Support recursive show rules

src/eval/args.rs

@@ -26,19 +26,22 @@ pub struct Arg {
 }
 
 impl Args {
+    /// Create empty arguments from a span.
+    pub fn new(span: Span) -> Self {
+        Self { span, items: vec![] }
+    }
+
     /// Create positional arguments from a span and values.
     pub fn from_values(span: Span, values: impl IntoIterator<Item = Value>) -> Self {
-        Self {
+        let items = values
-            span,
-            items: values
             .into_iter()
             .map(|value| Arg {
                 span,
                 name: None,
                 value: Spanned::new(value, span),
             })
-                .collect(),
+            .collect();
-        }
+        Self { span, items }
     }
 
     /// Consume and cast the first positional argument.

src/eval/capture.rs

@@ -91,8 +91,10 @@ impl<'a> CapturesVisitor<'a> {
             // A show rule contains a binding, but that binding is only active
             // after the target has been evaluated.
             Some(Expr::Show(show)) => {
-                self.visit(show.target().as_red());
+                self.visit(show.pattern().as_red());
-                self.bind(show.binding());
+                if let Some(binding) = show.binding() {
+                    self.bind(binding);
+                }
                 self.visit(show.body().as_red());
             }
 

src/eval/dict.rs

@@ -46,8 +46,8 @@ impl Dict {
     }
 
     /// Borrow the value the given `key` maps to.
-    pub fn get(&self, key: EcoString) -> StrResult<&Value> {
+    pub fn get(&self, key: &EcoString) -> StrResult<&Value> {
-        self.0.get(&key).ok_or_else(|| missing_key(&key))
+        self.0.get(key).ok_or_else(|| missing_key(key))
     }
 
     /// Mutably borrow the value the given `key` maps to.
@@ -59,7 +59,7 @@ impl Dict {
     }
 
     /// Whether the dictionary contains a specific key.
-    pub fn contains(&self, key: &str) -> bool {
+    pub fn contains(&self, key: &EcoString) -> bool {
         self.0.contains_key(key)
     }
 
@@ -69,10 +69,10 @@ impl Dict {
     }
 
     /// Remove a mapping by `key`.
-    pub fn remove(&mut self, key: EcoString) -> StrResult<()> {
+    pub fn remove(&mut self, key: &EcoString) -> StrResult<()> {
-        match Arc::make_mut(&mut self.0).remove(&key) {
+        match Arc::make_mut(&mut self.0).remove(key) {
             Some(_) => Ok(()),
-            None => Err(missing_key(&key)),
+            None => Err(missing_key(key)),
         }
     }
 

src/eval/func.rs

@@ -4,9 +4,8 @@ use std::sync::Arc;
 
 use super::{Args, Control, Eval, Scope, Scopes, Value};
 use crate::diag::{StrResult, TypResult};
-use crate::model::{Content, StyleMap};
+use crate::model::{Content, NodeId, StyleMap};
 use crate::syntax::ast::Expr;
-use crate::syntax::Span;
 use crate::util::EcoString;
 use crate::Context;
 
@@ -35,7 +34,7 @@ impl Func {
             name,
             func,
             set: None,
-            show: None,
+            node: None,
         })))
     }
 
@@ -49,15 +48,7 @@ impl Func {
                 Ok(Value::Content(content.styled_with_map(styles.scoped())))
             },
             set: Some(T::set),
-            show: if T::SHOWABLE {
+            node: T::SHOWABLE.then(|| NodeId::of::<T>()),
-                Some(|recipe, span| {
-                    let mut styles = StyleMap::new();
-                    styles.set_recipe::<T>(recipe, span);
-                    styles
-                })
-            } else {
-                None
-            },
         })))
     }
 
@@ -80,7 +71,20 @@ impl Func {
         }
     }
 
-    /// Call the function with a virtual machine and arguments.
+    /// The number of positional arguments this function takes, if known.
+    pub fn argc(&self) -> Option<usize> {
+        match self.0.as_ref() {
+            Repr::Closure(closure) => Some(
+                closure.params.iter().filter(|(_, default)| default.is_none()).count(),
+            ),
+            Repr::With(wrapped, applied) => Some(wrapped.argc()?.saturating_sub(
+                applied.items.iter().filter(|arg| arg.name.is_none()).count(),
+            )),
+            _ => None,
+        }
+    }
+
+    /// Call the function with the given arguments.
     pub fn call(&self, ctx: &mut Context, mut args: Args) -> TypResult<Value> {
         let value = match self.0.as_ref() {
             Repr::Native(native) => (native.func)(ctx, &mut args)?,
@@ -104,10 +108,10 @@ impl Func {
         Ok(styles)
     }
 
-    /// Execute the function's show rule.
+    /// The id of the node to customize with this function's show rule.
-    pub fn show(&self, recipe: Func, span: Span) -> StrResult<StyleMap> {
+    pub fn node(&self) -> StrResult<NodeId> {
         match self.0.as_ref() {
-            Repr::Native(Native { show: Some(show), .. }) => Ok(show(recipe, span)),
+            Repr::Native(Native { node: Some(id), .. }) => Ok(*id),
             _ => Err("this function cannot be customized with show")?,
         }
     }
@@ -138,8 +142,8 @@ struct Native {
     pub func: fn(&mut Context, &mut Args) -> TypResult<Value>,
     /// The set rule.
     pub set: Option<fn(&mut Args) -> TypResult<StyleMap>>,
-    /// The show rule.
+    /// The id of the node to customize with this function's show rule.
-    pub show: Option<fn(Func, Span) -> StyleMap>,
+    pub node: Option<NodeId>,
 }
 
 impl Hash for Native {
@@ -147,7 +151,7 @@ impl Hash for Native {
         self.name.hash(state);
         (self.func as usize).hash(state);
         self.set.map(|set| set as usize).hash(state);
-        self.show.map(|show| show as usize).hash(state);
+        self.node.hash(state);
     }
 }
 

src/eval/methods.rs

@@ -96,7 +96,7 @@ pub fn call_mut(
         },
 
         Value::Dict(dict) => match method {
-            "remove" => dict.remove(args.expect("key")?).at(span)?,
+            "remove" => dict.remove(&args.expect("key")?).at(span)?,
             _ => missing()?,
         },
 

src/eval/mod.rs

@@ -38,7 +38,7 @@ use unicode_segmentation::UnicodeSegmentation;
 use crate::diag::{At, StrResult, Trace, Tracepoint, TypResult};
 use crate::geom::{Angle, Em, Fraction, Length, Ratio};
 use crate::library;
-use crate::model::{Content, StyleMap};
+use crate::model::{Content, Pattern, Recipe, StyleEntry, StyleMap};
 use crate::syntax::ast::*;
 use crate::syntax::{Span, Spanned};
 use crate::util::EcoString;
@@ -79,8 +79,9 @@ fn eval_markup(
                 eval_markup(ctx, scp, nodes)?.styled_with_map(styles)
             }
             MarkupNode::Expr(Expr::Show(show)) => {
-                let styles = show.eval(ctx, scp)?;
+                let recipe = show.eval(ctx, scp)?;
-                eval_markup(ctx, scp, nodes)?.styled_with_map(styles)
+                eval_markup(ctx, scp, nodes)?
+                    .styled_with_entry(StyleEntry::Recipe(recipe).into())
             }
             MarkupNode::Expr(Expr::Wrap(wrap)) => {
                 let tail = eval_markup(ctx, scp, nodes)?;
@@ -434,8 +435,17 @@ impl Eval for FieldAccess {
 
     fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         let object = self.object().eval(ctx, scp)?;
+        let span = self.field().span();
+        let field = self.field().take();
+
         Ok(match object {
-            Value::Dict(dict) => dict.get(self.field().take()).at(self.span())?.clone(),
+            Value::Dict(dict) => dict.get(&field).at(span)?.clone(),
+
+            Value::Content(Content::Show(_, Some(dict))) => dict
+                .get(&field)
+                .map_err(|_| format!("unknown field {field:?}"))
+                .at(span)?
+                .clone(),
 
             v => bail!(
                 self.object().span(),
@@ -455,7 +465,7 @@ impl Eval for FuncCall {
 
         Ok(match callee {
             Value::Array(array) => array.get(args.into_index()?).at(self.span())?.clone(),
-            Value::Dict(dict) => dict.get(args.into_key()?).at(self.span())?.clone(),
+            Value::Dict(dict) => dict.get(&args.into_key()?).at(self.span())?.clone(),
             Value::Func(func) => {
                 let point = || Tracepoint::Call(func.name().map(ToString::to_string));
                 func.call(ctx, args).trace(point, self.span())?
@@ -615,13 +625,12 @@ impl Eval for SetExpr {
 }
 
 impl Eval for ShowExpr {
-    type Output = StyleMap;
+    type Output = Recipe;
 
     fn eval(&self, ctx: &mut Context, scp: &mut Scopes) -> EvalResult<Self::Output> {
         // Evaluate the target function.
-        let target = self.target();
+        let pattern = self.pattern();
-        let target_span = target.span();
+        let pattern = pattern.eval(ctx, scp)?.cast::<Pattern>().at(pattern.span())?;
-        let target = target.eval(ctx, scp)?.cast::<Func>().at(target_span)?;
 
         // Collect captured variables.
         let captured = {
@@ -630,18 +639,24 @@ impl Eval for ShowExpr {
             visitor.finish()
         };
 
+        // Define parameters.
+        let mut params = vec![];
+        if let Some(binding) = self.binding() {
+            params.push((binding.take(), None));
+        }
+
         // Define the recipe function.
         let body = self.body();
-        let body_span = body.span();
+        let span = body.span();
-        let recipe = Func::from_closure(Closure {
+        let func = Func::from_closure(Closure {
             name: None,
             captured,
-            params: vec![(self.binding().take(), None)],
+            params,
             sink: None,
             body,
         });
 
-        Ok(target.show(recipe, body_span).at(target_span)?)
+        Ok(Recipe { pattern, func, span })
     }
 }
 

src/eval/value.rs

@@ -11,7 +11,7 @@ use crate::geom::{
     Angle, Color, Dir, Em, Fraction, Length, Paint, Ratio, Relative, RgbaColor, Sides,
 };
 use crate::library::text::RawNode;
-use crate::model::{Content, Layout, LayoutNode};
+use crate::model::{Content, Layout, LayoutNode, Pattern};
 use crate::syntax::Spanned;
 use crate::util::EcoString;
 
@@ -617,13 +617,13 @@ where
                 }
 
                 let sides = Sides {
-                    left: dict.get("left".into()).or_else(|_| dict.get("x".into())),
+                    left: dict.get(&"left".into()).or_else(|_| dict.get(&"x".into())),
-                    top: dict.get("top".into()).or_else(|_| dict.get("y".into())),
+                    top: dict.get(&"top".into()).or_else(|_| dict.get(&"y".into())),
-                    right: dict.get("right".into()).or_else(|_| dict.get("x".into())),
+                    right: dict.get(&"right".into()).or_else(|_| dict.get(&"x".into())),
-                    bottom: dict.get("bottom".into()).or_else(|_| dict.get("y".into())),
+                    bottom: dict.get(&"bottom".into()).or_else(|_| dict.get(&"y".into())),
                 }
                 .map(|side| {
-                    side.or_else(|_| dict.get("rest".into()))
+                    side.or_else(|_| dict.get(&"rest".into()))
                         .and_then(|v| T::cast(v.clone()))
                         .unwrap_or_default()
                 });
@@ -684,6 +684,12 @@ castable! {
     Value::Content(content) => content.pack(),
 }
 
+castable! {
+    Pattern,
+    Expected: "function",
+    Value::Func(func) => Pattern::Node(func.node()?),
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/geom/relative.rs

@@ -52,7 +52,11 @@ impl<T: Numeric> Relative<T> {
 
 impl<T: Numeric> Debug for Relative<T> {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "{:?} + {:?}", self.rel, self.abs)
+        match (self.rel.is_zero(), self.abs.is_zero()) {
+            (false, false) => write!(f, "{:?} + {:?}", self.rel, self.abs),
+            (false, true) => self.rel.fmt(f),
+            (true, _) => self.abs.fmt(f),
+        }
     }
 }
 

src/library/math/mod.rs

@@ -36,6 +36,10 @@ impl MathNode {
 }
 
 impl Show for MathNode {
+    fn unguard(&self, _: Selector) -> ShowNode {
+        Self { formula: self.formula.clone(), ..*self }.pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! {
             "formula" => Value::Str(self.formula.clone()),

src/library/prelude.rs

@@ -15,8 +15,8 @@ pub use crate::eval::{
 pub use crate::frame::*;
 pub use crate::geom::*;
 pub use crate::model::{
-    Content, Fold, Key, Layout, LayoutNode, Regions, Resolve, Show, ShowNode, StyleChain,
+    Content, Fold, Key, Layout, LayoutNode, Regions, Resolve, Selector, Show, ShowNode,
-    StyleMap, StyleVec,
+    StyleChain, StyleMap, StyleVec,
 };
 pub use crate::syntax::{Span, Spanned};
 pub use crate::util::{EcoString, OptionExt};

src/library/structure/heading.rs

@@ -64,6 +64,10 @@ impl HeadingNode {
 }
 
 impl Show for HeadingNode {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self { body: self.body.unguard(sel), ..*self }.pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! {
             "level" => Value::Int(self.level.get() as i64),

src/library/structure/list.rs

@@ -75,6 +75,17 @@ impl<const L: ListKind> ListNode<L> {
 }
 
 impl<const L: ListKind> Show for ListNode<L> {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self {
+            items: self.items.map(|item| ListItem {
+                body: Box::new(item.body.unguard(sel)),
+                ..*item
+            }),
+            ..*self
+        }
+        .pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! {
             "start" => Value::Int(self.start as i64),
@@ -83,7 +94,7 @@ impl<const L: ListKind> Show for ListNode<L> {
             "items" => Value::Array(
                 self.items
                     .items()
-                    .map(|item| Value::Content((*item.body).clone()))
+                    .map(|item| Value::Content(item.body.as_ref().clone()))
                     .collect()
             ),
         }

src/library/structure/table.rs

@@ -51,6 +51,15 @@ impl TableNode {
 }
 
 impl Show for TableNode {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self {
+            tracks: self.tracks.clone(),
+            gutter: self.gutter.clone(),
+            cells: self.cells.iter().map(|cell| cell.unguard(sel)).collect(),
+        }
+        .pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! {
             "cells" => Value::Array(

src/library/text/deco.rs

@@ -36,11 +36,15 @@ impl<const L: DecoLine> DecoNode<L> {
     pub const EVADE: bool = true;
 
     fn construct(_: &mut Context, args: &mut Args) -> TypResult<Content> {
-        Ok(Content::show(Self(args.expect::<Content>("body")?)))
+        Ok(Content::show(Self(args.expect("body")?)))
     }
 }
 
 impl<const L: DecoLine> Show for DecoNode<L> {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self(self.0.unguard(sel)).pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! { "body" => Value::Content(self.0.clone()) }
     }

src/library/text/link.rs

@@ -28,6 +28,14 @@ impl LinkNode {
 }
 
 impl Show for LinkNode {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self {
+            url: self.url.clone(),
+            body: self.body.as_ref().map(|body| body.unguard(sel)),
+        }
+        .pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! {
             "url" => Value::Str(self.url.clone()),

src/library/text/mod.rs

@@ -463,6 +463,10 @@ impl StrongNode {
 }
 
 impl Show for StrongNode {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self(self.0.unguard(sel)).pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! { "body" => Value::Content(self.0.clone()) }
     }
@@ -484,6 +488,10 @@ impl EmphNode {
 }
 
 impl Show for EmphNode {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        Self(self.0.unguard(sel)).pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! { "body" => Value::Content(self.0.clone()) }
     }

src/library/text/par.rs

@@ -618,7 +618,10 @@ fn shared_get<'a, K: Key<'a>>(
     children: &StyleVec<ParChild>,
     key: K,
 ) -> Option<K::Output> {
-    children.maps().all(|map| !map.contains(key)).then(|| styles.get(key))
+    children
+        .styles()
+        .all(|map| !map.contains(key))
+        .then(|| styles.get(key))
 }
 
 /// Find suitable linebreaks.

src/library/text/raw.rs

@@ -51,6 +51,10 @@ impl RawNode {
 }
 
 impl Show for RawNode {
+    fn unguard(&self, _: Selector) -> ShowNode {
+        Self { text: self.text.clone(), ..*self }.pack()
+    }
+
     fn encode(&self) -> Dict {
         dict! {
             "text" => Value::Str(self.text.clone()),

src/model/content.rs

@@ -7,8 +7,8 @@ use std::ops::{Add, AddAssign};
 use typed_arena::Arena;
 
 use super::{
-    CollapsingBuilder, Interruption, Key, Layout, LayoutNode, Show, ShowNode, StyleMap,
+    CollapsingBuilder, Interruption, Key, Layout, LayoutNode, Property, Show, ShowNode,
-    StyleVecBuilder,
+    StyleEntry, StyleMap, StyleVecBuilder, Target,
 };
 use crate::diag::StrResult;
 use crate::library::layout::{FlowChild, FlowNode, PageNode, PlaceNode, Spacing};
@@ -38,6 +38,8 @@ use crate::util::EcoString;
 ///    sequences.
 #[derive(PartialEq, Clone, Hash)]
 pub enum Content {
+    /// Empty content.
+    Empty,
     /// A word space.
     Space,
     /// A forced line break.
@@ -68,8 +70,9 @@ pub enum Content {
     Pagebreak { weak: bool },
     /// A page node.
     Page(PageNode),
-    /// A node that can be realized with styles.
+    /// A node that can be realized with styles, optionally with attached
-    Show(ShowNode),
+    /// properties.
+    Show(ShowNode, Option<Dict>),
     /// Content with attached styles.
     Styled(Arc<(Self, StyleMap)>),
     /// A sequence of multiple nodes.
@@ -79,7 +82,7 @@ pub enum Content {
 impl Content {
     /// Create empty content.
     pub fn new() -> Self {
-        Self::sequence(vec![])
+        Self::Empty
     }
 
     /// Create content from an inline-level node.
@@ -103,15 +106,15 @@ impl Content {
     where
         T: Show + Debug + Hash + Sync + Send + 'static,
     {
-        Self::Show(node.pack())
+        Self::Show(node.pack(), None)
     }
 
     /// Create a new sequence nodes from multiples nodes.
     pub fn sequence(seq: Vec<Self>) -> Self {
-        if seq.len() == 1 {
+        match seq.as_slice() {
-            seq.into_iter().next().unwrap()
+            [] => Self::Empty,
-        } else {
+            [_] => seq.into_iter().next().unwrap(),
-            Self::Sequence(Arc::new(seq))
+            _ => Self::Sequence(Arc::new(seq)),
         }
     }
 
@@ -124,15 +127,20 @@ impl Content {
     }
 
     /// Style this content with a single style property.
-    pub fn styled<'k, K: Key<'k>>(mut self, key: K, value: K::Value) -> Self {
+    pub fn styled<'k, K: Key<'k>>(self, key: K, value: K::Value) -> Self {
+        self.styled_with_entry(StyleEntry::Property(Property::new(key, value)))
+    }
+
+    /// Style this content with a style entry.
+    pub fn styled_with_entry(mut self, entry: StyleEntry) -> Self {
         if let Self::Styled(styled) = &mut self {
             if let Some((_, map)) = Arc::get_mut(styled) {
-                map.apply(key, value);
+                map.apply(entry);
                 return self;
             }
         }
 
-        Self::Styled(Arc::new((self, StyleMap::with(key, value))))
+        Self::Styled(Arc::new((self, entry.into())))
     }
 
     /// Style this content with a full style map.
@@ -151,6 +159,11 @@ impl Content {
         Self::Styled(Arc::new((self, styles)))
     }
 
+    /// Reenable the show rule identified by the selector.
+    pub fn unguard(&self, sel: Selector) -> Self {
+        self.clone().styled_with_entry(StyleEntry::Unguard(sel))
+    }
+
     /// Underline this content.
     pub fn underlined(self) -> Self {
         Self::show(DecoNode::<UNDERLINE>(self))
@@ -228,6 +241,7 @@ impl Default for Content {
 impl Debug for Content {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         match self {
+            Self::Empty => f.pad("Empty"),
             Self::Space => f.pad("Space"),
             Self::Linebreak { justified } => write!(f, "Linebreak({justified})"),
             Self::Horizontal { amount, weak } => {
@@ -245,7 +259,7 @@ impl Debug for Content {
             Self::Item(item) => item.fmt(f),
             Self::Pagebreak { weak } => write!(f, "Pagebreak({weak})"),
             Self::Page(page) => page.fmt(f),
-            Self::Show(node) => node.fmt(f),
+            Self::Show(node, _) => node.fmt(f),
             Self::Styled(styled) => {
                 let (sub, map) = styled.as_ref();
                 map.fmt(f)?;
@@ -261,6 +275,8 @@ impl Add for Content {
 
     fn add(self, rhs: Self) -> Self::Output {
         Self::Sequence(match (self, rhs) {
+            (Self::Empty, rhs) => return rhs,
+            (lhs, Self::Empty) => return lhs,
             (Self::Sequence(mut lhs), Self::Sequence(rhs)) => {
                 let mutable = Arc::make_mut(&mut lhs);
                 match Arc::try_unwrap(rhs) {
@@ -352,7 +368,8 @@ impl<'a, 'ctx> Builder<'a, 'ctx> {
     fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> TypResult<()> {
         // Handle special content kinds.
         match content {
-            Content::Show(node) => return self.show(node, styles),
+            Content::Empty => return Ok(()),
+            Content::Show(node, _) => return self.show(node, styles),
             Content::Styled(styled) => return self.styled(styled, styles),
             Content::Sequence(seq) => return self.sequence(seq, styles),
             _ => {}
@@ -388,15 +405,11 @@ impl<'a, 'ctx> Builder<'a, 'ctx> {
     }
 
     fn show(&mut self, node: &ShowNode, styles: StyleChain<'a>) -> TypResult<()> {
-        let id = node.id();
+        if let Some(realized) = styles.apply(self.ctx, Target::Node(node))? {
-        let realized = match styles.realize(self.ctx, node)? {
+            let stored = self.scratch.templates.alloc(realized);
-            Some(content) => content,
+            self.accept(stored, styles.unscoped(node.id()))?;
-            None => node.realize(self.ctx, styles)?,
+        }
-        };
+        Ok(())
-
-        let content = node.finalize(self.ctx, styles, realized)?;
-        let stored = self.scratch.templates.alloc(content);
-        self.accept(stored, styles.unscoped(id))
     }
 
     fn styled(

src/model/recipe.rs

@@ -1,14 +1,17 @@
 use std::fmt::{self, Debug, Formatter};
 
-use super::NodeId;
+use super::{Content, Interruption, NodeId, Show, ShowNode, StyleEntry};
-use crate::eval::{Func, Node};
+use crate::diag::{At, TypResult};
+use crate::eval::{Args, Func, Value};
+use crate::library::structure::{EnumNode, ListNode};
 use crate::syntax::Span;
+use crate::Context;
 
 /// A show rule recipe.
 #[derive(Clone, PartialEq, Hash)]
 pub struct Recipe {
-    /// The affected node.
+    /// The patterns to customize.
-    pub node: NodeId,
+    pub pattern: Pattern,
     /// The function that defines the recipe.
     pub func: Func,
     /// The span to report all erros with.
@@ -16,14 +19,87 @@ pub struct Recipe {
 }
 
 impl Recipe {
-    /// Create a new recipe for the node `T`.
+    /// Whether the recipe is applicable to the target.
-    pub fn new<T: Node>(func: Func, span: Span) -> Self {
+    pub fn applicable(&self, target: Target) -> bool {
-        Self { node: NodeId::of::<T>(), func, span }
+        match (&self.pattern, target) {
+            (Pattern::Node(id), Target::Node(node)) => *id == node.id(),
+            _ => false,
+        }
+    }
+
+    /// Try to apply the recipe to the target.
+    pub fn apply(
+        &self,
+        ctx: &mut Context,
+        sel: Selector,
+        target: Target,
+    ) -> TypResult<Option<Content>> {
+        let content = match (target, &self.pattern) {
+            (Target::Node(node), &Pattern::Node(id)) if node.id() == id => {
+                let node = node.unguard(sel);
+                self.call(ctx, || {
+                    let dict = node.encode();
+                    Value::Content(Content::Show(node, Some(dict)))
+                })?
+            }
+
+            _ => return Ok(None),
+        };
+
+        Ok(Some(content.styled_with_entry(StyleEntry::Guard(sel))))
+    }
+
+    /// Call the recipe function, with the argument if desired.
+    fn call<F>(&self, ctx: &mut Context, arg: F) -> TypResult<Content>
+    where
+        F: FnOnce() -> Value,
+    {
+        let args = if self.func.argc() == Some(0) {
+            Args::new(self.span)
+        } else {
+            Args::from_values(self.span, [arg()])
+        };
+
+        self.func.call(ctx, args)?.cast().at(self.span)
+    }
+
+    /// What kind of structure the property interrupts.
+    pub fn interruption(&self) -> Option<Interruption> {
+        if let Pattern::Node(id) = self.pattern {
+            if id == NodeId::of::<ListNode>() || id == NodeId::of::<EnumNode>() {
+                return Some(Interruption::List);
+            }
+        }
+
+        None
     }
 }
 
 impl Debug for Recipe {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "Recipe for {:?} from {:?}", self.node, self.span)
+        write!(f, "Recipe matching {:?} from {:?}", self.pattern, self.span)
     }
 }
+
+/// A show rule pattern that may match a target.
+#[derive(Debug, Clone, PartialEq, Hash)]
+pub enum Pattern {
+    /// Defines the appearence of some node.
+    Node(NodeId),
+}
+
+/// A target for a show rule recipe.
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub enum Target<'a> {
+    /// A showable node.
+    Node(&'a ShowNode),
+}
+
+/// Identifies a show rule recipe.
+#[derive(Debug, Copy, Clone, PartialEq, Hash)]
+pub enum Selector {
+    /// The nth recipe from the top of the chain.
+    Nth(usize),
+    /// The base recipe for a kind of node.
+    Base(NodeId),
+}

src/model/show.rs

@@ -2,7 +2,7 @@ use std::fmt::{self, Debug, Formatter, Write};
 use std::hash::Hash;
 use std::sync::Arc;
 
-use super::{Content, NodeId, StyleChain};
+use super::{Content, NodeId, Selector, StyleChain};
 use crate::diag::TypResult;
 use crate::eval::Dict;
 use crate::util::Prehashed;
@@ -10,6 +10,9 @@ use crate::Context;
 
 /// A node that can be realized given some styles.
 pub trait Show: 'static {
+    /// Unguard nested content against recursive show rules.
+    fn unguard(&self, sel: Selector) -> ShowNode;
+
     /// Encode this node into a dictionary.
     fn encode(&self) -> Dict;
 
@@ -63,6 +66,10 @@ impl ShowNode {
 }
 
 impl Show for ShowNode {
+    fn unguard(&self, sel: Selector) -> ShowNode {
+        self.0.unguard(sel)
+    }
+
     fn encode(&self) -> Dict {
         self.0.encode()
     }

src/model/styles.rs

@@ -3,11 +3,9 @@ use std::hash::Hash;
 use std::iter;
 use std::marker::PhantomData;
 
-use super::{Barrier, Content, Key, Property, Recipe, Show, ShowNode};
+use super::{Barrier, Content, Key, Property, Recipe, Selector, Show, Target};
-use crate::diag::{At, TypResult};
+use crate::diag::TypResult;
-use crate::eval::{Args, Func, Node, Value};
 use crate::library::text::{FontFamily, TextNode};
-use crate::syntax::Span;
 use crate::util::ReadableTypeId;
 use crate::Context;
 
@@ -65,11 +63,6 @@ impl StyleMap {
         );
     }
 
-    /// Set a show rule recipe for a node.
-    pub fn set_recipe<T: Node>(&mut self, func: Func, span: Span) {
-        self.push(StyleEntry::Recipe(Recipe::new::<T>(func, span)));
-    }
-
     /// Whether the map contains a style property for the given key.
     pub fn contains<'a, K: Key<'a>>(&self, _: K) -> bool {
         self.0
@@ -91,16 +84,12 @@ impl StyleMap {
         }
     }
 
-    /// Set an outer value for a style property.
+    /// Set an outer style property.
-    ///
-    /// If the property needs folding and the value is already contained in the
-    /// style map, `self` contributes the inner values and `value` is the outer
-    /// one.
     ///
     /// Like [`chain`](Self::chain) or [`apply_map`](Self::apply_map), but with
-    /// only a single property.
+    /// only a entry.
-    pub fn apply<'a, K: Key<'a>>(&mut self, key: K, value: K::Value) {
+    pub fn apply(&mut self, entry: StyleEntry) {
-        self.0.insert(0, StyleEntry::Property(Property::new(key, value)));
+        self.0.insert(0, entry);
     }
 
     /// Apply styles from `tail` in-place. The resulting style map is equivalent
@@ -126,11 +115,7 @@ impl StyleMap {
 
     /// The highest-level kind of of structure the map interrupts.
     pub fn interruption(&self) -> Option<Interruption> {
-        self.0
+        self.0.iter().filter_map(|entry| entry.interruption()).max()
-            .iter()
-            .filter_map(|entry| entry.property())
-            .filter_map(|property| property.interruption())
-            .max()
     }
 }
 
@@ -182,29 +167,17 @@ pub enum Interruption {
 pub enum StyleEntry {
     /// A style property originating from a set rule or constructor.
     Property(Property),
-    /// A barrier for scoped styles.
-    Barrier(Barrier),
     /// A show rule recipe.
     Recipe(Recipe),
+    /// A barrier for scoped styles.
+    Barrier(Barrier),
+    /// Guards against recursive show rules.
+    Guard(Selector),
+    /// Allows recursive show rules again.
+    Unguard(Selector),
 }
 
 impl StyleEntry {
-    /// If this is a property, return it.
-    pub fn property(&self) -> Option<&Property> {
-        match self {
-            Self::Property(property) => Some(property),
-            _ => None,
-        }
-    }
-
-    /// If this is a recipe, return it.
-    pub fn recipe(&self) -> Option<&Recipe> {
-        match self {
-            Self::Recipe(recipe) => Some(recipe),
-            _ => None,
-        }
-    }
-
     /// Make this style the first link of the `tail` chain.
     pub fn chain<'a>(&'a self, tail: &'a StyleChain) -> StyleChain<'a> {
         if let StyleEntry::Barrier(barrier) = self {
@@ -222,6 +195,31 @@ impl StyleEntry {
             tail: Some(tail),
         }
     }
+
+    /// If this is a property, return it.
+    pub fn property(&self) -> Option<&Property> {
+        match self {
+            Self::Property(property) => Some(property),
+            _ => None,
+        }
+    }
+
+    /// If this is a recipe, return it.
+    pub fn recipe(&self) -> Option<&Recipe> {
+        match self {
+            Self::Recipe(recipe) => Some(recipe),
+            _ => None,
+        }
+    }
+
+    /// The highest-level kind of of structure the entry interrupts.
+    pub fn interruption(&self) -> Option<Interruption> {
+        match self {
+            Self::Property(property) => property.interruption(),
+            Self::Recipe(recipe) => recipe.interruption(),
+            _ => None,
+        }
+    }
 }
 
 impl Debug for StyleEntry {
@@ -231,6 +229,8 @@ impl Debug for StyleEntry {
             Self::Property(property) => property.fmt(f)?,
             Self::Recipe(recipe) => recipe.fmt(f)?,
             Self::Barrier(barrier) => barrier.fmt(f)?,
+            Self::Guard(sel) => write!(f, "Guard against {sel:?}")?,
+            Self::Unguard(sel) => write!(f, "Unguard against {sel:?}")?,
         }
         f.write_str("]")
     }
@@ -262,35 +262,6 @@ impl<'a> StyleChain<'a> {
         Self { head: &root.0, tail: None }
     }
 
-    /// Get the output value of a style property.
-    ///
-    /// Returns the property's default value if no map in the chain contains an
-    /// entry for it. Also takes care of resolving and folding and returns
-    /// references where applicable.
-    pub fn get<K: Key<'a>>(self, key: K) -> K::Output {
-        K::get(self, self.values(key))
-    }
-
-    /// Realize a node with a user recipe.
-    pub fn realize(
-        self,
-        ctx: &mut Context,
-        node: &ShowNode,
-    ) -> TypResult<Option<Content>> {
-        let id = node.id();
-        if let Some(recipe) = self
-            .entries()
-            .filter_map(StyleEntry::recipe)
-            .find(|recipe| recipe.node == id)
-        {
-            let dict = node.encode();
-            let args = Args::from_values(recipe.span, [Value::Dict(dict)]);
-            Ok(Some(recipe.func.call(ctx, args)?.cast().at(recipe.span)?))
-        } else {
-            Ok(None)
-        }
-    }
-
     /// Return the chain, but without trailing scoped properties for the given
     /// `node`.
     pub fn unscoped(mut self, node: NodeId) -> Self {
@@ -306,6 +277,80 @@ impl<'a> StyleChain<'a> {
         self
     }
 
+    /// Get the output value of a style property.
+    ///
+    /// Returns the property's default value if no map in the chain contains an
+    /// entry for it. Also takes care of resolving and folding and returns
+    /// references where applicable.
+    pub fn get<K: Key<'a>>(self, key: K) -> K::Output {
+        K::get(self, self.values(key))
+    }
+
+    /// Apply show recipes in this style chain to a target.
+    pub fn apply(self, ctx: &mut Context, target: Target) -> TypResult<Option<Content>> {
+        // Find out how many recipes there any and whether any of their patterns
+        // match.
+        let mut n = 0;
+        let mut any = true;
+        for recipe in self.entries().filter_map(StyleEntry::recipe) {
+            n += 1;
+            any |= recipe.applicable(target);
+        }
+
+        // Find an applicable recipe.
+        let mut realized = None;
+        let mut guarded = false;
+        if any {
+            for recipe in self.entries().filter_map(StyleEntry::recipe) {
+                if recipe.applicable(target) {
+                    let sel = Selector::Nth(n);
+                    if self.guarded(sel) {
+                        guarded = true;
+                    } else if let Some(content) = recipe.apply(ctx, sel, target)? {
+                        realized = Some(content);
+                        break;
+                    }
+                }
+                n -= 1;
+            }
+        }
+
+        if let Target::Node(node) = target {
+            // Realize if there was no matching recipe.
+            if realized.is_none() {
+                let sel = Selector::Base(node.id());
+                if self.guarded(sel) {
+                    guarded = true;
+                } else {
+                    let content = node.unguard(sel).realize(ctx, self)?;
+                    realized = Some(content.styled_with_entry(StyleEntry::Guard(sel)));
+                }
+            }
+
+            // Finalize only if guarding didn't stop any recipe.
+            if !guarded {
+                if let Some(content) = realized {
+                    realized = Some(node.finalize(ctx, self, content)?);
+                }
+            }
+        }
+
+        Ok(realized)
+    }
+
+    /// Whether the recipe identified by the selector is guarded.
+    fn guarded(&self, sel: Selector) -> bool {
+        for entry in self.entries() {
+            match *entry {
+                StyleEntry::Guard(s) if s == sel => return true,
+                StyleEntry::Unguard(s) if s == sel => return false,
+                _ => {}
+            }
+        }
+
+        false
+    }
+
     /// Remove the last link from the chain.
     fn pop(&mut self) {
         *self = self.tail.copied().unwrap_or_default();
@@ -386,7 +431,7 @@ impl<'a, K: Key<'a>> Iterator for Values<'a, K> {
                 StyleEntry::Barrier(barrier) => {
                     self.depth += barrier.is_for(K::node()) as usize;
                 }
-                StyleEntry::Recipe(_) => {}
+                _ => {}
             }
         }
 
@@ -459,13 +504,15 @@ impl<T> StyleVec<T> {
         }
     }
 
-    /// Iterate over the contained maps. Note that zipping this with `items()`
+    /// Map the contained items.
-    /// does not yield the same result as calling `iter()` because this method
+    pub fn map<F, U>(&self, f: F) -> StyleVec<U>
-    /// only returns maps once that are shared by consecutive items. This method
+    where
-    /// is designed for use cases where you want to check, for example, whether
+        F: FnMut(&T) -> U,
-    /// any of the maps fulfills a specific property.
+    {
-    pub fn maps(&self) -> impl Iterator<Item = &StyleMap> {
+        StyleVec {
-        self.maps.iter().map(|(map, _)| map)
+            items: self.items.iter().map(f).collect(),
+            maps: self.maps.clone(),
+        }
     }
 
     /// Iterate over the contained items.
@@ -473,6 +520,15 @@ impl<T> StyleVec<T> {
         self.items.iter()
     }
 
+    /// Iterate over the contained maps. Note that zipping this with `items()`
+    /// does not yield the same result as calling `iter()` because this method
+    /// only returns maps once that are shared by consecutive items. This method
+    /// is designed for use cases where you want to check, for example, whether
+    /// any of the maps fulfills a specific property.
+    pub fn styles(&self) -> impl Iterator<Item = &StyleMap> {
+        self.maps.iter().map(|(map, _)| map)
+    }
+
     /// Iterate over references to the contained items and associated style maps.
     pub fn iter(&self) -> impl Iterator<Item = (&T, &StyleMap)> + '_ {
         self.items().zip(

src/parse/mod.rs

@@ -807,9 +807,15 @@ fn set_expr(p: &mut Parser) -> ParseResult {
 fn show_expr(p: &mut Parser) -> ParseResult {
     p.perform(NodeKind::ShowExpr, |p| {
         p.assert(NodeKind::Show);
-        ident(p)?;
+        let marker = p.marker();
-        p.expect(NodeKind::Colon)?;
+        expr(p)?;
-        ident(p)?;
+        if p.eat_if(NodeKind::Colon) {
+            marker.filter_children(p, |child| match child.kind() {
+                NodeKind::Ident(_) | NodeKind::Colon => Ok(()),
+                _ => Err("expected identifier"),
+            });
+            expr(p)?;
+        }
         p.expect(NodeKind::As)?;
         expr(p)
     })

src/syntax/ast.rs

@@ -1012,17 +1012,21 @@ node! {
 
 impl ShowExpr {
     /// The binding to assign to.
-    pub fn binding(&self) -> Ident {
+    pub fn binding(&self) -> Option<Ident> {
-        self.0.cast_first_child().expect("show rule is missing binding")
+        let mut children = self.0.children();
+        children
+            .find_map(RedRef::cast)
+            .filter(|_| children.any(|child| child.kind() == &NodeKind::Colon))
     }
 
-    /// The function to customize with this show rule.
+    /// The pattern that this rule matches.
-    pub fn target(&self) -> Ident {
+    pub fn pattern(&self) -> Expr {
         self.0
             .children()
-            .filter_map(RedRef::cast)
+            .rev()
-            .nth(1)
+            .skip_while(|child| child.kind() != &NodeKind::As)
-            .expect("show rule is missing target")
+            .find_map(RedRef::cast)
+            .expect("show rule is missing pattern")
     }
 
     /// The expression that realizes the node.

tests/typ/code/field.typ

@@ -12,7 +12,7 @@
 }
 
 ---
-// Error: 2-13 dictionary does not contain key: "invalid"
+// Error: 6-13 dictionary does not contain key: "invalid"
 {(:).invalid}
 
 ---