diff --git a/library/src/basics/enum.rs b/library/src/basics/enum.rs
index f1784a7ef..c5d4d8aef 100644
--- a/library/src/basics/enum.rs
+++ b/library/src/basics/enum.rs
@@ -1,9 +1,8 @@
 use std::str::FromStr;
 
-use crate::compute::NumberingPattern;
+use crate::compute::{Numbering, NumberingPattern};
 use crate::layout::{BlockNode, GridNode, ParNode, Spacing, TrackSizing};
 use crate::prelude::*;
-use crate::text::TextNode;
 
 /// # Numbered List
 /// A numbered list.
@@ -98,7 +97,7 @@ pub struct EnumNode {
 #[node]
 impl EnumNode {
     /// How to number the enumeration. Accepts a
-    /// [numbering pattern](@numbering).
+    /// [numbering pattern or function](@numbering).
     ///
     /// # Example
     /// ```
@@ -109,8 +108,8 @@ impl EnumNode {
     /// + Style
     /// ```
     #[property(referenced)]
-    pub const NUMBERING: EnumNumbering =
-        EnumNumbering::Pattern(NumberingPattern::from_str("1.").unwrap());
+    pub const NUMBERING: Numbering =
+        Numbering::Pattern(NumberingPattern::from_str("1.").unwrap());
 
     /// The indentation of each item's label.
     #[property(resolve)]
@@ -188,7 +187,7 @@ impl Layout for EnumNode {
         let mut number = NonZeroUsize::new(1).unwrap();
         for ((n, item), map) in self.items.iter() {
             number = n.unwrap_or(number);
-            let resolved = numbering.resolve(vt, number)?;
+            let resolved = numbering.apply(vt.world(), &[number])?.display();
             cells.push(Content::empty());
             cells.push(resolved.styled_with_map(map.clone()));
             cells.push(Content::empty());
@@ -209,43 +208,3 @@ impl Layout for EnumNode {
         .layout(vt, styles, regions)
     }
 }
-
-/// How to number an enumeration.
-#[derive(Debug, Clone, Hash)]
-pub enum EnumNumbering {
-    /// A pattern with prefix, numbering, lower / upper case and suffix.
-    Pattern(NumberingPattern),
-    /// A closure mapping from an item's number to content.
-    Func(Func, Span),
-}
-
-impl EnumNumbering {
-    /// Resolve the marker based on the number.
-    pub fn resolve(&self, vt: &Vt, number: NonZeroUsize) -> SourceResult<Content> {
-        Ok(match self {
-            Self::Pattern(pattern) => TextNode::packed(pattern.apply(&[number])),
-            Self::Func(func, span) => {
-                let args = Args::new(*span, [Value::Int(number.get() as i64)]);
-                func.call_detached(vt.world(), args)?.display()
-            }
-        })
-    }
-}
-
-impl Cast<Spanned<Value>> for EnumNumbering {
-    fn is(value: &Spanned<Value>) -> bool {
-        matches!(&value.v, Value::Content(_) | Value::Func(_))
-    }
-
-    fn cast(value: Spanned<Value>) -> StrResult<Self> {
-        match value.v {
-            Value::Str(v) => Ok(Self::Pattern(v.parse()?)),
-            Value::Func(v) => Ok(Self::Func(v, value.span)),
-            v => Self::error(v),
-        }
-    }
-
-    fn describe() -> CastInfo {
-        CastInfo::Union(vec![CastInfo::Type("string"), CastInfo::Type("function")])
-    }
-}
diff --git a/library/src/basics/heading.rs b/library/src/basics/heading.rs
index 925d23a20..c2696dd6e 100644
--- a/library/src/basics/heading.rs
+++ b/library/src/basics/heading.rs
@@ -1,6 +1,6 @@
 use typst::font::FontWeight;
 
-use crate::compute::NumberingPattern;
+use crate::compute::Numbering;
 use crate::layout::{BlockNode, VNode};
 use crate::prelude::*;
 use crate::text::{SpaceNode, TextNode, TextSize};
@@ -15,8 +15,8 @@ use crate::text::{SpaceNode, TextNode, TextSize};
 /// (not the document's title).
 ///
 /// Typst can automatically number your headings for you. To enable numbering,
-/// specify how you want your headings to be numbered with a [numbering
-/// pattern](@numbering).
+/// specify how you want your headings to be numbered with a
+/// [numbering pattern or function](@numbering).
 ///
 /// Independently from the numbering, Typst can also automatically generate an
 /// [outline](@outline) of all headings for you. To exclude one or more headings
@@ -60,7 +60,8 @@ pub struct HeadingNode {
 
 #[node]
 impl HeadingNode {
-    /// How to number the heading. Accepts a [numbering pattern](@numbering).
+    /// How to number the heading. Accepts a
+    /// [numbering pattern or function](@numbering).
     ///
     /// # Example
     /// ```
@@ -71,7 +72,7 @@ impl HeadingNode {
     /// === A sub-subsection
     /// ```
     #[property(referenced)]
-    pub const NUMBERING: Option<NumberingPattern> = None;
+    pub const NUMBERING: Option<Numbering> = None;
 
     /// Whether the heading should appear in the outline.
     ///
@@ -106,7 +107,12 @@ impl HeadingNode {
 }
 
 impl Prepare for HeadingNode {
-    fn prepare(&self, vt: &mut Vt, mut this: Content, styles: StyleChain) -> Content {
+    fn prepare(
+        &self,
+        vt: &mut Vt,
+        mut this: Content,
+        styles: StyleChain,
+    ) -> SourceResult<Content> {
         let my_id = vt.identify(&this);
 
         let mut counter = HeadingCounter::new();
@@ -115,30 +121,32 @@ impl Prepare for HeadingNode {
                 break;
             }
 
-            if matches!(node.field("numbers"), Some(Value::Str(_))) {
+            let numbers = node.field("numbers").unwrap();
+            if numbers != Value::None {
                 let heading = node.to::<Self>().unwrap();
                 counter.advance(heading);
             }
         }
 
         let mut numbers = Value::None;
-        if let Some(pattern) = styles.get(Self::NUMBERING) {
-            numbers = Value::Str(pattern.apply(counter.advance(self)).into());
+        if let Some(numbering) = styles.get(Self::NUMBERING) {
+            numbers = numbering.apply(vt.world(), counter.advance(self))?;
         }
 
         this.push_field("outlined", Value::Bool(styles.get(Self::OUTLINED)));
         this.push_field("numbers", numbers);
 
         let meta = Meta::Node(my_id, this.clone());
-        this.styled(Meta::DATA, vec![meta])
+        Ok(this.styled(Meta::DATA, vec![meta]))
     }
 }
 
 impl Show for HeadingNode {
     fn show(&self, _: &mut Vt, this: &Content, _: StyleChain) -> SourceResult<Content> {
         let mut realized = self.title.clone();
-        if let Some(Value::Str(numbering)) = this.field("numbers") {
-            realized = TextNode::packed(numbering) + SpaceNode.pack() + realized;
+        let numbers = this.field("numbers").unwrap();
+        if numbers != Value::None {
+            realized = numbers.display() + SpaceNode.pack() + realized;
         }
         Ok(BlockNode(realized).pack())
     }
diff --git a/library/src/basics/table.rs b/library/src/basics/table.rs
index 01b16898e..208818301 100644
--- a/library/src/basics/table.rs
+++ b/library/src/basics/table.rs
@@ -199,7 +199,7 @@ pub enum Celled<T> {
     /// A bare value, the same for all cells.
     Value(T),
     /// A closure mapping from cell coordinates to a value.
-    Func(Func, Span),
+    Func(Func),
 }
 
 impl<T: Cast + Clone> Celled<T> {
@@ -207,24 +207,25 @@ impl<T: Cast + Clone> Celled<T> {
     pub fn resolve(&self, vt: &Vt, x: usize, y: usize) -> SourceResult<T> {
         Ok(match self {
             Self::Value(value) => value.clone(),
-            Self::Func(func, span) => {
-                let args = Args::new(*span, [Value::Int(x as i64), Value::Int(y as i64)]);
-                func.call_detached(vt.world(), args)?.cast().at(*span)?
+            Self::Func(func) => {
+                let args =
+                    Args::new(func.span(), [Value::Int(x as i64), Value::Int(y as i64)]);
+                func.call_detached(vt.world(), args)?.cast().at(func.span())?
             }
         })
     }
 }
 
-impl<T: Cast> Cast<Spanned<Value>> for Celled<T> {
-    fn is(value: &Spanned<Value>) -> bool {
-        matches!(&value.v, Value::Func(_)) || T::is(&value.v)
+impl<T: Cast> Cast for Celled<T> {
+    fn is(value: &Value) -> bool {
+        matches!(value, Value::Func(_)) || T::is(value)
     }
 
-    fn cast(value: Spanned<Value>) -> StrResult<Self> {
-        match value.v {
-            Value::Func(v) => Ok(Self::Func(v, value.span)),
+    fn cast(value: Value) -> StrResult<Self> {
+        match value {
+            Value::Func(v) => Ok(Self::Func(v)),
             v if T::is(&v) => Ok(Self::Value(T::cast(v)?)),
-            v => Self::error(v),
+            v => <Self as Cast>::error(v),
         }
     }
 
diff --git a/library/src/compute/data.rs b/library/src/compute/data.rs
index bcf64360d..e3013389b 100644
--- a/library/src/compute/data.rs
+++ b/library/src/compute/data.rs
@@ -218,10 +218,7 @@ fn convert_json(value: serde_json::Value) -> Value {
 /// Format the user-facing JSON error message.
 fn format_json_error(error: serde_json::Error) -> String {
     assert!(error.is_syntax() || error.is_eof());
-    format!(
-        "failed to parse json file: syntax error in line {}",
-        error.line()
-    )
+    format!("failed to parse json file: syntax error in line {}", error.line())
 }
 
 /// # XML
@@ -252,22 +249,22 @@ fn format_json_error(error: serde_json::Error) -> String {
 ///   let author = findChild(elem, "author")
 ///   let pars = findChild(elem, "content")
 ///
-///   heading((title.children)(0))
+///   heading(title.children.first())
 ///   text(10pt, weight: "medium")[
 ///     Published by
-///     {(author.children)(0)}
+///     {author.children.first()}
 ///   ]
 ///
 ///   for p in pars.children {
 ///     if (type(p) == "dictionary") {
 ///       parbreak()
-///       (p.children)(0)
+///       p.children.first()
 ///     }
 ///   }
 /// }
 ///
-/// #let file = xml("example.xml")
-/// #for child in file(0).children {
+/// #let data = xml("example.xml")
+/// #for child in data.first().children {
 ///   if (type(child) == "dictionary") {
 ///     article(child)
 ///   }
diff --git a/library/src/compute/utility.rs b/library/src/compute/utility.rs
index 78cf39533..414a62f50 100644
--- a/library/src/compute/utility.rs
+++ b/library/src/compute/utility.rs
@@ -35,53 +35,105 @@ pub fn lorem(args: &mut Args) -> SourceResult<Value> {
 }
 
 /// # Numbering
-/// Apply a numbering pattern to a sequence of numbers.
+/// Apply a numbering to a sequence of numbers.
 ///
-/// Numbering patterns are strings that define how a sequence of numbers should
-/// be rendered as text. The patterns consist of [counting
-/// symbols](#parameters--pattern) for which the actual number is substituted,
-/// their prefixes, and one suffix. The prefixes and the suffix are repeated as-is.
+/// A numbering defines how a sequence of numbers should be displayed as
+/// content. It is defined either through a pattern string or an arbitrary
+/// function.
+///
+/// A numbering pattern consists of [counting symbols](#parameters--numbering)
+/// for which the actual number is substituted, their prefixes, and one suffix.
+/// The prefixes and the suffix are repeated as-is.
 ///
 /// ## Example
 /// ```
 /// #numbering("1.1)", 1, 2, 3) \
 /// #numbering("1.a.i", 1, 2) \
-/// #numbering("I – 1", 12, 2)
+/// #numbering("I – 1", 12, 2) \
+/// #numbering(
+///   (..nums) => nums
+///     .pos()
+///     .map(str)
+///     .join(".") + ")",
+///   1, 2, 3,
+/// )
 /// ```
 ///
 /// ## Parameters
-/// - pattern: NumberingPattern (positional, required)
-///   A string that defines how the numbering works.
+/// - numbering: Numbering (positional, required)
+///   Defines how the numbering works.
 ///
-///   **Counting symbols** are `1`, `a`, `A`, `i`, `I` and `*`. They are replaced
-///   by the number in the sequence, in the given case.
+///   **Counting symbols** are `1`, `a`, `A`, `i`, `I` and `*`. They are
+///   replaced by the number in the sequence, in the given case.
 ///
-///   The `*` character means that symbols should be used to count, in the order
-///   of `*`, `†`, `‡`, `§`, `¶`, and `‖`. If there are more than six items, the
-///   number is represented using multiple symbols.
+///   The `*` character means that symbols should be used to count, in the
+///   order of `*`, `†`, `‡`, `§`, `¶`, and `‖`. If there are more than six
+///   items, the number is represented using multiple symbols.
 ///
 ///   **Suffixes** are all characters after the last counting symbol. They are
 ///   repeated as-is at the end of any rendered number.
 ///
 ///   **Prefixes** are all characters that are neither counting symbols nor
-///   suffixes. They are repeated as-is at in front of their rendered equivalent
-///   of their counting symbol.
+///   suffixes. They are repeated as-is at in front of their rendered
+///   equivalent of their counting symbol.
+///
+///   This parameter can also be an arbitrary function that gets each number as
+///   an individual argument. When given a function, the `numbering` function
+///   just forwards the arguments to that function. While this is not
+///   particularly useful in itself, it means that you can just give arbitrary
+///   numberings to the `numbering` function without caring whether they are
+///   defined as a pattern or function.
 ///
 /// - numbers: NonZeroUsize (positional, variadic)
-///   The numbers to apply the pattern to. Must be positive.
+///   The numbers to apply the numbering to. Must be positive.
 ///
-///   If more numbers than counting symbols are given, the last counting symbol
-///   with its prefix is repeated.
+///   If `numbering` is a pattern and more numbers than counting symbols are
+///   given, the last counting symbol with its prefix is repeated.
 ///
-/// - returns: string
+/// - returns: any
 ///
 /// ## Category
 /// utility
 #[func]
-pub fn numbering(args: &mut Args) -> SourceResult<Value> {
-    let pattern = args.expect::<NumberingPattern>("pattern")?;
+pub fn numbering(vm: &Vm, args: &mut Args) -> SourceResult<Value> {
+    let numbering = args.expect::<Numbering>("pattern or function")?;
     let numbers = args.all::<NonZeroUsize>()?;
-    Ok(Value::Str(pattern.apply(&numbers).into()))
+    numbering.apply(vm.world(), &numbers)
+}
+
+/// How to number an enumeration.
+#[derive(Debug, Clone, Hash)]
+pub enum Numbering {
+    /// A pattern with prefix, numbering, lower / upper case and suffix.
+    Pattern(NumberingPattern),
+    /// A closure mapping from an item's number to content.
+    Func(Func),
+}
+
+impl Numbering {
+    /// Apply the pattern to the given numbers.
+    pub fn apply(
+        &self,
+        world: Tracked<dyn World>,
+        numbers: &[NonZeroUsize],
+    ) -> SourceResult<Value> {
+        Ok(match self {
+            Self::Pattern(pattern) => Value::Str(pattern.apply(numbers).into()),
+            Self::Func(func) => {
+                let args = Args::new(
+                    func.span(),
+                    numbers.iter().map(|n| Value::Int(n.get() as i64)),
+                );
+                func.call_detached(world, args)?
+            }
+        })
+    }
+}
+
+castable! {
+    Numbering,
+    v: Str => Self::Pattern(v.parse()?),
+    v: Func => Self::Func(v),
 }
 
 /// How to turn a number into text.
@@ -157,11 +209,6 @@ impl FromStr for NumberingPattern {
     }
 }
 
-castable! {
-    NumberingPattern,
-    string: EcoString => string.parse()?,
-}
-
 /// Different kinds of numberings.
 #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
 enum NumberingKind {
diff --git a/library/src/layout/mod.rs b/library/src/layout/mod.rs
index 91822be83..330db7166 100644
--- a/library/src/layout/mod.rs
+++ b/library/src/layout/mod.rs
@@ -317,7 +317,8 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         // Prepare only if this is the first application for this node.
         if let Some(node) = content.with::<dyn Prepare>() {
             if !content.is_prepared() {
-                let prepared = node.prepare(self.vt, content.clone().prepared(), styles);
+                let prepared =
+                    node.prepare(self.vt, content.clone().prepared(), styles)?;
                 let stored = self.scratch.content.alloc(prepared);
                 return self.accept(stored, styles);
             }
diff --git a/library/src/layout/page.rs b/library/src/layout/page.rs
index f4f9620c0..4f5231def 100644
--- a/library/src/layout/page.rs
+++ b/library/src/layout/page.rs
@@ -2,7 +2,6 @@ use std::str::FromStr;
 
 use super::ColumnsNode;
 use crate::prelude::*;
-use crate::text::TextNode;
 
 /// # Page
 /// Layouts its child onto one or multiple pages.
@@ -174,7 +173,7 @@ impl PageNode {
     /// #lorem(18)
     /// ```
     #[property(referenced)]
-    pub const HEADER: Marginal = Marginal::None;
+    pub const HEADER: Option<Marginal> = None;
 
     /// The page's footer.
     ///
@@ -199,7 +198,7 @@ impl PageNode {
     /// #lorem(18)
     /// ```
     #[property(referenced)]
-    pub const FOOTER: Marginal = Marginal::None;
+    pub const FOOTER: Option<Marginal> = None;
 
     /// Content in the page's background.
     ///
@@ -221,7 +220,7 @@ impl PageNode {
     /// (of typesetting).
     /// ```
     #[property(referenced)]
-    pub const BACKGROUND: Marginal = Marginal::None;
+    pub const BACKGROUND: Option<Marginal> = None;
 
     /// Content in the page's foreground.
     ///
@@ -239,7 +238,7 @@ impl PageNode {
     /// not understand our approach...
     /// ```
     #[property(referenced)]
-    pub const FOREGROUND: Marginal = Marginal::None;
+    pub const FOREGROUND: Option<Marginal> = None;
 
     fn construct(_: &Vm, args: &mut Args) -> SourceResult<Content> {
         Ok(Self(args.expect("body")?).pack())
@@ -323,14 +322,15 @@ impl PageNode {
                 (foreground, Point::zero(), size),
                 (background, Point::zero(), size),
             ] {
-                if let Some(content) = marginal.resolve(vt, page)? {
-                    let pod = Regions::one(area, area, Axes::splat(true));
-                    let sub = content.layout(vt, styles, pod)?.into_frame();
-                    if std::ptr::eq(marginal, background) {
-                        frame.prepend_frame(pos, sub);
-                    } else {
-                        frame.push_frame(pos, sub);
-                    }
+                let in_background = std::ptr::eq(marginal, background);
+                let Some(marginal) = marginal else { continue };
+                let content = marginal.resolve(vt, page)?;
+                let pod = Regions::one(area, area, Axes::splat(true));
+                let sub = content.layout(vt, styles, pod)?.into_frame();
+                if in_background {
+                    frame.prepend_frame(pos, sub);
+                } else {
+                    frame.push_frame(pos, sub);
                 }
             }
 
@@ -396,53 +396,29 @@ impl PagebreakNode {
 /// A header, footer, foreground or background definition.
 #[derive(Debug, Clone, Hash)]
 pub enum Marginal {
-    /// Nothing,
-    None,
     /// Bare content.
     Content(Content),
     /// A closure mapping from a page number to content.
-    Func(Func, Span),
+    Func(Func),
 }
 
 impl Marginal {
     /// Resolve the marginal based on the page number.
-    pub fn resolve(&self, vt: &Vt, page: usize) -> SourceResult<Option<Content>> {
+    pub fn resolve(&self, vt: &Vt, page: usize) -> SourceResult<Content> {
         Ok(match self {
-            Self::None => None,
-            Self::Content(content) => Some(content.clone()),
-            Self::Func(func, span) => {
-                let args = Args::new(*span, [Value::Int(page as i64)]);
-                Some(func.call_detached(vt.world(), args)?.display())
+            Self::Content(content) => content.clone(),
+            Self::Func(func) => {
+                let args = Args::new(func.span(), [Value::Int(page as i64)]);
+                func.call_detached(vt.world(), args)?.display()
             }
         })
     }
 }
 
-impl Cast<Spanned<Value>> for Marginal {
-    fn is(value: &Spanned<Value>) -> bool {
-        matches!(
-            &value.v,
-            Value::None | Value::Str(_) | Value::Content(_) | Value::Func(_)
-        )
-    }
-
-    fn cast(value: Spanned<Value>) -> StrResult<Self> {
-        match value.v {
-            Value::None => Ok(Self::None),
-            Value::Str(v) => Ok(Self::Content(TextNode::packed(v))),
-            Value::Content(v) => Ok(Self::Content(v)),
-            Value::Func(v) => Ok(Self::Func(v, value.span)),
-            v => Self::error(v),
-        }
-    }
-
-    fn describe() -> CastInfo {
-        CastInfo::Union(vec![
-            CastInfo::Type("none"),
-            CastInfo::Type("content"),
-            CastInfo::Type("function"),
-        ])
-    }
+castable! {
+    Marginal,
+    v: Content => Self::Content(v),
+    v: Func => Self::Func(v),
 }
 
 /// Specification of a paper.
diff --git a/library/src/meta/outline.rs b/library/src/meta/outline.rs
index c3d17196b..4b70666b6 100644
--- a/library/src/meta/outline.rs
+++ b/library/src/meta/outline.rs
@@ -84,7 +84,12 @@ impl OutlineNode {
 }
 
 impl Prepare for OutlineNode {
-    fn prepare(&self, vt: &mut Vt, mut this: Content, _: StyleChain) -> Content {
+    fn prepare(
+        &self,
+        vt: &mut Vt,
+        mut this: Content,
+        _: StyleChain,
+    ) -> SourceResult<Content> {
         let headings = vt
             .locate(Selector::node::<HeadingNode>())
             .into_iter()
@@ -94,7 +99,7 @@ impl Prepare for OutlineNode {
             .collect();
 
         this.push_field("headings", Value::Array(Array::from_vec(headings)));
-        this
+        Ok(this)
     }
 }
 
@@ -151,32 +156,27 @@ impl Show for OutlineNode {
 
             // Add hidden ancestors numberings to realize the indent.
             if indent {
-                let text = ancestors
+                let hidden: Vec<_> = ancestors
                     .iter()
-                    .filter_map(|node| match node.field("numbers").unwrap() {
-                        Value::Str(numbering) => {
-                            Some(EcoString::from(numbering) + ' '.into())
-                        }
-                        _ => None,
-                    })
-                    .collect::<EcoString>();
+                    .map(|node| node.field("numbers").unwrap())
+                    .filter(|numbers| *numbers != Value::None)
+                    .map(|numbers| numbers.display() + SpaceNode.pack())
+                    .collect();
 
-                if !text.is_empty() {
-                    seq.push(HideNode(TextNode::packed(text)).pack());
+                if !hidden.is_empty() {
+                    seq.push(HideNode(Content::sequence(hidden)).pack());
                     seq.push(SpaceNode.pack());
                 }
             }
 
             // Format the numbering.
-            let numbering = match node.field("numbers").unwrap() {
-                Value::Str(numbering) => {
-                    TextNode::packed(EcoString::from(numbering) + ' '.into())
-                }
-                _ => Content::empty(),
+            let mut start = heading.title.clone();
+            let numbers = node.field("numbers").unwrap();
+            if numbers != Value::None {
+                start = numbers.display() + SpaceNode.pack() + start;
             };
 
             // Add the numbering and section name.
-            let start = numbering + heading.title.clone();
             seq.push(start.linked(Destination::Internal(loc)));
 
             // Add filler symbols between the section name and page number.
diff --git a/library/src/text/raw.rs b/library/src/text/raw.rs
index c6026f898..2f0ec79ec 100644
--- a/library/src/text/raw.rs
+++ b/library/src/text/raw.rs
@@ -132,7 +132,12 @@ impl RawNode {
 }
 
 impl Prepare for RawNode {
-    fn prepare(&self, _: &mut Vt, mut this: Content, styles: StyleChain) -> Content {
+    fn prepare(
+        &self,
+        _: &mut Vt,
+        mut this: Content,
+        styles: StyleChain,
+    ) -> SourceResult<Content> {
         this.push_field(
             "lang",
             match styles.get(Self::LANG) {
@@ -140,7 +145,7 @@ impl Prepare for RawNode {
                 None => Value::None,
             },
         );
-        this
+        Ok(this)
     }
 }
 
diff --git a/src/diag.rs b/src/diag.rs
index 55f16b5fc..e0015fcc6 100644
--- a/src/diag.rs
+++ b/src/diag.rs
@@ -66,7 +66,9 @@ pub struct SourceError {
 
 impl SourceError {
     /// Create a new, bare error.
+    #[track_caller]
     pub fn new(span: Span, message: impl Into<EcoString>) -> Self {
+        assert!(!span.is_detached());
         Self {
             span,
             pos: ErrorPos::Full,
diff --git a/src/model/args.rs b/src/model/args.rs
index 4aaaded4f..9ab07ed8c 100644
--- a/src/model/args.rs
+++ b/src/model/args.rs
@@ -150,7 +150,7 @@ impl Args {
     }
 
     /// Extract the positional arguments as an array.
-    pub fn to_positional(&self) -> Array {
+    pub fn to_pos(&self) -> Array {
         self.items
             .iter()
             .filter(|item| item.name.is_none())
diff --git a/src/model/array.rs b/src/model/array.rs
index fb740a13c..28b9d1a03 100644
--- a/src/model/array.rs
+++ b/src/model/array.rs
@@ -5,7 +5,6 @@ use std::sync::Arc;
 
 use super::{ops, Args, Func, Value, Vm};
 use crate::diag::{bail, At, SourceResult, StrResult};
-use crate::syntax::Spanned;
 use crate::util::{format_eco, ArcExt, EcoString};
 
 /// Create a new [`Array`] from values.
@@ -137,13 +136,13 @@ impl Array {
     }
 
     /// Return the first matching element.
-    pub fn find(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<Option<Value>> {
-        if f.v.argc().map_or(false, |count| count != 1) {
-            bail!(f.span, "function must have exactly one parameter");
+    pub fn find(&self, vm: &Vm, func: Func) -> SourceResult<Option<Value>> {
+        if func.argc().map_or(false, |count| count != 1) {
+            bail!(func.span(), "function must have exactly one parameter");
         }
         for item in self.iter() {
-            let args = Args::new(f.span, [item.clone()]);
-            if f.v.call(vm, args)?.cast::<bool>().at(f.span)? {
+            let args = Args::new(func.span(), [item.clone()]);
+            if func.call(vm, args)?.cast::<bool>().at(func.span())? {
                 return Ok(Some(item.clone()));
             }
         }
@@ -152,13 +151,13 @@ impl Array {
     }
 
     /// Return the index of the first matching element.
-    pub fn position(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<Option<i64>> {
-        if f.v.argc().map_or(false, |count| count != 1) {
-            bail!(f.span, "function must have exactly one parameter");
+    pub fn position(&self, vm: &Vm, func: Func) -> SourceResult<Option<i64>> {
+        if func.argc().map_or(false, |count| count != 1) {
+            bail!(func.span(), "function must have exactly one parameter");
         }
         for (i, item) in self.iter().enumerate() {
-            let args = Args::new(f.span, [item.clone()]);
-            if f.v.call(vm, args)?.cast::<bool>().at(f.span)? {
+            let args = Args::new(func.span(), [item.clone()]);
+            if func.call(vm, args)?.cast::<bool>().at(func.span())? {
                 return Ok(Some(i as i64));
             }
         }
@@ -168,14 +167,14 @@ impl Array {
 
     /// Return a new array with only those elements for which the function
     /// returns true.
-    pub fn filter(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<Self> {
-        if f.v.argc().map_or(false, |count| count != 1) {
-            bail!(f.span, "function must have exactly one parameter");
+    pub fn filter(&self, vm: &Vm, func: Func) -> SourceResult<Self> {
+        if func.argc().map_or(false, |count| count != 1) {
+            bail!(func.span(), "function must have exactly one parameter");
         }
         let mut kept = vec![];
         for item in self.iter() {
-            let args = Args::new(f.span, [item.clone()]);
-            if f.v.call(vm, args)?.cast::<bool>().at(f.span)? {
+            let args = Args::new(func.span(), [item.clone()]);
+            if func.call(vm, args)?.cast::<bool>().at(func.span())? {
                 kept.push(item.clone())
             }
         }
@@ -183,45 +182,45 @@ impl Array {
     }
 
     /// Transform each item in the array with a function.
-    pub fn map(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<Self> {
-        if f.v.argc().map_or(false, |count| count < 1 || count > 2) {
-            bail!(f.span, "function must have one or two parameters");
+    pub fn map(&self, vm: &Vm, func: Func) -> SourceResult<Self> {
+        if func.argc().map_or(false, |count| count < 1 || count > 2) {
+            bail!(func.span(), "function must have one or two parameters");
         }
-        let enumerate = f.v.argc() == Some(2);
+        let enumerate = func.argc() == Some(2);
         self.iter()
             .enumerate()
             .map(|(i, item)| {
-                let mut args = Args::new(f.span, []);
+                let mut args = Args::new(func.span(), []);
                 if enumerate {
-                    args.push(f.span, Value::Int(i as i64));
+                    args.push(func.span(), Value::Int(i as i64));
                 }
-                args.push(f.span, item.clone());
-                f.v.call(vm, args)
+                args.push(func.span(), item.clone());
+                func.call(vm, args)
             })
             .collect()
     }
 
     /// Fold all of the array's elements into one with a function.
-    pub fn fold(&self, vm: &Vm, init: Value, f: Spanned<Func>) -> SourceResult<Value> {
-        if f.v.argc().map_or(false, |count| count != 2) {
-            bail!(f.span, "function must have exactly two parameters");
+    pub fn fold(&self, vm: &Vm, init: Value, func: Func) -> SourceResult<Value> {
+        if func.argc().map_or(false, |count| count != 2) {
+            bail!(func.span(), "function must have exactly two parameters");
         }
         let mut acc = init;
         for item in self.iter() {
-            let args = Args::new(f.span, [acc, item.clone()]);
-            acc = f.v.call(vm, args)?;
+            let args = Args::new(func.span(), [acc, item.clone()]);
+            acc = func.call(vm, args)?;
         }
         Ok(acc)
     }
 
     /// Whether any element matches.
-    pub fn any(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<bool> {
-        if f.v.argc().map_or(false, |count| count != 1) {
-            bail!(f.span, "function must have exactly one parameter");
+    pub fn any(&self, vm: &Vm, func: Func) -> SourceResult<bool> {
+        if func.argc().map_or(false, |count| count != 1) {
+            bail!(func.span(), "function must have exactly one parameter");
         }
         for item in self.iter() {
-            let args = Args::new(f.span, [item.clone()]);
-            if f.v.call(vm, args)?.cast::<bool>().at(f.span)? {
+            let args = Args::new(func.span(), [item.clone()]);
+            if func.call(vm, args)?.cast::<bool>().at(func.span())? {
                 return Ok(true);
             }
         }
@@ -230,13 +229,13 @@ impl Array {
     }
 
     /// Whether all elements match.
-    pub fn all(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<bool> {
-        if f.v.argc().map_or(false, |count| count != 1) {
-            bail!(f.span, "function must have exactly one parameter");
+    pub fn all(&self, vm: &Vm, func: Func) -> SourceResult<bool> {
+        if func.argc().map_or(false, |count| count != 1) {
+            bail!(func.span(), "function must have exactly one parameter");
         }
         for item in self.iter() {
-            let args = Args::new(f.span, [item.clone()]);
-            if !f.v.call(vm, args)?.cast::<bool>().at(f.span)? {
+            let args = Args::new(func.span(), [item.clone()]);
+            if !func.call(vm, args)?.cast::<bool>().at(func.span())? {
                 return Ok(false);
             }
         }
diff --git a/src/model/dict.rs b/src/model/dict.rs
index 83c16824f..a5dbeeae2 100644
--- a/src/model/dict.rs
+++ b/src/model/dict.rs
@@ -6,7 +6,6 @@ use std::sync::Arc;
 use super::{Args, Array, Func, Str, Value, Vm};
 use crate::diag::{bail, SourceResult, StrResult};
 use crate::syntax::is_ident;
-use crate::syntax::Spanned;
 use crate::util::{format_eco, ArcExt, EcoString};
 
 /// Create a new [`Dict`] from key-value pairs.
@@ -107,14 +106,15 @@ impl Dict {
     }
 
     /// Transform each pair in the dictionary with a function.
-    pub fn map(&self, vm: &Vm, f: Spanned<Func>) -> SourceResult<Array> {
-        if f.v.argc().map_or(false, |count| count != 1) {
-            bail!(f.span, "function must have exactly two parameters");
+    pub fn map(&self, vm: &Vm, func: Func) -> SourceResult<Array> {
+        if func.argc().map_or(false, |count| count != 2) {
+            bail!(func.span(), "function must have exactly two parameters");
         }
         self.iter()
             .map(|(key, value)| {
-                let args = Args::new(f.span, [Value::Str(key.clone()), value.clone()]);
-                f.v.call(vm, args)
+                let args =
+                    Args::new(func.span(), [Value::Str(key.clone()), value.clone()]);
+                func.call(vm, args)
             })
             .collect()
     }
diff --git a/src/model/eval.rs b/src/model/eval.rs
index ab89f9c23..e9134114e 100644
--- a/src/model/eval.rs
+++ b/src/model/eval.rs
@@ -563,7 +563,11 @@ impl Eval for ast::Ident {
     type Output = Value;
 
     fn eval(&self, vm: &mut Vm) -> SourceResult<Self::Output> {
-        vm.scopes.get(self).cloned().at(self.span())
+        let value = vm.scopes.get(self).cloned().at(self.span())?;
+        Ok(match value {
+            Value::Func(func) => Value::Func(func.spanned(self.span())),
+            value => value,
+        })
     }
 }
 
@@ -912,15 +916,17 @@ impl Eval for ast::Closure {
             }
         }
 
-        // Define the actual function.
-        Ok(Value::Func(Func::from_closure(Closure {
+        // Define the closure function.
+        let closure = Closure {
             location: vm.location,
             name,
             captured,
             params,
             sink,
             body: self.body(),
-        })))
+        };
+
+        Ok(Value::Func(Func::from_closure(closure, self.span())))
     }
 }
 
diff --git a/src/model/func.rs b/src/model/func.rs
index 3fb8f4d4d..878b717f4 100644
--- a/src/model/func.rs
+++ b/src/model/func.rs
@@ -10,13 +10,13 @@ use super::{
 };
 use crate::diag::{bail, SourceResult, StrResult};
 use crate::syntax::ast::{self, AstNode, Expr};
-use crate::syntax::{SourceId, SyntaxNode};
+use crate::syntax::{SourceId, Span, SyntaxNode};
 use crate::util::EcoString;
 use crate::World;
 
 /// An evaluatable function.
 #[derive(Clone, Hash)]
-pub struct Func(Arc<Repr>);
+pub struct Func(Arc<Repr>, Span);
 
 /// The different kinds of function representations.
 #[derive(Hash)]
@@ -40,27 +40,33 @@ impl Func {
         func: fn(&Vm, &mut Args) -> SourceResult<Value>,
         info: FuncInfo,
     ) -> Self {
-        Self(Arc::new(Repr::Native(Native { func, set: None, node: None, info })))
+        Self(
+            Arc::new(Repr::Native(Native { func, set: None, node: None, info })),
+            Span::detached(),
+        )
     }
 
     /// Create a new function from a native rust node.
     pub fn from_node<T: Node>(mut info: FuncInfo) -> Self {
         info.params.extend(T::properties());
-        Self(Arc::new(Repr::Native(Native {
-            func: |ctx, args| {
-                let styles = T::set(args, true)?;
-                let content = T::construct(ctx, args)?;
-                Ok(Value::Content(content.styled_with_map(styles.scoped())))
-            },
-            set: Some(|args| T::set(args, false)),
-            node: Some(NodeId::of::<T>()),
-            info,
-        })))
+        Self(
+            Arc::new(Repr::Native(Native {
+                func: |ctx, args| {
+                    let styles = T::set(args, true)?;
+                    let content = T::construct(ctx, args)?;
+                    Ok(Value::Content(content.styled_with_map(styles.scoped())))
+                },
+                set: Some(|args| T::set(args, false)),
+                node: Some(NodeId::of::<T>()),
+                info,
+            })),
+            Span::detached(),
+        )
     }
 
     /// Create a new function from a closure.
-    pub(super) fn from_closure(closure: Closure) -> Self {
-        Self(Arc::new(Repr::Closure(closure)))
+    pub(super) fn from_closure(closure: Closure, span: Span) -> Self {
+        Self(Arc::new(Repr::Closure(closure)), span)
     }
 
     /// The name of the function.
@@ -81,6 +87,17 @@ impl Func {
         }
     }
 
+    /// The function's span.
+    pub fn span(&self) -> Span {
+        self.1
+    }
+
+    /// Attach a span to the function.
+    pub fn spanned(mut self, span: Span) -> Self {
+        self.1 = span;
+        self
+    }
+
     /// The number of positional arguments this function takes, if known.
     pub fn argc(&self) -> Option<usize> {
         match self.0.as_ref() {
@@ -121,7 +138,8 @@ impl Func {
 
     /// Apply the given arguments to the function.
     pub fn with(self, args: Args) -> Self {
-        Self(Arc::new(Repr::With(self, args)))
+        let span = self.1;
+        Self(Arc::new(Repr::With(self, args)), span)
     }
 
     /// Create a selector for this function's node type, filtering by node's
diff --git a/src/model/methods.rs b/src/model/methods.rs
index 8155685c6..3823df415 100644
--- a/src/model/methods.rs
+++ b/src/model/methods.rs
@@ -118,7 +118,7 @@ pub fn call(
         },
 
         Value::Args(args) => match method {
-            "positional" => Value::Array(args.to_positional()),
+            "pos" => Value::Array(args.to_pos()),
             "named" => Value::Dict(args.to_named()),
             _ => return missing(),
         },
diff --git a/src/model/realize.rs b/src/model/realize.rs
index 862bc8b6f..39c1fd422 100644
--- a/src/model/realize.rs
+++ b/src/model/realize.rs
@@ -136,7 +136,12 @@ fn try_apply(
 #[capability]
 pub trait Prepare {
     /// Prepare the node for show rule application.
-    fn prepare(&self, vt: &mut Vt, this: Content, styles: StyleChain) -> Content;
+    fn prepare(
+        &self,
+        vt: &mut Vt,
+        this: Content,
+        styles: StyleChain,
+    ) -> SourceResult<Content>;
 }
 
 /// The base recipe for a node.
diff --git a/tests/typ/compiler/for.typ b/tests/typ/compiler/for.typ
index 822f7423c..f63b870e1 100644
--- a/tests/typ/compiler/for.typ
+++ b/tests/typ/compiler/for.typ
@@ -33,7 +33,7 @@
 #for v in (1, 2, 3, 4, 5, 6, 7) [#if v >= 2 and v <= 5 { repr(v) }]
 
 // Map captured arguments.
-#let f1(..args) = args.positional().map(repr)
+#let f1(..args) = args.pos().map(repr)
 #let f2(..args) = args.named().pairs((k, v) => repr(k) + ": " + repr(v))
 #let f(..args) = (f1(..args) + f2(..args)).join(", ")
 #f(1, a: 2)
diff --git a/tests/typ/compiler/return.typ b/tests/typ/compiler/return.typ
index 0eea394e2..779a56ddc 100644
--- a/tests/typ/compiler/return.typ
+++ b/tests/typ/compiler/return.typ
@@ -57,7 +57,7 @@
 // Test that the expression is evaluated to the end.
 #let sum(..args) = {
   let s = 0
-  for v in args.positional() {
+  for v in args.pos() {
     s += v
   }
   s