
-            other => {
-                error!(
-                    @f, expr.span,
-                    "expected font width (name or number), found {}",
-                    other.name(),
-                );
-                None
-            }
-        }
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn parse_color_strings() {
-        fn test(hex: &str, r: u8, g: u8, b: u8, a: u8) {
-            assert_eq!(
-                RgbaColor::from_str(hex),
-                Ok(RgbaColor::new(r, g, b, a)),
-            );
-        }
-
-        test("f61243ff", 0xf6, 0x12, 0x43, 0xff);
-        test("b3d8b3", 0xb3, 0xd8, 0xb3, 0xff);
-        test("fCd2a9AD", 0xfc, 0xd2, 0xa9, 0xad);
-        test("233", 0x22, 0x33, 0x33, 0xff);
-        test("111b", 0x11, 0x11, 0x11, 0xbb);
-    }
-
-    fn entry(expr: Expr) -> TableExprEntry {
-        TableExprEntry {
-            key: Span::ZERO,
-            val: Spanned::zero(expr),
-        }
-    }
-
-    #[test]
-    fn test_table_take_removes_correct_entry() {
-        let mut table = TableExpr::new();
-        table.insert(1, entry(Expr::Bool(false)));
-        table.insert(2, entry(Expr::Str("hi".to_string())));
-        assert_eq!(table.take::<String>(), Some("hi".to_string()));
-        assert_eq!(table.len(), 1);
-        assert_eq!(table.take::<bool>(), Some(false));
-        assert!(table.is_empty());
-    }
-
-    #[test]
-    fn test_table_expect_errors_about_previous_entries() {
-        let mut f = Feedback::new();
-        let mut table = TableExpr::new();
-        table.insert(1, entry(Expr::Bool(false)));
-        table.insert(3, entry(Expr::Str("hi".to_string())));
-        table.insert(5, entry(Expr::Bool(true)));
-        assert_eq!(table.expect::<String>(&mut f), Some("hi".to_string()));
-        assert_eq!(f.diagnostics, [error!(Span::ZERO, "expected string, found bool")]);
-        assert_eq!(table.len(), 1);
-    }
-
-    #[test]
-    fn test_table_take_with_key_removes_the_entry() {
-        let mut f = Feedback::new();
-        let mut table = TableExpr::new();
-        table.insert(1, entry(Expr::Bool(false)));
-        table.insert("hi", entry(Expr::Bool(true)));
-        assert_eq!(table.take_with_key::<_, bool>(1, &mut f), Some(false));
-        assert_eq!(table.take_with_key::<_, f64>("hi", &mut f), None);
-        assert_eq!(f.diagnostics, [error!(Span::ZERO, "expected number, found bool")]);
-        assert!(table.is_empty());
-    }
-
-    #[test]
-    fn test_table_take_all_removes_the_correct_entries() {
-        let mut table = TableExpr::new();
-        table.insert(1, entry(Expr::Bool(false)));
-        table.insert(3, entry(Expr::Number(0.0)));
-        table.insert(7, entry(Expr::Bool(true)));
-        assert_eq!(
-            table.take_all_num::<bool>().collect::<Vec<_>>(),
-            [(1, false), (7, true)],
-        );
-        assert_eq!(table.len(), 1);
-        assert_eq!(table[3].val.v, Expr::Number(0.0));
-    }
-}
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index fdc105a3..596291a5 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -1,17 +1,44 @@
 //! Syntax trees, parsing and tokenization.
 
 pub mod decoration;
-pub mod expr;
 pub mod parsing;
-pub mod scope;
 pub mod span;
 pub mod tokens;
 pub mod tree;
 
+use std::fmt::{self, Debug, Formatter};
+use tokens::is_identifier;
+
+/// An identifier as defined by unicode with a few extra permissible characters.
+#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct Ident(pub String);
+
+impl Ident {
+    /// Create a new identifier from a string checking that it is a valid.
+    pub fn new(ident: impl AsRef<str> + Into<String>) -> Option<Self> {
+        if is_identifier(ident.as_ref()) {
+            Some(Self(ident.into()))
+        } else {
+            None
+        }
+    }
+
+    /// Return a reference to the underlying string.
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+impl Debug for Ident {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "`{}`", self.0)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::fmt::Debug;
-    use crate::func::prelude::*;
+    use crate::prelude::*;
     use super::span;
 
     /// Assert that expected and found are equal, printing both and panicking
@@ -49,18 +76,4 @@ mod tests {
             Spanned::zero(t)
         }
     }
-
-    pub fn debug_func(call: FuncCall, _: &ParseState) -> Pass<SyntaxNode> {
-        Pass::node(DebugNode(call), Feedback::new())
-    }
-
-    #[derive(Debug, Clone, PartialEq)]
-    pub struct DebugNode(pub FuncCall);
-
-    #[async_trait(?Send)]
-    impl Layout for DebugNode {
-        async fn layout<'a>(&'a self, _: LayoutContext<'_>) -> Pass<Commands<'a>> {
-            unimplemented!()
-        }
-    }
 }
diff --git a/src/syntax/parsing.rs b/src/syntax/parsing.rs
index 88c930cb..8ed778e1 100644
--- a/src/syntax/parsing.rs
+++ b/src/syntax/parsing.rs
@@ -3,31 +3,13 @@
 use std::str::FromStr;
 
 use crate::{Feedback, Pass};
+use crate::color::RgbaColor;
+use crate::compute::table::SpannedEntry;
 use super::decoration::Decoration;
-use super::expr::*;
-use super::scope::Scope;
 use super::span::{Pos, Span, Spanned};
 use super::tokens::{is_newline_char, Token, TokenMode, Tokens};
-use super::tree::{SyntaxNode, SyntaxTree};
-
-/// A function which parses a function call into a dynamic node.
-pub type CallParser = dyn Fn(FuncCall, &ParseState) -> Pass<SyntaxNode>;
-
-/// An invocation of a function.
-#[derive(Debug, Clone, PartialEq)]
-pub struct FuncCall {
-    pub name: Spanned<Ident>,
-    pub args: TableExpr,
-}
-
-/// The state which can influence how a string of source code is parsed.
-///
-/// Parsing is pure - when passed in the same state and source code, the output
-/// must be the same.
-pub struct ParseState {
-    /// The scope containing all function definitions.
-    pub scope: Scope,
-}
+use super::tree::{CallExpr, Expr, SyntaxNode, SyntaxTree, TableExpr};
+use super::Ident;
 
 /// Parse a string of source code.
 ///
@@ -35,7 +17,7 @@ pub struct ParseState {
 /// `offset` position. This is used to make spans of a function body relative to
 /// the start of the function as a whole as opposed to the start of the
 /// function's body.
-pub fn parse(src: &str, offset: Pos, state: &ParseState) -> Pass<SyntaxTree> {
+pub fn parse(src: &str, offset: Pos) -> Pass<SyntaxTree> {
     let mut tree = SyntaxTree::new();
     let mut par = SyntaxTree::new();
     let mut feedback = Feedback::new();
@@ -58,12 +40,12 @@ pub fn parse(src: &str, offset: Pos, state: &ParseState) -> Pass<SyntaxTree> {
             }
 
             Token::Function { header, body, terminated } => {
-                let parsed = FuncParser::new(header, body, state).parse();
+                let parsed = FuncParser::new(header, body).parse();
                 feedback.extend_offset(parsed.feedback, span.start);
                 if !terminated {
                     error!(@feedback, Span::at(span.end), "expected closing bracket");
                 }
-                parsed.output
+                SyntaxNode::Call(parsed.output)
             }
 
             Token::Star => SyntaxNode::ToggleBolder,
@@ -97,8 +79,6 @@ pub fn parse(src: &str, offset: Pos, state: &ParseState) -> Pass<SyntaxTree> {
 }
 
 struct FuncParser<'s> {
-    state: &'s ParseState,
-    /// The tokens inside the header.
     tokens: Tokens<'s>,
     peeked: Option<Option<Spanned<Token<'s>>>>,
     body: Option<Spanned<&'s str>>,
@@ -106,13 +86,8 @@ struct FuncParser<'s> {
 }
 
 impl<'s> FuncParser<'s> {
-    fn new(
-        header: &'s str,
-        body: Option<Spanned<&'s str>>,
-        state: &'s ParseState,
-    ) -> Self {
+    fn new(header: &'s str, body: Option<Spanned<&'s str>>) -> Self {
         Self {
-            state,
             // Start at column 1 because the opening bracket is also part of
             // the function, but not part of the `header` string.
             tokens: Tokens::new(header, Pos::new(0, 1), TokenMode::Header),
@@ -122,65 +97,17 @@ impl<'s> FuncParser<'s> {
         }
     }
 
-    fn parse(mut self) -> Pass<SyntaxNode> {
-        let (parser, mut call) = if let Some(call) = self.parse_func_header() {
-            let name = call.name.v.as_str();
-            let (parser, deco) = match self.state.scope.get_parser(name) {
-                // The function exists in the scope.
-                Some(parser) => (parser, Decoration::ResolvedFunc),
-
-                // The function does not exist in the scope. The parser that is
-                // returned here is a fallback parser which exists to make sure
-                // the content of the function is not totally dropped (on a best
-                // effort basis).
-                None => {
-                    error!(@self.feedback, call.name.span, "unknown function");
-                    let parser = self.state.scope.get_fallback_parser();
-                    (parser, Decoration::UnresolvedFunc)
-                }
-            };
-
-            self.feedback.decorations.push(Spanned::new(deco, call.name.span));
-            (parser, call)
-        } else {
-            // Parse the call with the fallback parser even when the header is
-            // completely unparsable.
-            let parser = self.state.scope.get_fallback_parser();
-            let call = FuncCall {
-                name: Spanned::new(Ident(String::new()), Span::ZERO),
-                args: TableExpr::new(),
-            };
-            (parser, call)
-        };
-
-        if let Some(body) = self.body {
-            call.args.push(TableExprEntry {
-                key: Span::ZERO,
-                val: body.map(|src| {
-                    let parsed = parse(src, body.span.start, &self.state);
-                    self.feedback.extend(parsed.feedback);
-                    Expr::Tree(parsed.output)
-                }),
-            });
-        }
-
-        let parsed = parser(call, self.state);
-        self.feedback.extend(parsed.feedback);
-
-        Pass::new(parsed.output, self.feedback)
-    }
-
-    fn parse_func_header(&mut self) -> Option<FuncCall> {
+    fn parse(mut self) -> Pass<CallExpr> {
         let after_bracket = self.pos();
 
         self.skip_white();
-        let name = try_opt_or!(self.parse_ident(), {
+        let name = self.parse_ident().unwrap_or_else(|| {
             self.expected_found_or_at("function name", after_bracket);
-            return None;
+            Spanned::zero(Ident(String::new()))
         });
 
         self.skip_white();
-        let args = match self.eat().map(Spanned::value) {
+        let mut args = match self.eat().map(Spanned::value) {
             Some(Token::Colon) => self.parse_table(false).0.v,
             Some(_) => {
                 self.expected_at("colon", name.span.end);
@@ -189,7 +116,15 @@ impl<'s> FuncParser<'s> {
             None => TableExpr::new(),
         };
 
-        Some(FuncCall { name, args })
+        if let Some(body) = self.body {
+            args.push(SpannedEntry::val(body.map(|src| {
+                let parsed = parse(src, body.span.start);
+                self.feedback.extend(parsed.feedback);
+                Expr::Tree(parsed.output)
+            })));
+        }
+
+        Pass::new(CallExpr { name, args }, self.feedback)
     }
 }
 
@@ -325,14 +260,20 @@ impl FuncParser<'_> {
         }
     }
 
-    fn parse_func_call(&mut self, name: Spanned<Ident>) -> Spanned<FuncCall> {
+    fn parse_func_call(&mut self, name: Spanned<Ident>) -> Spanned<CallExpr> {
         let args = self.parse_table(true).0;
         let span = Span::merge(name.span, args.span);
-        Spanned::new(FuncCall { name, args: args.v }, span)
+        Spanned::new(CallExpr { name, args: args.v }, span)
     }
 
-    /// The boolean tells you whether the table can be coerced into an expression
-    /// (this is the case when it's length 1 and has no trailing comma).
+    /// Set `parens` to true, when this should expect an opening paren and stop
+    /// at the balanced closing paren (this is the case for normal tables and
+    /// round-paren function calls). Set it to false, when this is used to parse
+    /// the top-level function arguments.
+    ///
+    /// The returned boolean tells you whether the table can be coerced into an
+    /// expression (this is the case when it's length 1 and has no trailing
+    /// comma).
     fn parse_table(&mut self, parens: bool) -> (Spanned<TableExpr>, bool) {
         let start = self.pos();
         if parens {
@@ -369,19 +310,19 @@ impl FuncParser<'_> {
 
                     coercable = false;
                     behind_arg = val.span.end;
-                    table.insert(key.v.0, TableExprEntry::new(key.span, val));
+                    table.insert(key.v.0, SpannedEntry::new(key.span, val));
 
                 } else if self.check(Token::LeftParen) {
                     let call = self.parse_func_call(ident);
                     let expr = call.map(|call| Expr::Call(call));
 
                     behind_arg = expr.span.end;
-                    table.push(TableExprEntry::val(expr));
+                    table.push(SpannedEntry::val(expr));
                 } else {
                     let expr = ident.map(|id| Expr::Ident(id));
 
                     behind_arg = expr.span.end;
-                    table.push(TableExprEntry::val(expr));
+                    table.push(SpannedEntry::val(expr));
                 }
             } else {
                 // It's a positional argument.
@@ -390,7 +331,7 @@ impl FuncParser<'_> {
                     continue;
                 });
                 behind_arg = expr.span.end;
-                table.push(TableExprEntry::val(expr));
+                table.push(SpannedEntry::val(expr));
             }
 
             self.skip_white();
@@ -593,7 +534,7 @@ mod tests {
     }
 
     macro_rules! F {
-        ($($tts:tt)*) => { SyntaxNode::boxed(DebugNode(Call!(@$($tts)*))) }
+        ($($tts:tt)*) => { SyntaxNode::Call(Call!(@$($tts)*)) }
     }
 
     // ------------------------ Construct Expressions ----------------------- //
@@ -603,24 +544,17 @@ mod tests {
     fn Id(ident: &str) -> Expr { Expr::Ident(Ident(ident.to_string())) }
     fn Str(string: &str) -> Expr { Expr::Str(string.to_string()) }
 
-    macro_rules! Tree {
-        (@$($node:expr),* $(,)?) => {
-            vec![$(Into::<Spanned<SyntaxNode>>::into($node)),*]
-        };
-        ($($tts:tt)*) => { Expr::Tree(Tree![@$($tts)*]) };
-    }
-
     macro_rules! Table {
         (@table=$table:expr,) => {};
         (@table=$table:expr, $key:expr => $value:expr $(, $($tts:tt)*)?) => {{
             let key = Into::<Spanned<&str>>::into($key);
             let val = Into::<Spanned<Expr>>::into($value);
-            $table.insert(key.v, TableExprEntry::new(key.span, val));
+            $table.insert(key.v, SpannedEntry::new(key.span, val));
             Table![@table=$table, $($($tts)*)?];
         }};
         (@table=$table:expr, $value:expr $(, $($tts:tt)*)?) => {
             let val = Into::<Spanned<Expr>>::into($value);
-            $table.push(TableExprEntry::val(val));
+            $table.push(SpannedEntry::val(val));
             Table![@table=$table, $($($tts)*)?];
         };
         (@$($tts:tt)*) => {{
@@ -632,6 +566,24 @@ mod tests {
         ($($tts:tt)*) => { Expr::Table(Table![@$($tts)*]) };
     }
 
+    macro_rules! Tree {
+        (@$($node:expr),* $(,)?) => {
+            vec![$(Into::<Spanned<SyntaxNode>>::into($node)),*]
+        };
+        ($($tts:tt)*) => { Expr::Tree(Tree![@$($tts)*]) };
+    }
+
+    macro_rules! Call {
+        (@$name:expr $(; $($tts:tt)*)?) => {{
+            let name = Into::<Spanned<&str>>::into($name);
+            CallExpr {
+                name: name.map(|n| Ident(n.to_string())),
+                args: Table![@$($($tts)*)?],
+            }
+        }};
+        ($($tts:tt)*) => { Expr::Call(Call![@$($tts)*]) };
+    }
+
     fn Neg<T: Into<Spanned<Expr>>>(e1: T) -> Expr {
         Expr::Neg(Box::new(e1.into()))
     }
@@ -648,17 +600,6 @@ mod tests {
         Expr::Div(Box::new(e1.into()), Box::new(e2.into()))
     }
 
-    macro_rules! Call {
-        (@$name:expr $(; $($tts:tt)*)?) => {{
-            let name = Into::<Spanned<&str>>::into($name);
-            FuncCall {
-                name: name.map(|n| Ident(n.to_string())),
-                args: Table![@$($($tts)*)?],
-            }
-        }};
-        ($($tts:tt)*) => { Expr::Call(Call![@$($tts)*]) };
-    }
-
     // ----------------------------- Test Macros ---------------------------- //
 
     // Test syntax trees with or without spans.
@@ -667,7 +608,7 @@ mod tests {
     macro_rules! test {
         (@spans=$spans:expr, $src:expr => $($tts:tt)*) => {
             let exp = Tree![@$($tts)*];
-            let pass = parse_default($src);
+            let pass = parse($src, Pos::ZERO);
             check($src, exp, pass.output, $spans);
         };
     }
@@ -683,7 +624,7 @@ mod tests {
     macro_rules! e {
         ($src:expr => $($tts:tt)*) => {
             let exp = vec![$($tts)*];
-            let pass = parse_default($src);
+            let pass = parse($src, Pos::ZERO);
             let found = pass.feedback.diagnostics.iter()
                 .map(|s| s.as_ref().map(|e| e.message.as_str()))
                 .collect::<Vec<_>>();
@@ -695,20 +636,11 @@ mod tests {
     macro_rules! d {
         ($src:expr => $($tts:tt)*) => {
             let exp = vec![$($tts)*];
-            let pass = parse_default($src);
+            let pass = parse($src, Pos::ZERO);
             check($src, exp, pass.feedback.decorations, true);
         };
     }
 
-    fn parse_default(src: &str) -> Pass<SyntaxTree> {
-        let mut scope = Scope::new(Box::new(debug_func));
-        scope.insert("box", Box::new(debug_func));
-        scope.insert("val", Box::new(debug_func));
-        scope.insert("f", Box::new(debug_func));
-        let state = ParseState { scope };
-        parse(src, Pos::ZERO, &state)
-    }
-
     // -------------------------------- Tests ------------------------------- //
 
     #[test]
@@ -797,15 +729,9 @@ mod tests {
         e!("[\"]" => s(0,1, 0,3, "expected function name, found string"),
                      s(0,3, 0,3, "expected closing bracket"));
 
-        // An unknown name.
-        t!("[hi]" => P![F!("hi")]);
-        e!("[hi]" => s(0,1, 0,3, "unknown function"));
-        d!("[hi]" => s(0,1, 0,3, UnresolvedFunc));
-
         // A valid name.
-        t!("[f]"   => P![F!("f")]);
+        t!("[hi]"  => P![F!("hi")]);
         t!("[  f]" => P![F!("f")]);
-        d!("[  f]" => s(0,3, 0,4, ResolvedFunc));
 
         // An invalid name.
         e!("[12]"   => s(0,1, 0,3, "expected function name, found number"));
@@ -821,7 +747,6 @@ mod tests {
         t!("[val=]"     => P![F!("val")]);
         e!("[val=]"     => s(0,4, 0,4, "expected colon"));
         e!("[val/🌎:$]" => s(0,4, 0,4, "expected colon"));
-        d!("[val=]"     => s(0,1, 0,4, ResolvedFunc));
 
         // String in invalid header without colon still parsed as string
         // Note: No "expected quote" error because not even the string was
@@ -939,9 +864,9 @@ mod tests {
         v!("(1, key=\"value\")" => Table![Num(1.0), "key" => Str("value")]);
 
         // Decorations.
-        d!("[val: key=hi]"    => s(0,6, 0,9, TableKey), s(0,1, 0,4, ResolvedFunc));
-        d!("[val: (key=hi)]"  => s(0,7, 0,10, TableKey), s(0,1, 0,4, ResolvedFunc));
-        d!("[val: f(key=hi)]" => s(0,8, 0,11, TableKey), s(0,1, 0,4, ResolvedFunc));
+        d!("[val: key=hi]"    => s(0,6, 0,9, TableKey));
+        d!("[val: (key=hi)]"  => s(0,7, 0,10, TableKey));
+        d!("[val: f(key=hi)]" => s(0,8, 0,11, TableKey));
 
         // Spanned with spacing around keyword arguments.
         ts!("[val: \n hi \n = /* //\n */ \"s\n\"]" => s(0,0, 4,2, P![
diff --git a/src/syntax/scope.rs b/src/syntax/scope.rs
deleted file mode 100644
index c17ff64d..00000000
--- a/src/syntax/scope.rs
+++ /dev/null
@@ -1,44 +0,0 @@
-//! Mapping of function names to function parsers.
-
-use std::collections::HashMap;
-use std::fmt::{self, Debug, Formatter};
-
-use super::parsing::CallParser;
-
-/// A map from identifiers to function parsers.
-pub struct Scope {
-    parsers: HashMap<String, Box<CallParser>>,
-    fallback: Box<CallParser>,
-}
-
-impl Scope {
-    /// Create a new empty scope with a fallback parser that is invoked when no
-    /// match is found.
-    pub fn new(fallback: Box<CallParser>) -> Self {
-        Self {
-            parsers: HashMap::new(),
-            fallback,
-        }
-    }
-
-    /// Associate the given function name with a dynamic node type.
-    pub fn insert(&mut self, name: impl Into<String>, parser: Box<CallParser>) {
-        self.parsers.insert(name.into(), parser);
-    }
-
-    /// Return the parser with the given name if there is one.
-    pub fn get_parser(&self, name: &str) -> Option<&CallParser> {
-        self.parsers.get(name).map(AsRef::as_ref)
-    }
-
-    /// Return the fallback parser.
-    pub fn get_fallback_parser(&self) -> &CallParser {
-        &*self.fallback
-    }
-}
-
-impl Debug for Scope {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.debug_set().entries(self.parsers.keys()).finish()
-    }
-}
diff --git a/src/syntax/span.rs b/src/syntax/span.rs
index 923d13c2..89f773c7 100644
--- a/src/syntax/span.rs
+++ b/src/syntax/span.rs
@@ -39,13 +39,11 @@ impl<T> Offset for SpanVec<T> {
 }
 
 /// A value with the span it corresponds to in the source code.
-#[derive(Copy, Clone, Eq, PartialEq, Hash)]
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 #[cfg_attr(feature = "serialize", derive(Serialize))]
 pub struct Spanned<T> {
-    /// The value.
-    pub v: T,
-    /// The corresponding span.
     pub span: Span,
+    pub v: T,
 }
 
 impl<T> Spanned<T> {
@@ -99,7 +97,7 @@ impl<T: Debug> Debug for Spanned<T> {
 }
 
 /// Locates a slice of source code.
-#[derive(Copy, Clone, Hash)]
+#[derive(Copy, Clone, Ord, PartialOrd, Hash)]
 #[cfg_attr(feature = "serialize", derive(Serialize))]
 pub struct Span {
     /// The inclusive start position.
diff --git a/src/syntax/tree.rs b/src/syntax/tree.rs
index 4ce39cd4..e7a1eaf1 100644
--- a/src/syntax/tree.rs
+++ b/src/syntax/tree.rs
@@ -1,17 +1,20 @@
 //! The syntax tree.
 
-use std::any::Any;
-use std::fmt::Debug;
+use std::fmt::{self, Debug, Formatter};
 
-use crate::layout::Layout;
-use super::span::SpanVec;
+use crate::color::RgbaColor;
+use crate::compute::table::{SpannedEntry, Table};
+use crate::compute::value::{TableValue, Value};
+use crate::length::Length;
+use super::span::{Spanned, SpanVec};
+use super::Ident;
 
 /// A collection of nodes which form a tree together with the nodes' children.
 pub type SyntaxTree = SpanVec<SyntaxNode>;
 
 /// A syntax node, which encompasses a single logical entity of parsed source
 /// code.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum SyntaxNode {
     /// Whitespace containing less than two newlines.
     Spacing,
@@ -27,84 +30,138 @@ pub enum SyntaxNode {
     Raw(Vec<String>),
     /// A paragraph of child nodes.
     Par(SyntaxTree),
-    /// A dynamic node, created through function invocations in source code.
-    Dyn(Box<dyn DynamicNode>),
+    /// A function call.
+    Call(CallExpr),
 }
 
-impl SyntaxNode {
-    /// Create a `Dyn` variant from an unboxed dynamic node.
-    pub fn boxed<T: DynamicNode + 'static>(node: T) -> SyntaxNode {
-        SyntaxNode::Dyn(Box::new(node))
-    }
+/// An expression.
+#[derive(Clone, PartialEq)]
+pub enum Expr {
+    /// An identifier: `ident`.
+    Ident(Ident),
+    /// A string: `"string"`.
+    Str(String),
+    /// A boolean: `true, false`.
+    Bool(bool),
+    /// A number: `1.2, 200%`.
+    Number(f64),
+    /// A length: `2cm, 5.2in`.
+    Length(Length),
+    /// A color value with alpha channel: `#f79143ff`.
+    Color(RgbaColor),
+    /// A table expression: `(false, 12cm, greeting="hi")`.
+    Table(TableExpr),
+    /// A syntax tree containing typesetting content.
+    Tree(SyntaxTree),
+    /// A function call expression: `cmyk(37.7, 0, 3.9, 1.1)`.
+    Call(CallExpr),
+    /// An operation that negates the contained expression.
+    Neg(Box<Spanned<Expr>>),
+    /// An operation that adds the contained expressions.
+    Add(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
+    /// An operation that subtracts the contained expressions.
+    Sub(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
+    /// An operation that multiplies the contained expressions.
+    Mul(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
+    /// An operation that divides the contained expressions.
+    Div(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
 }
 
-impl PartialEq for SyntaxNode {
-    fn eq(&self, other: &SyntaxNode) -> bool {
-        use SyntaxNode::*;
-        match (self, other) {
-            (Spacing, Spacing) => true,
-            (Linebreak, Linebreak) => true,
-            (ToggleItalic, ToggleItalic) => true,
-            (ToggleBolder, ToggleBolder) => true,
-            (Text(a), Text(b)) => a == b,
-            (Raw(a), Raw(b)) => a == b,
-            (Par(a), Par(b)) => a == b,
-            (Dyn(a), Dyn(b)) => a == b,
-            _ => false,
+impl Expr {
+    /// A natural-language name of the type of this expression, e.g.
+    /// "identifier".
+    pub fn name(&self) -> &'static str {
+        use Expr::*;
+        match self {
+            Ident(_) => "identifier",
+            Str(_) => "string",
+            Bool(_) => "bool",
+            Number(_) => "number",
+            Length(_) => "length",
+            Color(_) => "color",
+            Table(_) => "table",
+            Tree(_) => "syntax tree",
+            Call(_) => "function call",
+            Neg(_) => "negation",
+            Add(_, _) => "addition",
+            Sub(_, _) => "subtraction",
+            Mul(_, _) => "multiplication",
+            Div(_, _) => "division",
         }
     }
-}
-
-/// Dynamic syntax nodes.
-///
-/// *Note*: This is automatically implemented for all types which are
-/// `Debug + Clone + PartialEq`, `Layout` and `'static`.
-pub trait DynamicNode: Debug + Layout {
-    /// Convert into a `dyn Any`.
-    fn as_any(&self) -> &dyn Any;
-
-    /// Check for equality with another dynamic node.
-    fn dyn_eq(&self, other: &dyn DynamicNode) -> bool;
 
-    /// Clone into a boxed node trait object.
-    fn box_clone(&self) -> Box<dyn DynamicNode>;
-}
-
-impl dyn DynamicNode {
-    /// Downcast this dynamic node to a concrete node.
-    pub fn downcast<T: DynamicNode + 'static>(&self) -> Option<&T> {
-        self.as_any().downcast_ref::<T>()
+    /// Evaluate the expression to a value.
+    pub fn eval(&self) -> Value {
+        use Expr::*;
+        match self {
+            Ident(i) => Value::Ident(i.clone()),
+            Str(s) => Value::Str(s.clone()),
+            &Bool(b) => Value::Bool(b),
+            &Number(n) => Value::Number(n),
+            &Length(s) => Value::Length(s),
+            &Color(c) => Value::Color(c),
+            Table(t) => Value::Table(t.eval()),
+            Tree(t) => Value::Tree(t.clone()),
+            Call(_) => todo!("eval call"),
+            Neg(_) => todo!("eval neg"),
+            Add(_, _) => todo!("eval add"),
+            Sub(_, _) => todo!("eval sub"),
+            Mul(_, _) => todo!("eval mul"),
+            Div(_, _) => todo!("eval div"),
+        }
     }
 }
 
-impl PartialEq for dyn DynamicNode {
-    fn eq(&self, other: &Self) -> bool {
-        self.dyn_eq(other)
+impl Debug for Expr {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        use Expr::*;
+        match self {
+            Ident(i) => i.fmt(f),
+            Str(s) => s.fmt(f),
+            Bool(b) => b.fmt(f),
+            Number(n) => n.fmt(f),
+            Length(s) => s.fmt(f),
+            Color(c) => c.fmt(f),
+            Table(t) => t.fmt(f),
+            Tree(t) => t.fmt(f),
+            Call(c) => c.fmt(f),
+            Neg(e) => write!(f, "-{:?}", e),
+            Add(a, b) => write!(f, "({:?} + {:?})", a, b),
+            Sub(a, b) => write!(f, "({:?} - {:?})", a, b),
+            Mul(a, b) => write!(f, "({:?} * {:?})", a, b),
+            Div(a, b) => write!(f, "({:?} / {:?})", a, b),
+        }
     }
 }
 
-impl Clone for Box<dyn DynamicNode> {
-    fn clone(&self) -> Self {
-        self.box_clone()
-    }
-}
+/// A table of expressions.
+///
+/// # Example
+/// ```typst
+/// (false, 12cm, greeting="hi")
+/// ```
+pub type TableExpr = Table<SpannedEntry<Expr>>;
 
-impl<T> DynamicNode for T
-where
-    T: Debug + PartialEq + Clone + Layout + 'static,
-{
-    fn as_any(&self) -> &dyn Any {
-        self
-    }
+impl TableExpr {
+    /// Evaluate the table expression to a table value.
+    pub fn eval(&self) -> TableValue {
+        let mut table = TableValue::new();
 
-    fn dyn_eq(&self, other: &dyn DynamicNode) -> bool {
-        match other.as_any().downcast_ref::<Self>() {
-            Some(other) => self == other,
-            None => false,
+        for (&key, entry) in self.nums() {
+            table.insert(key, entry.as_ref().map(|val| val.eval()));
         }
-    }
 
-    fn box_clone(&self) -> Box<dyn DynamicNode> {
-        Box::new(self.clone())
+        for (key, entry) in self.strs() {
+            table.insert(key.clone(), entry.as_ref().map(|val| val.eval()));
+        }
+
+        table
     }
 }
+
+/// An invocation of a function.
+#[derive(Debug, Clone, PartialEq)]
+pub struct CallExpr {
+    pub name: Spanned<Ident>,
+    pub args: TableExpr,
+}