Basic show rules

25 files changed, 480 insertions, 1026 deletions

diff --git a/src/eval/capture.rs b/src/eval/capture.rs
index d62ec55c..fd6b0101 100644
--- a/src/eval/capture.rs
+++ b/src/eval/capture.rs
@@ -50,7 +50,7 @@ impl<'a> CapturesVisitor<'a> {
             Some(Expr::Ident(ident)) => self.capture(ident),
 
             // A closure contains parameter bindings, which are bound before the
-            // body is evaluated. Take must be taken so that the default values
+            // body is evaluated. Care must be taken so that the default values
             // of named parameters cannot access previous parameter bindings.
             Some(Expr::Closure(expr)) => {
                 for param in expr.params() {
diff --git a/src/eval/class.rs b/src/eval/class.rs
index 28e49a99..5601fb0b 100644
--- a/src/eval/class.rs
+++ b/src/eval/class.rs
@@ -1,3 +1,4 @@
+use std::any::TypeId;
 use std::fmt::{self, Debug, Formatter, Write};
 use std::hash::{Hash, Hasher};
 
@@ -36,6 +37,7 @@ use crate::diag::TypResult;
 #[derive(Clone)]
 pub struct Class {
     name: &'static str,
+    id: TypeId,
     construct: fn(&mut Vm, &mut Args) -> TypResult<Value>,
     set: fn(&mut Args, &mut StyleMap) -> TypResult<()>,
 }
@@ -48,6 +50,7 @@ impl Class {
     {
         Self {
             name,
+            id: TypeId::of::<T>(),
             construct: |ctx, args| {
                 let mut styles = StyleMap::new();
                 T::set(args, &mut styles)?;
@@ -63,6 +66,11 @@ impl Class {
         self.name
     }
 
+    /// The type id of the class.
+    pub fn id(&self) -> TypeId {
+        self.id
+    }
+
     /// Return the class constructor as a function.
     pub fn constructor(&self) -> Func {
         Func::native(self.name, self.construct)
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 32ffb0c9..1b61ac15 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -36,7 +36,6 @@ use unicode_segmentation::UnicodeSegmentation;
 
 use crate::diag::{At, Error, StrResult, Trace, Tracepoint, TypResult};
 use crate::geom::{Angle, Fractional, Length, Relative};
-use crate::layout::Layout;
 use crate::library;
 use crate::syntax::ast::*;
 use crate::syntax::{Span, Spanned};
@@ -80,15 +79,12 @@ fn eval_markup(
     while let Some(node) = nodes.next() {
         seq.push(match node {
             MarkupNode::Expr(Expr::Set(set)) => {
-                let class = set.class();
-                let class = class.eval(vm)?.cast::<Class>().at(class.span())?;
-                let args = set.args().eval(vm)?;
-                let styles = class.set(args)?;
-                let tail = eval_markup(vm, nodes)?;
-                tail.styled_with_map(styles)
+                let styles = set.eval(vm)?;
+                eval_markup(vm, nodes)?.styled_with_map(styles)
             }
             MarkupNode::Expr(Expr::Show(show)) => {
-                return Err("show rules are not yet implemented").at(show.span());
+                let styles = show.eval(vm)?;
+                eval_markup(vm, nodes)?.styled_with_map(styles)
             }
             MarkupNode::Expr(Expr::Wrap(wrap)) => {
                 let tail = eval_markup(vm, nodes)?;
@@ -182,7 +178,7 @@ impl Eval for ListNode {
     fn eval(&self, vm: &mut Vm) -> TypResult<Self::Output> {
         Ok(Template::List(library::ListItem {
             number: None,
-            body: self.body().eval(vm)?.pack(),
+            body: Box::new(self.body().eval(vm)?),
         }))
     }
 }
@@ -193,7 +189,7 @@ impl Eval for EnumNode {
     fn eval(&self, vm: &mut Vm) -> TypResult<Self::Output> {
         Ok(Template::Enum(library::ListItem {
             number: self.number(),
-            body: self.body().eval(vm)?.pack(),
+            body: Box::new(self.body().eval(vm)?),
         }))
     }
 }
@@ -216,9 +212,10 @@ impl Eval for Expr {
             Self::Unary(v) => v.eval(vm),
             Self::Binary(v) => v.eval(vm),
             Self::Let(v) => v.eval(vm),
-            Self::Set(v) => v.eval(vm),
-            Self::Show(v) => v.eval(vm),
-            Self::Wrap(v) => v.eval(vm),
+            Self::Set(_) | Self::Show(_) | Self::Wrap(_) => {
+                Err("set, show and wrap are only allowed directly in markup")
+                    .at(self.span())
+            }
             Self::If(v) => v.eval(vm),
             Self::While(v) => v.eval(vm),
             Self::For(v) => v.eval(vm),
@@ -551,26 +548,27 @@ impl Eval for LetExpr {
 }
 
 impl Eval for SetExpr {
-    type Output = Value;
+    type Output = StyleMap;
 
-    fn eval(&self, _: &mut Vm) -> TypResult<Self::Output> {
-        Err("set is only allowed directly in markup").at(self.span())
+    fn eval(&self, vm: &mut Vm) -> TypResult<Self::Output> {
+        let class = self.class();
+        let class = class.eval(vm)?.cast::<Class>().at(class.span())?;
+        let args = self.args().eval(vm)?;
+        class.set(args)
     }
 }
 
 impl Eval for ShowExpr {
-    type Output = Value;
+    type Output = StyleMap;
 
-    fn eval(&self, _: &mut Vm) -> TypResult<Self::Output> {
-        Err("show is only allowed directly in markup").at(self.span())
-    }
-}
-
-impl Eval for WrapExpr {
-    type Output = Value;
-
-    fn eval(&self, _: &mut Vm) -> TypResult<Self::Output> {
-        Err("wrap is only allowed directly in markup").at(self.span())
+    fn eval(&self, vm: &mut Vm) -> TypResult<Self::Output> {
+        let class = self.class();
+        let class = class.eval(vm)?.cast::<Class>().at(class.span())?;
+        let closure = self.closure();
+        let func = closure.eval(vm)?.cast::<Func>().at(closure.span())?;
+        let mut styles = StyleMap::new();
+        styles.set_recipe(class.id(), func, self.span());
+        Ok(styles)
     }
 }
 
diff --git a/src/eval/styles.rs b/src/eval/styles.rs
index 2bf3f239..346eb784 100644
--- a/src/eval/styles.rs
+++ b/src/eval/styles.rs
@@ -3,21 +3,28 @@ use std::fmt::{self, Debug, Formatter};
 use std::hash::{Hash, Hasher};
 use std::sync::Arc;
 
+use super::{Args, Func, Span, Template, Value, Vm};
+use crate::diag::{At, TypResult};
 use crate::library::{PageNode, ParNode};
 
 /// A map of style properties.
 #[derive(Default, Clone, PartialEq, Hash)]
-pub struct StyleMap(Vec<Entry>);
+pub struct StyleMap {
+    /// Settable properties.
+    props: Vec<Entry>,
+    /// Show rule recipes.
+    recipes: Vec<Recipe>,
+}
 
 impl StyleMap {
     /// Create a new, empty style map.
     pub fn new() -> Self {
-        Self(vec![])
+        Self { props: vec![], recipes: vec![] }
     }
 
     /// Whether this map contains no styles.
     pub fn is_empty(&self) -> bool {
-        self.0.is_empty()
+        self.props.is_empty() && self.recipes.is_empty()
     }
 
     /// Create a style map from a single property-value pair.
@@ -29,7 +36,7 @@ impl StyleMap {
 
     /// Set the value for a style property.
     pub fn set<P: Property>(&mut self, key: P, value: P::Value) {
-        self.0.push(Entry::new(key, value));
+        self.props.push(Entry::new(key, value));
     }
 
     /// Set a value for a style property if it is `Some(_)`.
@@ -39,11 +46,16 @@ impl StyleMap {
         }
     }
 
+    /// Set a recipe.
+    pub fn set_recipe(&mut self, node: TypeId, func: Func, span: Span) {
+        self.recipes.push(Recipe { node, func, span });
+    }
+
     /// Mark all contained properties as _scoped_. This means that they only
     /// apply to the first descendant node (of their type) in the hierarchy and
     /// not its children, too. This is used by class constructors.
     pub fn scoped(mut self) -> Self {
-        for entry in &mut self.0 {
+        for entry in &mut self.props {
             entry.scoped = true;
         }
         self
@@ -51,7 +63,7 @@ impl StyleMap {
 
     /// Whether this map contains scoped styles.
     pub fn has_scoped(&self) -> bool {
-        self.0.iter().any(|e| e.scoped)
+        self.props.iter().any(|e| e.scoped)
     }
 
     /// Make `self` the first link of the style chain `outer`.
@@ -78,20 +90,24 @@ impl StyleMap {
     /// lifetime, for example, because you want to store the style map inside a
     /// packed node.
     pub fn apply(&mut self, outer: &Self) {
-        self.0.splice(0 .. 0, outer.0.clone());
+        self.props.splice(0 .. 0, outer.props.iter().cloned());
+        self.recipes.splice(0 .. 0, outer.recipes.iter().cloned());
     }
 
     /// The highest-level interruption of the map.
     pub fn interruption(&self) -> Option<Interruption> {
-        self.0.iter().filter_map(|entry| entry.interruption()).max()
+        self.props.iter().filter_map(|entry| entry.interruption()).max()
     }
 }
 
 impl Debug for StyleMap {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        for entry in self.0.iter().rev() {
+        for entry in self.props.iter().rev() {
             writeln!(f, "{:#?}", entry)?;
         }
+        for recipe in self.recipes.iter().rev() {
+            writeln!(f, "#[Recipe for {:?} from {:?}]", recipe.node, recipe.span)?;
+        }
         Ok(())
     }
 }
@@ -105,6 +121,169 @@ pub enum Interruption {
     Page,
 }
 
+/// Style property keys.
+///
+/// This trait is not intended to be implemented manually, but rather through
+/// the `#[class]` proc-macro.
+pub trait Property: Sync + Send + 'static {
+    /// The type of value that is returned when getting this property from a
+    /// style map. For example, this could be [`Length`](crate::geom::Length)
+    /// for a `WIDTH` property.
+    type Value: Debug + Clone + PartialEq + Hash + Sync + Send + 'static;
+
+    /// The name of the property, used for debug printing.
+    const NAME: &'static str;
+
+    /// Whether the property needs folding.
+    const FOLDING: bool = false;
+
+    /// The type id of the node this property belongs to.
+    fn node_id() -> TypeId;
+
+    /// The default value of the property.
+    fn default() -> Self::Value;
+
+    /// A static reference to the default value of the property.
+    ///
+    /// This is automatically implemented through lazy-initialization in the
+    /// `#[class]` macro. This way, expensive defaults don't need to be
+    /// recreated all the time.
+    fn default_ref() -> &'static Self::Value;
+
+    /// Fold the property with an outer value.
+    ///
+    /// For example, this would fold a relative font size with an outer
+    /// absolute font size.
+    #[allow(unused_variables)]
+    fn fold(inner: Self::Value, outer: Self::Value) -> Self::Value {
+        inner
+    }
+}
+
+/// Marker trait that indicates that a property doesn't need folding.
+pub trait Nonfolding {}
+
+/// An entry for a single style property.
+#[derive(Clone)]
+struct Entry {
+    pair: Arc<dyn Bounds>,
+    scoped: bool,
+}
+
+impl Entry {
+    fn new<P: Property>(key: P, value: P::Value) -> Self {
+        Self {
+            pair: Arc::new((key, value)),
+            scoped: false,
+        }
+    }
+
+    fn is<P: Property>(&self) -> bool {
+        self.pair.style_id() == TypeId::of::<P>()
+    }
+
+    fn is_of<T: 'static>(&self) -> bool {
+        self.pair.node_id() == TypeId::of::<T>()
+    }
+
+    fn is_of_id(&self, node: TypeId) -> bool {
+        self.pair.node_id() == node
+    }
+
+    fn downcast<P: Property>(&self) -> Option<&P::Value> {
+        self.pair.as_any().downcast_ref()
+    }
+
+    fn interruption(&self) -> Option<Interruption> {
+        if self.is_of::<PageNode>() {
+            Some(Interruption::Page)
+        } else if self.is_of::<ParNode>() {
+            Some(Interruption::Par)
+        } else {
+            None
+        }
+    }
+}
+
+impl Debug for Entry {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.write_str("#[")?;
+        self.pair.dyn_fmt(f)?;
+        if self.scoped {
+            f.write_str(" (scoped)")?;
+        }
+        f.write_str("]")
+    }
+}
+
+impl PartialEq for Entry {
+    fn eq(&self, other: &Self) -> bool {
+        self.pair.dyn_eq(other) && self.scoped == other.scoped
+    }
+}
+
+impl Hash for Entry {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        state.write_u64(self.pair.hash64());
+        state.write_u8(self.scoped as u8);
+    }
+}
+
+/// This trait is implemented for pairs of zero-sized property keys and their
+/// value types below. Although it is zero-sized, the property `P` must be part
+/// of the implementing type so that we can use it in the methods (it must be a
+/// constrained type parameter).
+trait Bounds: Sync + Send + 'static {
+    fn as_any(&self) -> &dyn Any;
+    fn dyn_fmt(&self, f: &mut Formatter) -> fmt::Result;
+    fn dyn_eq(&self, other: &Entry) -> bool;
+    fn hash64(&self) -> u64;
+    fn node_id(&self) -> TypeId;
+    fn style_id(&self) -> TypeId;
+}
+
+impl<P: Property> Bounds for (P, P::Value) {
+    fn as_any(&self) -> &dyn Any {
+        &self.1
+    }
+
+    fn dyn_fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{} = {:?}", P::NAME, self.1)
+    }
+
+    fn dyn_eq(&self, other: &Entry) -> bool {
+        self.style_id() == other.pair.style_id()
+            && if let Some(other) = other.downcast::<P>() {
+                &self.1 == other
+            } else {
+                false
+            }
+    }
+
+    fn hash64(&self) -> u64 {
+        let mut state = fxhash::FxHasher64::default();
+        self.style_id().hash(&mut state);
+        self.1.hash(&mut state);
+        state.finish()
+    }
+
+    fn node_id(&self) -> TypeId {
+        P::node_id()
+    }
+
+    fn style_id(&self) -> TypeId {
+        TypeId::of::<P>()
+    }
+}
+
+/// A show rule recipe.
+#[derive(Debug, Clone, PartialEq, Hash)]
+struct Recipe {
+    node: TypeId,
+    func: Func,
+    span: Span,
+}
+
 /// A chain of style maps, similar to a linked list.
 ///
 /// A style chain allows to conceptually merge (and fold) properties from
@@ -162,11 +341,18 @@ impl<'a> StyleChain<'a> {
         *self = self.outer.copied().unwrap_or_default();
     }
 
+    /// Return the span of a recipe for the given node.
+    pub(crate) fn recipe_span(self, node: TypeId) -> Option<Span> {
+        self.recipes(node).next().map(|recipe| recipe.span)
+    }
+
     /// Return the chain, but without the last link if that one contains only
     /// scoped styles. This is a hack.
     pub(crate) fn unscoped(mut self, node: TypeId) -> Self {
         if let Some(Link::Map(map)) = self.link {
-            if map.0.iter().all(|e| e.scoped && e.is_of_id(node)) {
+            if map.props.iter().all(|e| e.scoped && e.is_of_id(node))
+                && map.recipes.is_empty()
+            {
                 self.pop();
             }
         }
@@ -225,6 +411,21 @@ impl<'a> StyleChain<'a> {
         }
     }
 
+    /// Execute a user recipe for a node.
+    pub fn show(
+        self,
+        node: &dyn Any,
+        vm: &mut Vm,
+        values: impl IntoIterator<Item = Value>,
+    ) -> TypResult<Option<Template>> {
+        Ok(if let Some(recipe) = self.recipes(node.type_id()).next() {
+            let args = Args::from_values(recipe.span, values);
+            Some(recipe.func.call(vm, args)?.cast().at(recipe.span)?)
+        } else {
+            None
+        })
+    }
+
     /// Insert a barrier into the style chain.
     ///
     /// Barriers interact with [scoped](StyleMap::scoped) styles: A scoped style
@@ -233,7 +434,7 @@ impl<'a> StyleChain<'a> {
     pub fn barred<'b>(&'b self, node: TypeId) -> StyleChain<'b> {
         if self
             .maps()
-            .any(|map| map.0.iter().any(|entry| entry.scoped && entry.is_of_id(node)))
+            .any(|map| map.props.iter().any(|entry| entry.scoped && entry.is_of_id(node)))
         {
             StyleChain {
                 link: Some(Link::Barrier(node)),
@@ -252,7 +453,7 @@ impl<'a> StyleChain<'a> {
         self.links().flat_map(move |link| {
             let mut entries: &[Entry] = &[];
             match link {
-                Link::Map(map) => entries = &map.0,
+                Link::Map(map) => entries = &map.props,
                 Link::Barrier(id) => depth += (id == P::node_id()) as usize,
             }
             entries
@@ -263,6 +464,13 @@ impl<'a> StyleChain<'a> {
         })
     }
 
+    /// Iterate over the recipes for the given node.
+    fn recipes(self, node: TypeId) -> impl Iterator<Item = &'a Recipe> {
+        self.maps()
+            .flat_map(|map| map.recipes.iter().rev())
+            .filter(move |recipe| recipe.node == node)
+    }
+
     /// Iterate over the map links of the chain.
     fn maps(self) -> impl Iterator<Item = &'a StyleMap> {
         self.links().filter_map(|link| match link {
@@ -443,158 +651,3 @@ impl<'a, T> Default for StyleVecBuilder<'a, T> {
         Self::new()
     }
 }
-
-/// Style property keys.
-///
-/// This trait is not intended to be implemented manually, but rather through
-/// the `#[class]` proc-macro.
-pub trait Property: Sync + Send + 'static {
-    /// The type of value that is returned when getting this property from a
-    /// style map. For example, this could be [`Length`](crate::geom::Length)
-    /// for a `WIDTH` property.
-    type Value: Debug + Clone + PartialEq + Hash + Sync + Send + 'static;
-
-    /// The name of the property, used for debug printing.
-    const NAME: &'static str;
-
-    /// Whether the property needs folding.
-    const FOLDING: bool = false;
-
-    /// The type id of the node this property belongs to.
-    fn node_id() -> TypeId;
-
-    /// The default value of the property.
-    fn default() -> Self::Value;
-
-    /// A static reference to the default value of the property.
-    ///
-    /// This is automatically implemented through lazy-initialization in the
-    /// `#[class]` macro. This way, expensive defaults don't need to be
-    /// recreated all the time.
-    fn default_ref() -> &'static Self::Value;
-
-    /// Fold the property with an outer value.
-    ///
-    /// For example, this would fold a relative font size with an outer
-    /// absolute font size.
-    #[allow(unused_variables)]
-    fn fold(inner: Self::Value, outer: Self::Value) -> Self::Value {
-        inner
-    }
-}
-
-/// Marker trait that indicates that a property doesn't need folding.
-pub trait Nonfolding {}
-
-/// An entry for a single style property.
-#[derive(Clone)]
-struct Entry {
-    pair: Arc<dyn Bounds>,
-    scoped: bool,
-}
-
-impl Entry {
-    fn new<P: Property>(key: P, value: P::Value) -> Self {
-        Self {
-            pair: Arc::new((key, value)),
-            scoped: false,
-        }
-    }
-
-    fn is<P: Property>(&self) -> bool {
-        self.pair.style_id() == TypeId::of::<P>()
-    }
-
-    fn is_of<T: 'static>(&self) -> bool {
-        self.pair.node_id() == TypeId::of::<T>()
-    }
-
-    fn is_of_id(&self, node: TypeId) -> bool {
-        self.pair.node_id() == node
-    }
-
-    fn downcast<P: Property>(&self) -> Option<&P::Value> {
-        self.pair.as_any().downcast_ref()
-    }
-
-    fn interruption(&self) -> Option<Interruption> {
-        if self.is_of::<PageNode>() {
-            Some(Interruption::Page)
-        } else if self.is_of::<ParNode>() {
-            Some(Interruption::Par)
-        } else {
-            None
-        }
-    }
-}
-
-impl Debug for Entry {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.write_str("#[")?;
-        self.pair.dyn_fmt(f)?;
-        if self.scoped {
-            f.write_str(" (scoped)")?;
-        }
-        f.write_str("]")
-    }
-}
-
-impl PartialEq for Entry {
-    fn eq(&self, other: &Self) -> bool {
-        self.pair.dyn_eq(other) && self.scoped == other.scoped
-    }
-}
-
-impl Hash for Entry {
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        state.write_u64(self.pair.hash64());
-        state.write_u8(self.scoped as u8);
-    }
-}
-
-/// This trait is implemented for pairs of zero-sized property keys and their
-/// value types below. Although it is zero-sized, the property `P` must be part
-/// of the implementing type so that we can use it in the methods (it must be a
-/// constrained type parameter).
-trait Bounds: Sync + Send + 'static {
-    fn as_any(&self) -> &dyn Any;
-    fn dyn_fmt(&self, f: &mut Formatter) -> fmt::Result;
-    fn dyn_eq(&self, other: &Entry) -> bool;
-    fn hash64(&self) -> u64;
-    fn node_id(&self) -> TypeId;
-    fn style_id(&self) -> TypeId;
-}
-
-impl<P: Property> Bounds for (P, P::Value) {
-    fn as_any(&self) -> &dyn Any {
-        &self.1
-    }
-
-    fn dyn_fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "{} = {:?}", P::NAME, self.1)
-    }
-
-    fn dyn_eq(&self, other: &Entry) -> bool {
-        self.style_id() == other.pair.style_id()
-            && if let Some(other) = other.downcast::<P>() {
-                &self.1 == other
-            } else {
-                false
-            }
-    }
-
-    fn hash64(&self) -> u64 {
-        let mut state = fxhash::FxHasher64::default();
-        self.style_id().hash(&mut state);
-        self.1.hash(&mut state);
-        state.finish()
-    }
-
-    fn node_id(&self) -> TypeId {
-        P::node_id()
-    }
-
-    fn style_id(&self) -> TypeId {
-        TypeId::of::<P>()
-    }
-}
diff --git a/src/eval/template.rs b/src/eval/template.rs
index 1f1544e6..465e9195 100644
--- a/src/eval/template.rs
+++ b/src/eval/template.rs
@@ -403,9 +403,16 @@ impl<'a> Builder<'a> {
                 }
             }
             Template::Show(node) => {
+                let id = node.id();
+                if vm.rules.contains(&id) {
+                    let span = styles.recipe_span(id).unwrap();
+                    return Err("show rule is recursive").at(span)?;
+                }
+                vm.rules.push(id);
                 let template = node.show(vm, styles)?;
                 let stored = self.tpa.alloc(template);
-                self.process(vm, stored, styles.unscoped(node.id()))?;
+                self.process(vm, stored, styles.unscoped(id))?;
+                vm.rules.pop();
             }
             Template::Styled(styled) => {
                 let (sub, map) = styled.as_ref();
@@ -455,8 +462,8 @@ impl<'a> Builder<'a> {
         };
 
         let template = match kind {
-            UNORDERED => Template::show(ListNode::<UNORDERED> { items, wide, start: 1 }),
-            ORDERED | _ => Template::show(ListNode::<ORDERED> { items, wide, start: 1 }),
+            UNORDERED => Template::show(ListNode::<UNORDERED> { start: 1, wide, items }),
+            ORDERED | _ => Template::show(ListNode::<ORDERED> { start: 1, wide, items }),
         };
 
         let stored = self.tpa.alloc(template);
diff --git a/src/eval/value.rs b/src/eval/value.rs
index 2de210d5..952c7293 100644
--- a/src/eval/value.rs
+++ b/src/eval/value.rs
@@ -336,7 +336,7 @@ pub trait Cast<V = Value>: Sized {
 macro_rules! primitive {
     (
         $type:ty: $name:literal, $variant:ident
-        $(, $other:ident($binding:ident) => $out:expr)*
+        $(, $other:ident$(($binding:ident))? => $out:expr)*
     ) => {
         impl Type for $type {
             const TYPE_NAME: &'static str = $name;
@@ -344,13 +344,14 @@ macro_rules! primitive {
 
         impl Cast for $type {
             fn is(value: &Value) -> bool {
-                matches!(value, Value::$variant(_) $(| Value::$other(_))*)
+                matches!(value, Value::$variant(_)
+                    $(|  primitive!(@$other $(($binding))?))*)
             }
 
             fn cast(value: Value) -> StrResult<Self> {
                 match value {
                     Value::$variant(v) => Ok(v),
-                    $(Value::$other($binding) => Ok($out),)*
+                    $(Value::$other$(($binding))? => Ok($out),)*
                     v => Err(format!(
                         "expected {}, found {}",
                         Self::TYPE_NAME,
@@ -366,6 +367,9 @@ macro_rules! primitive {
             }
         }
     };
+
+    (@$other:ident($binding:ident)) => { Value::$other(_) };
+    (@$other:ident) => { Value::$other };
 }
 
 /// Implement traits for dynamic types.
@@ -440,7 +444,7 @@ primitive! { Color: "color", Color }
 primitive! { EcoString: "string", Str }
 primitive! { Array: "array", Array }
 primitive! { Dict: "dictionary", Dict }
-primitive! { Template: "template", Template }
+primitive! { Template: "template", Template, None => Template::new() }
 primitive! { Func: "function", Func, Class(v) => v.constructor() }
 primitive! { Args: "arguments", Args }
 primitive! { Class: "class", Class }
diff --git a/src/lib.rs b/src/lib.rs
index 6ad30d2f..be1bab0d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -25,7 +25,7 @@
 //! [evaluate]: Vm::evaluate
 //! [module]: eval::Module
 //! [template]: eval::Template
-//! [layouted]: eval::Template::layout
+//! [layouted]: eval::Template::layout_pages
 //! [cache]: layout::LayoutCache
 //! [PDF]: export::pdf
 
@@ -51,6 +51,7 @@ pub mod parse;
 pub mod source;
 pub mod syntax;
 
+use std::any::TypeId;
 use std::collections::HashMap;
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -218,6 +219,9 @@ pub struct Vm<'a> {
     pub modules: HashMap<SourceId, Module>,
     /// The active scopes.
     pub scopes: Scopes<'a>,
+    /// Currently evaluated show rules. This is used to prevent recursive show
+    /// rules.
+    pub rules: Vec<TypeId>,
     /// How deeply nested the current layout tree position is.
     #[cfg(feature = "layout-cache")]
     pub level: usize,
@@ -236,6 +240,7 @@ impl<'a> Vm<'a> {
             route: vec![],
             modules: HashMap::new(),
             scopes: Scopes::new(Some(&ctx.std)),
+            rules: vec![],
             level: 0,
         }
     }
diff --git a/src/library/deco.rs b/src/library/deco.rs
index 79026288..aac79d05 100644
--- a/src/library/deco.rs
+++ b/src/library/deco.rs
@@ -32,15 +32,19 @@ impl<const L: DecoLine> DecoNode<L> {
 }
 
 impl<const L: DecoLine> Show for DecoNode<L> {
-    fn show(&self, _: &mut Vm, styles: StyleChain) -> TypResult<Template> {
-        Ok(self.0.clone().styled(TextNode::LINES, vec![Decoration {
-            line: L,
-            stroke: styles.get(Self::STROKE),
-            thickness: styles.get(Self::THICKNESS),
-            offset: styles.get(Self::OFFSET),
-            extent: styles.get(Self::EXTENT),
-            evade: styles.get(Self::EVADE),
-        }]))
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        Ok(styles
+            .show(self, vm, [Value::Template(self.0.clone())])?
+            .unwrap_or_else(|| {
+                self.0.clone().styled(TextNode::LINES, vec![Decoration {
+                    line: L,
+                    stroke: styles.get(Self::STROKE),
+                    thickness: styles.get(Self::THICKNESS),
+                    offset: styles.get(Self::OFFSET),
+                    extent: styles.get(Self::EXTENT),
+                    evade: styles.get(Self::EVADE),
+                }])
+            }))
     }
 }
 
diff --git a/src/library/heading.rs b/src/library/heading.rs
index 1b8fcef9..49975655 100644
--- a/src/library/heading.rs
+++ b/src/library/heading.rs
@@ -34,6 +34,8 @@ impl HeadingNode {
     pub const ABOVE: Leveled<Length> = Leveled::Value(Length::zero());
     /// The extra padding below the heading.
     pub const BELOW: Leveled<Length> = Leveled::Value(Length::zero());
+    /// Whether the heading is block-level.
+    pub const BLOCK: Leveled<bool> = Leveled::Value(true);
 
     fn construct(_: &mut Vm, args: &mut Args) -> TypResult<Template> {
         Ok(Template::show(Self {
@@ -51,6 +53,14 @@ impl Show for HeadingNode {
             };
         }
 
+        // Resolve the user recipe.
+        let mut body = styles
+            .show(self, vm, [
+                Value::Int(self.level as i64),
+                Value::Template(self.body.clone()),
+            ])?
+            .unwrap_or_else(|| self.body.clone());
+
         let mut map = StyleMap::new();
         map.set(TextNode::SIZE, resolve!(Self::SIZE));
 
@@ -76,7 +86,6 @@ impl Show for HeadingNode {
         }
 
         let mut seq = vec![];
-        let mut body = self.body.clone();
         if resolve!(Self::UNDERLINE) {
             body = body.underlined();
         }
@@ -93,9 +102,12 @@ impl Show for HeadingNode {
             seq.push(Template::Vertical(below.into()));
         }
 
-        Ok(Template::block(
-            Template::sequence(seq).styled_with_map(map),
-        ))
+        let mut template = Template::sequence(seq).styled_with_map(map);
+        if resolve!(Self::BLOCK) {
+            template = Template::block(template);
+        }
+
+        Ok(template)
     }
 }
 
diff --git a/src/library/link.rs b/src/library/link.rs
index 95fff089..41209549 100644
--- a/src/library/link.rs
+++ b/src/library/link.rs
@@ -10,7 +10,7 @@ pub struct LinkNode {
     /// The url the link points to.
     pub url: EcoString,
     /// How the link is represented.
-    pub body: Template,
+    pub body: Option<Template>,
 }
 
 #[class]
@@ -22,22 +22,31 @@ impl LinkNode {
     pub const UNDERLINE: bool = true;
 
     fn construct(_: &mut Vm, args: &mut Args) -> TypResult<Template> {
-        let url = args.expect::<EcoString>("url")?;
-        let body = args.find()?.unwrap_or_else(|| {
-            let mut text = url.as_str();
-            for prefix in ["mailto:", "tel:"] {
-                text = text.trim_start_matches(prefix);
-            }
-            let shorter = text.len() < url.len();
-            Template::Text(if shorter { text.into() } else { url.clone() })
-        });
-
-        Ok(Template::show(Self { url, body }))
+        Ok(Template::show(Self {
+            url: args.expect::<EcoString>("url")?,
+            body: args.find()?,
+        }))
     }
 }
 
 impl Show for LinkNode {
-    fn show(&self, _: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        let mut body = styles
+            .show(self, vm, [Value::Str(self.url.clone()), match &self.body {
+                Some(body) => Value::Template(body.clone()),
+                None => Value::None,
+            }])?
+            .or_else(|| self.body.clone())
+            .unwrap_or_else(|| {
+                let url = &self.url;
+                let mut text = url.as_str();
+                for prefix in ["mailto:", "tel:"] {
+                    text = text.trim_start_matches(prefix);
+                }
+                let shorter = text.len() < url.len();
+                Template::Text(if shorter { text.into() } else { url.clone() })
+            });
+
         let mut map = StyleMap::new();
         map.set(TextNode::LINK, Some(self.url.clone()));
 
@@ -45,7 +54,6 @@ impl Show for LinkNode {
             map.set(TextNode::FILL, fill);
         }
 
-        let mut body = self.body.clone();
         if styles.get(Self::UNDERLINE) {
             body = body.underlined();
         }
diff --git a/src/library/list.rs b/src/library/list.rs
index 13f21a04..37dda843 100644
--- a/src/library/list.rs
+++ b/src/library/list.rs
@@ -8,13 +8,13 @@ use crate::parse::Scanner;
 /// An unordered or ordered list.
 #[derive(Debug, Hash)]
 pub struct ListNode<const L: ListKind> {
-    /// The individual bulleted or numbered items.
-    pub items: Vec<ListItem>,
+    /// Where the list starts.
+    pub start: usize,
     /// If true, there is paragraph spacing between the items, if false
     /// there is list spacing between the items.
     pub wide: bool,
-    /// Where the list starts.
-    pub start: usize,
+    /// The individual bulleted or numbered items.
+    pub items: Vec<ListItem>,
 }
 
 /// An item in a list.
@@ -23,7 +23,7 @@ pub struct ListItem {
     /// The number of the item.
     pub number: Option<usize>,
     /// The node that produces the item's body.
-    pub body: LayoutNode,
+    pub body: Box<Template>,
 }
 
 #[class]
@@ -39,19 +39,27 @@ impl<const L: ListKind> ListNode<L> {
 
     fn construct(_: &mut Vm, args: &mut Args) -> TypResult<Template> {
         Ok(Template::show(Self {
+            start: args.named("start")?.unwrap_or(0),
+            wide: args.named("wide")?.unwrap_or(false),
             items: args
                 .all()?
                 .into_iter()
-                .map(|body| ListItem { number: None, body })
+                .map(|body| ListItem { number: None, body: Box::new(body) })
                 .collect(),
-            wide: args.named("wide")?.unwrap_or(false),
-            start: args.named("start")?.unwrap_or(0),
         }))
     }
 }
 
 impl<const L: ListKind> Show for ListNode<L> {
     fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        if let Some(template) = styles.show(
+            self,
+            vm,
+            self.items.iter().map(|item| Value::Template((*item.body).clone())),
+        )? {
+            return Ok(template);
+        }
+
         let mut children = vec![];
         let mut number = self.start;
 
@@ -66,7 +74,7 @@ impl<const L: ListKind> Show for ListNode<L> {
             children.push(LayoutNode::default());
             children.push(label.resolve(vm, L, number)?.pack());
             children.push(LayoutNode::default());
-            children.push(item.body.clone());
+            children.push((*item.body).clone().pack());
             number += 1;
         }
 
@@ -119,8 +127,6 @@ pub enum Label {
     Pattern(EcoString, Numbering, bool, EcoString),
     /// A bare template.
     Template(Template),
-    /// A simple mapping from an item number to a template.
-    Mapping(fn(usize) -> Template),
     /// A closure mapping from an item number to a value.
     Func(Func, Span),
 }
@@ -144,10 +150,9 @@ impl Label {
                 Template::Text(format_eco!("{}{}{}", prefix, mid, suffix))
             }
             Self::Template(template) => template.clone(),
-            Self::Mapping(mapping) => mapping(number),
-            &Self::Func(ref func, span) => {
-                let args = Args::from_values(span, [Value::Int(number as i64)]);
-                func.call(vm, args)?.cast().at(span)?
+            Self::Func(func, span) => {
+                let args = Args::from_values(*span, [Value::Int(number as i64)]);
+                func.call(vm, args)?.cast().at(*span)?
             }
         })
     }
diff --git a/src/library/math.rs b/src/library/math.rs
index d3c8b5e5..40a1990e 100644
--- a/src/library/math.rs
+++ b/src/library/math.rs
@@ -22,11 +22,18 @@ impl MathNode {
 }
 
 impl Show for MathNode {
-    fn show(&self, _: &mut Vm, _: StyleChain) -> TypResult<Template> {
-        let mut template = Template::Text(self.formula.trim().into());
-        if self.display {
-            template = Template::Block(template.pack());
-        }
-        Ok(template.monospaced())
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        Ok(styles
+            .show(self, vm, [
+                Value::Str(self.formula.clone()),
+                Value::Bool(self.display),
+            ])?
+            .unwrap_or_else(|| {
+                let mut template = Template::Text(self.formula.trim().into());
+                if self.display {
+                    template = Template::Block(template.pack());
+                }
+                template.monospaced()
+            }))
     }
 }
diff --git a/src/library/raw.rs b/src/library/raw.rs
index da926679..bb4e2c96 100644
--- a/src/library/raw.rs
+++ b/src/library/raw.rs
@@ -40,8 +40,20 @@ impl RawNode {
 }
 
 impl Show for RawNode {
-    fn show(&self, _: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
         let lang = styles.get_ref(Self::LANG).as_ref();
+
+        if let Some(template) = styles.show(self, vm, [
+            Value::Str(self.text.clone()),
+            match lang {
+                Some(lang) => Value::Str(lang.clone()),
+                None => Value::None,
+            },
+            Value::Bool(self.block),
+        ])? {
+            return Ok(template);
+        }
+
         let foreground = THEME
             .settings
             .foreground
diff --git a/src/library/table.rs b/src/library/table.rs
index 8c088c09..f4de0f55 100644
--- a/src/library/table.rs
+++ b/src/library/table.rs
@@ -11,7 +11,7 @@ pub struct TableNode {
     /// Defines sizing of gutter rows and columns between content.
     pub gutter: Spec<Vec<TrackSizing>>,
     /// The nodes to be arranged in the table.
-    pub children: Vec<LayoutNode>,
+    pub children: Vec<Template>,
 }
 
 #[class]
@@ -55,7 +55,15 @@ impl TableNode {
 }
 
 impl Show for TableNode {
-    fn show(&self, _: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        if let Some(template) = styles.show(
+            self,
+            vm,
+            self.children.iter().map(|child| Value::Template(child.clone())),
+        )? {
+            return Ok(template);
+        }
+
         let primary = styles.get(Self::PRIMARY);
         let secondary = styles.get(Self::SECONDARY);
         let thickness = styles.get(Self::THICKNESS);
@@ -68,8 +76,8 @@ impl Show for TableNode {
             .iter()
             .cloned()
             .enumerate()
-            .map(|(i, mut child)| {
-                child = child.padded(Sides::splat(padding));
+            .map(|(i, child)| {
+                let mut child = child.pack().padded(Sides::splat(padding));
 
                 if let Some(stroke) = stroke {
                     child = child.stroked(stroke);
diff --git a/src/library/text.rs b/src/library/text.rs
index 721a8eac..a67fbcf5 100644
--- a/src/library/text.rs
+++ b/src/library/text.rs
@@ -123,8 +123,10 @@ impl StrongNode {
 }
 
 impl Show for StrongNode {
-    fn show(&self, _: &mut Vm, _: StyleChain) -> TypResult<Template> {
-        Ok(self.0.clone().styled(TextNode::STRONG, true))
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        Ok(styles
+            .show(self, vm, [Value::Template(self.0.clone())])?
+            .unwrap_or_else(|| self.0.clone().styled(TextNode::STRONG, true)))
     }
 }
 
@@ -140,8 +142,10 @@ impl EmphNode {
 }
 
 impl Show for EmphNode {
-    fn show(&self, _: &mut Vm, _: StyleChain) -> TypResult<Template> {
-        Ok(self.0.clone().styled(TextNode::EMPH, true))
+    fn show(&self, vm: &mut Vm, styles: StyleChain) -> TypResult<Template> {
+        Ok(styles
+            .show(self, vm, [Value::Template(self.0.clone())])?
+            .unwrap_or_else(|| self.0.clone().styled(TextNode::EMPH, true)))
     }
 }
 
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index fbace15b..c14c45cf 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -752,9 +752,20 @@ fn set_expr(p: &mut Parser) -> ParseResult {
 fn show_expr(p: &mut Parser) -> ParseResult {
     p.perform(NodeKind::ShowExpr, |p| {
         p.eat_assert(&NodeKind::Show);
-        expr(p)?;
-        p.eat_expect(&NodeKind::As)?;
-        expr(p)
+        ident(p)?;
+        if !p.at(&NodeKind::LeftParen) {
+            p.expected_found("parameter list");
+            return Err(ParseError);
+        }
+        p.perform(NodeKind::Closure, |p| {
+            let marker = p.marker();
+            p.start_group(Group::Paren);
+            collection(p);
+            p.end_group();
+            params(p, marker);
+            p.eat_expect(&NodeKind::As)?;
+            expr(p)
+        })
     })
 }
 
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
index 8b88096a..9d22121b 100644
--- a/src/syntax/ast.rs
+++ b/src/syntax/ast.rs
@@ -919,14 +919,14 @@ node! {
 }
 
 impl ShowExpr {
-    /// The pattern that decides which node's appearence to redefine.
-    pub fn pattern(&self) -> Expr {
-        self.0.cast_first_child().expect("show expression is missing pattern")
+    /// The class to set the show rule for.
+    pub fn class(&self) -> Ident {
+        self.0.cast_first_child().expect("show expression is missing class")
     }
 
-    /// The expression that defines the node's appearence.
-    pub fn body(&self) -> Expr {
-        self.0.cast_last_child().expect("show expression is missing body")
+    /// The closure that defines the rule.
+    pub fn closure(&self) -> ClosureExpr {
+        self.0.cast_first_child().expect("show expression is missing closure")
     }
 }
 
diff --git a/src/syntax/highlight.rs b/src/syntax/highlight.rs
index b806b4e4..af6fb0df 100644
--- a/src/syntax/highlight.rs
+++ b/src/syntax/highlight.rs
@@ -181,6 +181,7 @@ impl Category {
                 }
                 NodeKind::WithExpr => Some(Category::Function),
                 NodeKind::SetExpr => Some(Category::Function),
+                NodeKind::ShowExpr => Some(Category::Function),
                 NodeKind::Call => Some(Category::Function),
                 _ => Some(Category::Variable),
             },
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index c0de081d..c702199e 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -2,7 +2,6 @@
 
 pub mod ast;
 mod highlight;
-mod pretty;
 mod span;
 
 use std::fmt::{self, Debug, Display, Formatter};
@@ -11,7 +10,6 @@ use std::ops::Range;
 use std::sync::Arc;
 
 pub use highlight::*;
-pub use pretty::*;
 pub use span::*;
 
 use self::ast::{MathNode, RawNode, TypedNode};
diff --git a/src/syntax/pretty.rs b/src/syntax/pretty.rs
deleted file mode 100644
index 07ab979b..00000000
--- a/src/syntax/pretty.rs
+++ /dev/null
@@ -1,730 +0,0 @@
-//! Pretty printing.
-
-use std::fmt::{self, Arguments, Write};
-
-use super::ast::*;
-
-/// Pretty print an item and return the resulting string.
-pub fn pretty<T>(item: &T) -> String
-where
-    T: Pretty + ?Sized,
-{
-    let mut p = Printer::new();
-    item.pretty(&mut p);
-    p.finish()
-}
-
-/// Pretty print an item.
-pub trait Pretty {
-    /// Pretty print this item into the given printer.
-    fn pretty(&self, p: &mut Printer);
-}
-
-/// A buffer into which items can be pretty printed.
-#[derive(Default)]
-pub struct Printer {
-    buf: String,
-}
-
-impl Printer {
-    /// Create a new pretty printer.
-    pub fn new() -> Self {
-        Self::default()
-    }
-
-    /// Push a character into the buffer.
-    pub fn push(&mut self, c: char) {
-        self.buf.push(c);
-    }
-
-    /// Push a string into the buffer.
-    pub fn push_str(&mut self, string: &str) {
-        self.buf.push_str(string);
-    }
-
-    /// Write formatted items into the buffer.
-    pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> fmt::Result {
-        Write::write_fmt(self, fmt)
-    }
-
-    /// Write a list of items joined by a joiner and return how many there were.
-    pub fn join<T, I, F>(&mut self, items: I, joiner: &str, mut write_item: F) -> usize
-    where
-        I: IntoIterator<Item = T>,
-        F: FnMut(T, &mut Self),
-    {
-        let mut count = 0;
-        let mut iter = items.into_iter();
-        if let Some(first) = iter.next() {
-            write_item(first, self);
-            count += 1;
-        }
-        for item in iter {
-            self.push_str(joiner);
-            write_item(item, self);
-            count += 1;
-        }
-        count
-    }
-
-    /// Finish pretty printing and return the underlying buffer.
-    pub fn finish(self) -> String {
-        self.buf
-    }
-}
-
-impl Write for Printer {
-    fn write_str(&mut self, s: &str) -> fmt::Result {
-        self.push_str(s);
-        Ok(())
-    }
-}
-
-impl Pretty for Markup {
-    fn pretty(&self, p: &mut Printer) {
-        for node in self.nodes() {
-            node.pretty(p);
-        }
-    }
-}
-
-impl Pretty for MarkupNode {
-    fn pretty(&self, p: &mut Printer) {
-        match self {
-            // TODO: Handle escaping.
-            Self::Space => p.push(' '),
-            Self::Linebreak => p.push_str(r"\"),
-            Self::Parbreak => p.push_str("\n\n"),
-            Self::Strong(strong) => strong.pretty(p),
-            Self::Emph(emph) => emph.pretty(p),
-            Self::Text(text) => p.push_str(text),
-            Self::Raw(raw) => raw.pretty(p),
-            Self::Math(math) => math.pretty(p),
-            Self::Heading(heading) => heading.pretty(p),
-            Self::List(list) => list.pretty(p),
-            Self::Enum(enum_) => enum_.pretty(p),
-            Self::Expr(expr) => {
-                if expr.has_short_form() {
-                    p.push('#');
-                }
-                expr.pretty(p);
-            }
-        }
-    }
-}
-
-impl Pretty for StrongNode {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('*');
-        self.body().pretty(p);
-        p.push('*');
-    }
-}
-
-impl Pretty for EmphNode {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('_');
-        self.body().pretty(p);
-        p.push('_');
-    }
-}
-
-impl Pretty for RawNode {
-    fn pretty(&self, p: &mut Printer) {
-        // Find out how many backticks we need.
-        let mut backticks = 1;
-
-        // Language tag and block-level are only possible with 3+ backticks.
-        if self.lang.is_some() || self.block {
-            backticks = 3;
-        }
-
-        // More backticks may be required if there are lots of consecutive
-        // backticks.
-        let mut count = 0;
-        for c in self.text.chars() {
-            if c == '`' {
-                count += 1;
-                backticks = backticks.max(3).max(count + 1);
-            } else {
-                count = 0;
-            }
-        }
-
-        // Starting backticks.
-        for _ in 0 .. backticks {
-            p.push('`');
-        }
-
-        // Language tag.
-        if let Some(lang) = &self.lang {
-            p.push_str(lang);
-        }
-
-        // Start untrimming.
-        if self.block {
-            p.push('\n');
-        } else if backticks >= 3 {
-            p.push(' ');
-        }
-
-        // The lines.
-        p.push_str(&self.text);
-
-        // End untrimming.
-        if self.block {
-            p.push('\n');
-        } else if self.text.trim_end().ends_with('`') {
-            p.push(' ');
-        }
-
-        // Ending backticks.
-        for _ in 0 .. backticks {
-            p.push('`');
-        }
-    }
-}
-
-impl Pretty for MathNode {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('$');
-        if self.display {
-            p.push('[');
-        }
-        p.push_str(&self.formula);
-        if self.display {
-            p.push(']');
-        }
-        p.push('$');
-    }
-}
-
-impl Pretty for HeadingNode {
-    fn pretty(&self, p: &mut Printer) {
-        for _ in 0 .. self.level() {
-            p.push('=');
-        }
-        p.push(' ');
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for ListNode {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("- ");
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for EnumNode {
-    fn pretty(&self, p: &mut Printer) {
-        if let Some(number) = self.number() {
-            write!(p, "{}", number).unwrap();
-        }
-        p.push_str(". ");
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for Expr {
-    fn pretty(&self, p: &mut Printer) {
-        match self {
-            Self::Lit(v) => v.pretty(p),
-            Self::Ident(v) => v.pretty(p),
-            Self::Array(v) => v.pretty(p),
-            Self::Dict(v) => v.pretty(p),
-            Self::Template(v) => v.pretty(p),
-            Self::Group(v) => v.pretty(p),
-            Self::Block(v) => v.pretty(p),
-            Self::Unary(v) => v.pretty(p),
-            Self::Binary(v) => v.pretty(p),
-            Self::Call(v) => v.pretty(p),
-            Self::Closure(v) => v.pretty(p),
-            Self::With(v) => v.pretty(p),
-            Self::Let(v) => v.pretty(p),
-            Self::Set(v) => v.pretty(p),
-            Self::Show(v) => v.pretty(p),
-            Self::Wrap(v) => v.pretty(p),
-            Self::If(v) => v.pretty(p),
-            Self::While(v) => v.pretty(p),
-            Self::For(v) => v.pretty(p),
-            Self::Import(v) => v.pretty(p),
-            Self::Include(v) => v.pretty(p),
-            Self::Break(v) => v.pretty(p),
-            Self::Continue(v) => v.pretty(p),
-            Self::Return(v) => v.pretty(p),
-        }
-    }
-}
-
-impl Pretty for Lit {
-    fn pretty(&self, p: &mut Printer) {
-        match self.kind() {
-            LitKind::None => p.push_str("none"),
-            LitKind::Auto => p.push_str("auto"),
-            LitKind::Bool(v) => write!(p, "{}", v).unwrap(),
-            LitKind::Int(v) => write!(p, "{}", v).unwrap(),
-            LitKind::Float(v) => write!(p, "{}", v).unwrap(),
-            LitKind::Length(v, u) => write!(p, "{}{:?}", v, u).unwrap(),
-            LitKind::Angle(v, u) => write!(p, "{}{:?}", v, u).unwrap(),
-            LitKind::Percent(v) => write!(p, "{}%", v).unwrap(),
-            LitKind::Fractional(v) => write!(p, "{}fr", v).unwrap(),
-            LitKind::Str(v) => write!(p, "{:?}", v).unwrap(),
-        }
-    }
-}
-
-impl Pretty for ArrayExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('(');
-
-        let items = self.items();
-        let len = p.join(items, ", ", |item, p| item.pretty(p));
-        if len == 1 {
-            p.push(',');
-        }
-        p.push(')');
-    }
-}
-
-impl Pretty for DictExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('(');
-        let len = p.join(self.items(), ", ", |named, p| named.pretty(p));
-        if len == 0 {
-            p.push(':');
-        }
-        p.push(')');
-    }
-}
-
-impl Pretty for Named {
-    fn pretty(&self, p: &mut Printer) {
-        self.name().pretty(p);
-        p.push_str(": ");
-        self.expr().pretty(p);
-    }
-}
-
-impl Pretty for TemplateExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('[');
-        self.body().pretty(p);
-        p.push(']');
-    }
-}
-
-impl Pretty for GroupExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('(');
-        self.expr().pretty(p);
-        p.push(')');
-    }
-}
-
-impl Pretty for BlockExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push('{');
-        if self.exprs().count() > 1 {
-            p.push(' ');
-        }
-        let len = p.join(self.exprs(), "; ", |expr, p| expr.pretty(p));
-        if len > 1 {
-            p.push(' ');
-        }
-        p.push('}');
-    }
-}
-
-impl Pretty for UnaryExpr {
-    fn pretty(&self, p: &mut Printer) {
-        let op = self.op();
-        op.pretty(p);
-        if op == UnOp::Not {
-            p.push(' ');
-        }
-        self.expr().pretty(p);
-    }
-}
-
-impl Pretty for UnOp {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str(self.as_str());
-    }
-}
-
-impl Pretty for BinaryExpr {
-    fn pretty(&self, p: &mut Printer) {
-        self.lhs().pretty(p);
-        p.push(' ');
-        self.op().pretty(p);
-        p.push(' ');
-        self.rhs().pretty(p);
-    }
-}
-
-impl Pretty for BinOp {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str(self.as_str());
-    }
-}
-
-impl Pretty for CallExpr {
-    fn pretty(&self, p: &mut Printer) {
-        self.callee().pretty(p);
-
-        let mut write_args = |items: &[CallArg]| {
-            p.push('(');
-            p.join(items, ", ", |item, p| item.pretty(p));
-            p.push(')');
-        };
-
-        let args: Vec<_> = self.args().items().collect();
-        match args.as_slice() {
-            // This can be moved behind the arguments.
-            //
-            // Example: Transforms "#v(a, [b])" => "#v(a)[b]".
-            [head @ .., CallArg::Pos(Expr::Template(template))] => {
-                if !head.is_empty() {
-                    write_args(head);
-                }
-                template.pretty(p);
-            }
-            items => write_args(items),
-        }
-    }
-}
-
-impl Pretty for CallArgs {
-    fn pretty(&self, p: &mut Printer) {
-        p.join(self.items(), ", ", |item, p| item.pretty(p));
-    }
-}
-
-impl Pretty for CallArg {
-    fn pretty(&self, p: &mut Printer) {
-        match self {
-            Self::Pos(expr) => expr.pretty(p),
-            Self::Named(named) => named.pretty(p),
-            Self::Spread(expr) => {
-                p.push_str("..");
-                expr.pretty(p);
-            }
-        }
-    }
-}
-
-impl Pretty for ClosureExpr {
-    fn pretty(&self, p: &mut Printer) {
-        let params: Vec<_> = self.params().collect();
-        if let [param] = params.as_slice() {
-            param.pretty(p);
-        } else {
-            p.push('(');
-            p.join(params.iter(), ", ", |item, p| item.pretty(p));
-            p.push(')');
-        }
-        p.push_str(" => ");
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for ClosureParam {
-    fn pretty(&self, p: &mut Printer) {
-        match self {
-            Self::Pos(ident) => ident.pretty(p),
-            Self::Named(named) => named.pretty(p),
-            Self::Sink(ident) => {
-                p.push_str("..");
-                ident.pretty(p);
-            }
-        }
-    }
-}
-
-impl Pretty for WithExpr {
-    fn pretty(&self, p: &mut Printer) {
-        self.callee().pretty(p);
-        p.push_str(" with (");
-        self.args().pretty(p);
-        p.push(')');
-    }
-}
-
-impl Pretty for LetExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("let ");
-        self.binding().pretty(p);
-        if let Some(Expr::Closure(closure)) = self.init() {
-            p.push('(');
-            p.join(closure.params(), ", ", |item, p| item.pretty(p));
-            p.push_str(") = ");
-            closure.body().pretty(p);
-        } else if let Some(init) = self.init() {
-            p.push_str(" = ");
-            init.pretty(p);
-        }
-    }
-}
-
-impl Pretty for SetExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("set ");
-        self.class().pretty(p);
-        p.push_str("(");
-        self.args().pretty(p);
-        p.push(')');
-    }
-}
-
-impl Pretty for ShowExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("show ");
-        self.pattern().pretty(p);
-        p.push_str(" as ");
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for WrapExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("wrap ");
-        self.binding().pretty(p);
-        p.push_str(" in ");
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for IfExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("if ");
-        self.condition().pretty(p);
-        p.push(' ');
-        self.if_body().pretty(p);
-        if let Some(expr) = self.else_body() {
-            p.push_str(" else ");
-            expr.pretty(p);
-        }
-    }
-}
-
-impl Pretty for WhileExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("while ");
-        self.condition().pretty(p);
-        p.push(' ');
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for ForExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("for ");
-        self.pattern().pretty(p);
-        p.push_str(" in ");
-        self.iter().pretty(p);
-        p.push(' ');
-        self.body().pretty(p);
-    }
-}
-
-impl Pretty for ForPattern {
-    fn pretty(&self, p: &mut Printer) {
-        if let Some(key) = self.key() {
-            key.pretty(p);
-            p.push_str(", ");
-        }
-
-        self.value().pretty(p);
-    }
-}
-
-impl Pretty for ImportExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("import ");
-        self.imports().pretty(p);
-        p.push_str(" from ");
-        self.path().pretty(p);
-    }
-}
-
-impl Pretty for Imports {
-    fn pretty(&self, p: &mut Printer) {
-        match self {
-            Self::Wildcard => p.push('*'),
-            Self::Items(idents) => {
-                p.join(idents, ", ", |item, p| item.pretty(p));
-            }
-        }
-    }
-}
-
-impl Pretty for IncludeExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("include ");
-        self.path().pretty(p);
-    }
-}
-
-impl Pretty for BreakExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("break");
-    }
-}
-
-impl Pretty for ContinueExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("continue");
-    }
-}
-
-impl Pretty for ReturnExpr {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str("return");
-        if let Some(body) = self.body() {
-            p.push(' ');
-            body.pretty(p);
-        }
-    }
-}
-
-impl Pretty for Ident {
-    fn pretty(&self, p: &mut Printer) {
-        p.push_str(self);
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::source::SourceFile;
-
-    #[track_caller]
-    fn roundtrip(src: &str) {
-        test_parse(src, src);
-    }
-
-    #[track_caller]
-    fn test_parse(src: &str, expected: &str) {
-        let source = SourceFile::detached(src);
-        let ast = source.ast().unwrap();
-        let found = pretty(&ast);
-        if found != expected {
-            println!("tree:     {ast:#?}");
-            println!("expected: {expected}");
-            println!("found:    {found}");
-            panic!("test failed");
-        }
-    }
-
-    #[test]
-    fn test_pretty_print_markup() {
-        // Basic stuff.
-        roundtrip(" ");
-        roundtrip("*ab*");
-        roundtrip("\\ ");
-        roundtrip("\n\n");
-        roundtrip("hi");
-        roundtrip("_ab_");
-        roundtrip("= *Ok*");
-        roundtrip("- Ok");
-
-        // Raw node.
-        roundtrip("``");
-        roundtrip("`nolang 1`");
-        roundtrip("```lang 1```");
-        roundtrip("```lang 1 ```");
-        roundtrip("```hi  line  ```");
-        roundtrip("```py\ndef\n```");
-        roundtrip("```\n line \n```");
-        roundtrip("```\n`\n```");
-        roundtrip("``` ` ```");
-        roundtrip("````\n```\n```\n````");
-        test_parse("```lang```", "```lang ```");
-        test_parse("```1 ```", "``");
-        test_parse("``` 1```", "`1`");
-        test_parse("``` 1 ```", "`1 `");
-        test_parse("```` ` ````", "``` ` ```");
-
-        // Math node.
-        roundtrip("$$");
-        roundtrip("$a+b$");
-        roundtrip("$[ a^2 + b^2 = c^2 ]$");
-    }
-
-    #[test]
-    fn test_pretty_print_expr() {
-        // Basic expressions.
-        roundtrip("{none}");
-        roundtrip("{auto}");
-        roundtrip("{true}");
-        roundtrip("{10}");
-        roundtrip("{3.14}");
-        roundtrip("{10pt}");
-        roundtrip("{14.1deg}");
-        roundtrip("{20%}");
-        roundtrip("{0.5fr}");
-        roundtrip(r#"{"hi"}"#);
-        roundtrip(r#"{"let's \" go"}"#);
-        roundtrip("{hi}");
-
-        // Arrays.
-        roundtrip("{()}");
-        roundtrip("{(1)}");
-        roundtrip("{(1, 2, 3)}");
-
-        // Dictionaries.
-        roundtrip("{(:)}");
-        roundtrip("{(key: value)}");
-        roundtrip("{(a: 1, b: 2)}");
-
-        // Templates.
-        roundtrip("[]");
-        roundtrip("[*Ok*]");
-        roundtrip("{[f]}");
-
-        // Groups.
-        roundtrip("{(1)}");
-
-        // Blocks.
-        roundtrip("{}");
-        roundtrip("{1}");
-        roundtrip("{ let x = 1; x += 2; x + 1 }");
-        roundtrip("[{}]");
-
-        // Operators.
-        roundtrip("{-x}");
-        roundtrip("{not true}");
-        roundtrip("{1 + 3}");
-
-        // Functions.
-        roundtrip("{v()}");
-        roundtrip("{v()()}");
-        roundtrip("{v(1)}");
-        roundtrip("{v(a: 1, b)}");
-        roundtrip("#v()");
-        roundtrip("#v(1)");
-        roundtrip("#v(1, 2)[*Ok*]");
-        roundtrip("#v(1, f[2])");
-        roundtrip("{x => x + 1}");
-        roundtrip("{(a, b) => a + b}");
-
-        // Control flow.
-        roundtrip("#let x = 1 + 2");
-        roundtrip("#let f(x) = y");
-        roundtrip("#set text(size: 12pt)");
-        roundtrip("#show heading(body) as [*{body}*]");
-        roundtrip("#wrap body in columns(2, body)");
-        roundtrip("#if x [y] else [z]");
-        roundtrip("#if x {} else if y {} else {}");
-        roundtrip("#while x {y}");
-        roundtrip("#for x in y {z}");
-        roundtrip("#for k, x in y {z}");
-        roundtrip("#import * from \"file.typ\"");
-        roundtrip("#include \"chapter1.typ\"");
-        roundtrip("{break}");
-        roundtrip("{continue}");
-        roundtrip("{return}");
-        roundtrip("{return x + 1}");
-    }
-}
diff --git a/tests/ref/style/show.png b/tests/ref/style/show.png
new file mode 100644
index 00000000..0f1a16a3
--- /dev/null
+++ b/tests/ref/style/show.png
diff --git a/tests/typ/style/set.typ b/tests/typ/style/set.typ
index 0d4d47cc..6ce1d303 100644
--- a/tests/typ/style/set.typ
+++ b/tests/typ/style/set.typ
@@ -1,5 +1,5 @@
 // General tests for set.
 
 ---
-// Error: 2-10 set is only allowed directly in markup
+// Error: 2-10 set, show and wrap are only allowed directly in markup
 {set f(x)}
diff --git a/tests/typ/style/show.typ b/tests/typ/style/show.typ
index 533e50f5..1055f9c7 100644
--- a/tests/typ/style/show.typ
+++ b/tests/typ/style/show.typ
@@ -1,9 +1,53 @@
 // Test show rules.
 
+#set page("a8", footer: p => v(-5pt) + align(right, [#p]))
+
+#let i = 1
+#set heading(size: 100%)
+#show heading(level, body) as {
+  if level == 1 {
+    v(10pt)
+    underline(text(150%, blue)[{i}. #body])
+    i += 1
+  } else {
+    text(red, body)
+  }
+}
+
+#v(-10pt)
+
+= Aufgabe
+Some text.
+
+== Subtask
+Some more text.
+
+== Another subtask
+Some more text.
+
+= Aufgabe
+Another text.
+
+---
+#set heading(size: 100%, strong: false, block: false)
+#show heading(a, b) as [B]
+A [= Heading] C
+
+---
+// Error: 1-22 unexpected argument
+#show heading() as []
+= Heading
+
+---
+// Error: 1-28 expected template, found string
+#show heading(_, _) as "hi"
+= Heading
+
 ---
-// Error: 1-34 show rules are not yet implemented
-#show heading(body) as [*{body}*]
+// Error: 1-29 show rule is recursive
+#show strong(x) as strong(x)
+*Hi*
 
 ---
-// Error: 2-15 show is only allowed directly in markup
-{show (a) as b}
+// Error: 2-19 set, show and wrap are only allowed directly in markup
+{show list(a) as b}
diff --git a/tests/typ/style/wrap.typ b/tests/typ/style/wrap.typ
index 2fa7716e..9c94b9ae 100644
--- a/tests/typ/style/wrap.typ
+++ b/tests/typ/style/wrap.typ
@@ -25,5 +25,5 @@ A [_B #wrap c in [*#c*]; C_] D
 Forest
 
 ---
-// Error: 6-17 wrap is only allowed directly in markup
+// Error: 6-17 set, show and wrap are only allowed directly in markup
 {1 + wrap x in y}
diff --git a/tests/typeset.rs b/tests/typeset.rs
index dadcfed3..98c85df9 100644
--- a/tests/typeset.rs
+++ b/tests/typeset.rs
@@ -23,7 +23,7 @@ use typst::{Context, Vm};
 use {
     filedescriptor::{FileDescriptor, StdioDescriptor::*},
     std::fs::File,
-    typst::eval::Template,
+    typst::source::SourceId,
 };
 
 const TYP_DIR: &str = "./typ";
@@ -110,7 +110,7 @@ fn main() {
             &png_path,
             &ref_path,
             pdf_path.as_deref(),
-            args.debug,
+            args.syntax,
         ) as usize;
     }
 
@@ -126,7 +126,7 @@ fn main() {
 struct Args {
     filter: Vec<String>,
     exact: bool,
-    debug: bool,
+    syntax: bool,
     pdf: bool,
 }
 
@@ -134,7 +134,7 @@ impl Args {
     fn new(args: impl Iterator<Item = String>) -> Self {
         let mut filter = Vec::new();
         let mut exact = false;
-        let mut debug = false;
+        let mut syntax = false;
         let mut pdf = false;
 
         for arg in args {
@@ -146,13 +146,13 @@ impl Args {
                 // Generate PDFs.
                 "--pdf" => pdf = true,
                 // Debug print the layout trees.
-                "--debug" | "-d" => debug = true,
+                "--syntax" => syntax = true,
                 // Everything else is a file filter.
                 _ => filter.push(arg),
             }
         }
 
-        Self { filter, pdf, debug, exact }
+        Self { filter, pdf, syntax, exact }
     }
 
     fn matches(&self, path: &Path) -> bool {
@@ -172,7 +172,7 @@ fn test(
     png_path: &Path,
     ref_path: &Path,
     pdf_path: Option<&Path>,
-    debug: bool,
+    syntax: bool,
 ) -> bool {
     let name = src_path.strip_prefix(TYP_DIR).unwrap_or(src_path);
     println!("Testing {}", name.display());
@@ -208,7 +208,7 @@ fn test(
                 i,
                 compare_ref,
                 line,
-                debug,
+                syntax,
                 &mut rng,
             );
             ok &= part_ok;
@@ -242,7 +242,7 @@ fn test(
     }
 
     if ok {
-        if !debug {
+        if !syntax {
             print!("\x1b[1A");
         }
         println!("Testing {} ✔", name.display());
@@ -258,14 +258,14 @@ fn test_part(
     i: usize,
     compare_ref: bool,
     line: usize,
-    debug: bool,
+    syntax: bool,
     rng: &mut LinearShift,
 ) -> (bool, bool, Vec<Arc<Frame>>) {
     let mut ok = true;
 
     let id = ctx.sources.provide(src_path, src);
     let source = ctx.sources.get(id);
-    if debug {
+    if syntax {
         println!("Syntax Tree: {:#?}", source.root())
     }
 
@@ -277,13 +277,8 @@ fn test_part(
     let mut vm = Vm::new(ctx);
     let (frames, mut errors) = match vm.typeset(id) {
         Ok(mut frames) => {
-            let module = vm.evaluate(id).unwrap();
-            if debug {
-                println!("Template: {:#?}", module.template);
-            }
-
             #[cfg(feature = "layout-cache")]
-            (ok &= test_incremental(ctx, i, &module.template, &frames));
+            (ok &= test_incremental(ctx, i, id, &frames));
 
             if !compare_ref {
                 frames.clear();
@@ -483,7 +478,7 @@ fn test_reparse(src: &str, i: usize, rng: &mut LinearShift) -> bool {
 fn test_incremental(
     ctx: &mut Context,
     i: usize,
-    template: &Template,
+    id: SourceId,
     frames: &[Arc<Frame>],
 ) -> bool {
     let mut ok = true;
@@ -498,7 +493,7 @@ fn test_incremental(
 
         ctx.layout_cache.turnaround();
 
-        let cached = silenced(|| template.layout_pages(&mut Vm::new(ctx)).unwrap());
+        let cached = silenced(|| ctx.typeset(id).unwrap());
         let total = reference.levels() - 1;
         let misses = ctx
             .layout_cache