Set Rules Episode IX: The Rise of Testing

16 files changed, 474 insertions, 260 deletions

diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index ae330134..d05f2ddf 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -34,9 +34,10 @@ use std::path::PathBuf;
 use unicode_segmentation::UnicodeSegmentation;
 
 use crate::diag::{At, Error, StrResult, Trace, Tracepoint, TypResult};
-use crate::geom::{Angle, Fractional, Length, Relative, Spec};
+use crate::geom::{Angle, Fractional, Length, Relative};
 use crate::image::ImageStore;
-use crate::library::{GridNode, TextNode, TrackSizing};
+use crate::layout::RootNode;
+use crate::library::{self, TextNode};
 use crate::loading::Loader;
 use crate::source::{SourceId, SourceStore};
 use crate::syntax::ast::*;
@@ -44,15 +45,9 @@ use crate::syntax::{Span, Spanned};
 use crate::util::{EcoString, RefMutExt};
 use crate::Context;
 
-/// Evaluate a parsed source file into a module.
-pub fn eval(ctx: &mut Context, source: SourceId, markup: &Markup) -> TypResult<Module> {
-    let mut ctx = EvalContext::new(ctx, source);
-    let node = markup.eval(&mut ctx)?;
-    Ok(Module { scope: ctx.scopes.top, node })
-}
-
-/// An evaluated module, ready for importing or instantiation.
-#[derive(Debug, Clone)]
+/// An evaluated module, ready for importing or conversion to a root layout
+/// tree.
+#[derive(Debug, Default, Clone)]
 pub struct Module {
     /// The top-level definitions that were bound in this module.
     pub scope: Scope,
@@ -60,6 +55,22 @@ pub struct Module {
     pub node: Node,
 }
 
+impl Module {
+    /// Convert this module's node into a layout tree.
+    pub fn into_root(self) -> RootNode {
+        self.node.into_root()
+    }
+}
+
+/// Evaluate an expression.
+pub trait Eval {
+    /// The output of evaluating the expression.
+    type Output;
+
+    /// Evaluate the expression to the output value.
+    fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output>;
+}
+
 /// The context for evaluation.
 pub struct EvalContext<'a> {
     /// The loader from which resources (files and images) are loaded.
@@ -124,7 +135,7 @@ impl<'a> EvalContext<'a> {
         self.route.push(id);
 
         // Evaluate the module.
-        let template = ast.eval(self).trace(|| Tracepoint::Import, span)?;
+        let node = ast.eval(self).trace(|| Tracepoint::Import, span)?;
 
         // Restore the old context.
         let new_scopes = mem::replace(&mut self.scopes, prev_scopes);
@@ -132,7 +143,7 @@ impl<'a> EvalContext<'a> {
         self.route.pop().unwrap();
 
         // Save the evaluated module.
-        let module = Module { scope: new_scopes.top, node: template };
+        let module = Module { scope: new_scopes.top, node };
         self.modules.insert(id, module);
 
         Ok(id)
@@ -151,15 +162,6 @@ impl<'a> EvalContext<'a> {
     }
 }
 
-/// Evaluate an expression.
-pub trait Eval {
-    /// The output of evaluating the expression.
-    type Output;
-
-    /// Evaluate the expression to the output value.
-    fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output>;
-}
-
 impl Eval for Markup {
     type Output = Node;
 
@@ -231,13 +233,10 @@ impl Eval for HeadingNode {
     type Output = Node;
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
-        let upscale = (1.6 - 0.1 * self.level() as f64).max(0.75);
-        let mut styles = Styles::new();
-        styles.set(TextNode::STRONG, true);
-        styles.set(TextNode::SIZE, Relative::new(upscale).into());
-        Ok(Node::Block(
-            self.body().eval(ctx)?.into_block().styled(styles),
-        ))
+        Ok(Node::block(library::HeadingNode {
+            child: self.body().eval(ctx)?.into_block(),
+            level: self.level(),
+        }))
     }
 }
 
@@ -245,8 +244,10 @@ impl Eval for ListNode {
     type Output = Node;
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
-        let body = self.body().eval(ctx)?;
-        labelled(ctx, '•'.into(), body)
+        Ok(Node::block(library::ListNode {
+            child: self.body().eval(ctx)?.into_block(),
+            labelling: library::Unordered,
+        }))
     }
 }
 
@@ -254,22 +255,11 @@ impl Eval for EnumNode {
     type Output = Node;
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
-        let body = self.body().eval(ctx)?;
-        let label = format_eco!("{}.", self.number().unwrap_or(1));
-        labelled(ctx, label, body)
-    }
-}
-
-/// Evaluate a labelled list / enum.
-fn labelled(_: &mut EvalContext, label: EcoString, body: Node) -> TypResult<Node> {
-    // Create a grid containing the label, a bit of gutter space and then
-    // the item's body.
-    // TODO: Switch to em units for gutter once available.
-    Ok(Node::block(GridNode {
-        tracks: Spec::new(vec![TrackSizing::Auto; 2], vec![]),
-        gutter: Spec::new(vec![TrackSizing::Linear(Length::pt(5.0).into())], vec![]),
-        children: vec![Node::Text(label).into_block(), body.into_block()],
-    }))
+        Ok(Node::block(library::ListNode {
+            child: self.body().eval(ctx)?.into_block(),
+            labelling: library::Ordered(self.number()),
+        }))
+    }
 }
 
 impl Eval for Expr {
diff --git a/src/eval/node.rs b/src/eval/node.rs
index acdf4ed6..e2b02955 100644
--- a/src/eval/node.rs
+++ b/src/eval/node.rs
@@ -8,17 +8,18 @@ use std::ops::{Add, AddAssign};
 use super::Styles;
 use crate::diag::StrResult;
 use crate::geom::SpecAxis;
-use crate::layout::{Layout, PackedNode};
+use crate::layout::{Layout, PackedNode, RootNode};
 use crate::library::{
-    DocumentNode, FlowChild, FlowNode, PageNode, ParChild, ParNode, PlacedNode,
-    SpacingKind, SpacingNode, TextNode,
+    FlowChild, FlowNode, PageNode, ParChild, ParNode, PlacedNode, SpacingKind,
+    SpacingNode, TextNode,
 };
 use crate::util::EcoString;
 
 /// A partial representation of a layout node.
 ///
 /// A node is a composable intermediate representation that can be converted
-/// into a proper layout node by lifting it to a block-level or document node.
+/// into a proper layout node by lifting it to a [block-level](PackedNode) or
+/// [root node](RootNode).
 #[derive(Debug, PartialEq, Clone, Hash)]
 pub enum Node {
     /// A word space.
@@ -90,19 +91,19 @@ impl Node {
         }
     }
 
-    /// Lift to a document node, the root of the layout tree.
-    pub fn into_document(self) -> DocumentNode {
+    /// Lift to a root layout tree node.
+    pub fn into_root(self) -> RootNode {
         let mut packer = Packer::new(true);
         packer.walk(self, Styles::new());
-        packer.into_document()
+        packer.into_root()
     }
 
-    /// Repeat this template `n` times.
+    /// Repeat this node `n` times.
     pub fn repeat(&self, n: i64) -> StrResult<Self> {
         let count = usize::try_from(n)
             .map_err(|_| format!("cannot repeat this template {} times", n))?;
 
-        // TODO(set): Make more efficient.
+        // TODO(style): Make more efficient.
         Ok(Self::Sequence(vec![(self.clone(), Styles::new()); count]))
     }
 }
@@ -117,7 +118,7 @@ impl Add for Node {
     type Output = Self;
 
     fn add(self, rhs: Self) -> Self::Output {
-        // TODO(set): Make more efficient.
+        // TODO(style): Make more efficient.
         Self::Sequence(vec![(self, Styles::new()), (rhs, Styles::new())])
     }
 }
@@ -134,9 +135,9 @@ impl Sum for Node {
     }
 }
 
-/// Packs a [`Node`] into a flow or whole document.
+/// Packs a [`Node`] into a flow or root node.
 struct Packer {
-    /// Whether this packer produces the top-level document.
+    /// Whether this packer produces a root node.
     top: bool,
     /// The accumulated page nodes.
     pages: Vec<PageNode>,
@@ -163,10 +164,10 @@ impl Packer {
         FlowNode(self.flow.children).pack()
     }
 
-    /// Finish up and return the resulting document.
-    fn into_document(mut self) -> DocumentNode {
+    /// Finish up and return the resulting root node.
+    fn into_root(mut self) -> RootNode {
         self.pagebreak();
-        DocumentNode(self.pages)
+        RootNode(self.pages)
     }
 
     /// Consider a node with the given styles.
diff --git a/src/eval/styles.rs b/src/eval/styles.rs
index 2396646f..5304e0ad 100644
--- a/src/eval/styles.rs
+++ b/src/eval/styles.rs
@@ -20,6 +20,11 @@ impl Styles {
         Self { map: vec![] }
     }
 
+    /// Whether this map contains no styles.
+    pub fn is_empty(&self) -> bool {
+        self.map.is_empty()
+    }
+
     /// Create a style map with a single property-value pair.
     pub fn one<P: Property>(key: P, value: P::Value) -> Self {
         let mut styles = Self::new();
@@ -27,11 +32,6 @@ impl Styles {
         styles
     }
 
-    /// Whether this map contains no styles.
-    pub fn is_empty(&self) -> bool {
-        self.map.is_empty()
-    }
-
     /// Set the value for a style property.
     pub fn set<P: Property>(&mut self, key: P, value: P::Value) {
         let id = StyleId::of::<P>();
@@ -47,6 +47,13 @@ impl Styles {
         self.map.push((id, Entry::new(key, value)));
     }
 
+    /// Set a value for a style property if it is `Some(_)`.
+    pub fn set_opt<P: Property>(&mut self, key: P, value: Option<P::Value>) {
+        if let Some(value) = value {
+            self.set(key, value);
+        }
+    }
+
     /// Toggle a boolean style property.
     pub fn toggle<P: Property<Value = bool>>(&mut self, key: P) {
         let id = StyleId::of::<P>();
@@ -82,13 +89,22 @@ impl Styles {
     }
 
     /// Get a reference to a style directly in this map (no default value).
-    pub fn get_direct<P: Property>(&self, _: P) -> Option<&P::Value> {
+    fn get_direct<P: Property>(&self, _: P) -> Option<&P::Value> {
         self.map
             .iter()
             .find(|pair| pair.0 == StyleId::of::<P>())
             .and_then(|pair| pair.1.downcast())
     }
 
+    /// Create new styles combining `self` with `outer`.
+    ///
+    /// Properties from `self` take precedence over the ones from `outer`.
+    pub fn chain(&self, outer: &Self) -> Self {
+        let mut styles = self.clone();
+        styles.apply(outer);
+        styles
+    }
+
     /// Apply styles from `outer` in-place.
     ///
     /// Properties from `self` take precedence over the ones from `outer`.
@@ -105,13 +121,9 @@ impl Styles {
         }
     }
 
-    /// Create new styles combining `self` with `outer`.
-    ///
-    /// Properties from `self` take precedence over the ones from `outer`.
-    pub fn chain(&self, outer: &Self) -> Self {
-        let mut styles = self.clone();
-        styles.apply(outer);
-        styles
+    /// Keep only those styles that are not also in `other`.
+    pub fn erase(&mut self, other: &Self) {
+        self.map.retain(|a| other.map.iter().all(|b| a != b));
     }
 
     /// Keep only those styles that are also in `other`.
@@ -119,18 +131,13 @@ impl Styles {
         self.map.retain(|a| other.map.iter().any(|b| a == b));
     }
 
-    /// Keep only those styles that are not also in `other`.
-    pub fn erase(&mut self, other: &Self) {
-        self.map.retain(|a| other.map.iter().all(|b| a != b));
-    }
-
     /// Whether two style maps are equal when filtered down to the given
     /// properties.
     pub fn compatible<F>(&self, other: &Self, filter: F) -> bool
     where
         F: Fn(StyleId) -> bool,
     {
-        // TODO(set): Filtered length + one direction equal should suffice.
+        // TODO(style): Filtered length + one direction equal should suffice.
         let f = |e: &&(StyleId, Entry)| filter(e.0);
         self.map.iter().filter(f).all(|pair| other.map.contains(pair))
             && other.map.iter().filter(f).all(|pair| self.map.contains(pair))
@@ -177,7 +184,7 @@ impl Entry {
 
 impl Debug for Entry {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        self.0.fmt(f)
+        self.0.dyn_fmt(f)
     }
 }
 
@@ -195,22 +202,23 @@ impl Hash for Entry {
 
 trait Bounds: 'static {
     fn as_any(&self) -> &dyn Any;
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result;
+    fn dyn_fmt(&self, f: &mut Formatter) -> fmt::Result;
     fn dyn_eq(&self, other: &Entry) -> bool;
     fn hash64(&self) -> u64;
     fn combine(&self, outer: &Entry) -> Entry;
 }
 
-impl<P> Bounds for (P, P::Value)
-where
-    P: Property,
-    P::Value: Debug + Hash + PartialEq + 'static,
-{
+// `P` is always zero-sized. We only implement the trait for a pair of key and
+// associated value so that `P` is a constrained type parameter that we can use
+// in `dyn_fmt` to access the property's name. This way, we can effectively
+// store the property's name in its vtable instead of having an actual runtime
+// string somewhere in `Entry`.
+impl<P: Property> Bounds for (P, P::Value) {
     fn as_any(&self) -> &dyn Any {
         &self.1
     }
 
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    fn dyn_fmt(&self, f: &mut Formatter) -> fmt::Result {
         if f.alternate() {
             write!(f, "#[{} = {:?}]", P::NAME, self.1)
         } else {
@@ -242,11 +250,12 @@ where
 /// Style property keys.
 ///
 /// This trait is not intended to be implemented manually, but rather through
-/// the `properties!` macro.
+/// the `#[properties]` proc-macro.
 pub trait Property: Copy + 'static {
-    /// The type of this property, for example, this could be
-    /// [`Length`](crate::geom::Length) for a `WIDTH` property.
-    type Value: Debug + Clone + Hash + PartialEq + '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 + 'static;
 
     /// The name of the property, used for debug printing.
     const NAME: &'static str;
@@ -257,12 +266,16 @@ pub trait Property: Copy + 'static {
     /// A static reference to the default value of the property.
     ///
     /// This is automatically implemented through lazy-initialization in the
-    /// `properties!` macro. This way, expensive defaults don't need to be
+    /// `#[properties]` macro. This way, expensive defaults don't need to be
     /// recreated all the time.
     fn default_ref() -> &'static Self::Value;
 
-    /// Combine the property with an outer value.
-    fn combine(inner: Self::Value, _: Self::Value) -> Self::Value {
+    /// Fold the property with an outer value.
+    ///
+    /// For example, this would combine a relative font size with an outer
+    /// absolute font size.
+    #[allow(unused_variables)]
+    fn combine(inner: Self::Value, outer: Self::Value) -> Self::Value {
         inner
     }
 }
@@ -277,12 +290,3 @@ impl StyleId {
         Self(TypeId::of::<P>())
     }
 }
-
-/// Set a style property to a value if the value is `Some`.
-macro_rules! set {
-    ($styles:expr, $target:expr => $value:expr) => {
-        if let Some(v) = $value {
-            $styles.set($target, v);
-        }
-    };
-}
diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index cf714f88..bc28e893 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -20,13 +20,52 @@ use crate::font::FontStore;
 use crate::frame::Frame;
 use crate::geom::{Align, Linear, Point, Sides, Size, Spec, Transform};
 use crate::image::ImageStore;
-use crate::library::{AlignNode, DocumentNode, PadNode, SizedNode, TransformNode};
+use crate::library::{AlignNode, PadNode, PageNode, SizedNode, TransformNode};
 use crate::Context;
 
-/// Layout a document node into a collection of frames.
-pub fn layout(ctx: &mut Context, node: &DocumentNode) -> Vec<Rc<Frame>> {
-    let mut ctx = LayoutContext::new(ctx);
-    node.layout(&mut ctx)
+/// The root layout node, a document consisting of top-level page runs.
+#[derive(Hash)]
+pub struct RootNode(pub Vec<PageNode>);
+
+impl RootNode {
+    /// Layout the document into a sequence of frames, one per page.
+    pub fn layout(&self, ctx: &mut Context) -> Vec<Rc<Frame>> {
+        let mut ctx = LayoutContext::new(ctx);
+        self.0.iter().flat_map(|node| node.layout(&mut ctx)).collect()
+    }
+}
+
+impl Debug for RootNode {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.write_str("Root ")?;
+        f.debug_list().entries(&self.0).finish()
+    }
+}
+
+/// A node that can be layouted into a sequence of regions.
+///
+/// Layout return one frame per used region alongside constraints that define
+/// whether the result is reusable in other regions.
+pub trait Layout {
+    /// Layout the node into the given regions, producing constrained frames.
+    fn layout(
+        &self,
+        ctx: &mut LayoutContext,
+        regions: &Regions,
+    ) -> Vec<Constrained<Rc<Frame>>>;
+
+    /// Convert to a packed node.
+    fn pack(self) -> PackedNode
+    where
+        Self: Debug + Hash + Sized + 'static,
+    {
+        PackedNode {
+            #[cfg(feature = "layout-cache")]
+            hash: self.hash64(),
+            node: Rc::new(self),
+            styles: Styles::new(),
+        }
+    }
 }
 
 /// The context for layouting.
@@ -39,6 +78,7 @@ pub struct LayoutContext<'a> {
     #[cfg(feature = "layout-cache")]
     pub layouts: &'a mut LayoutCache,
     /// The inherited style properties.
+    // TODO(style): This probably shouldn't be here.
     pub styles: Styles,
     /// How deeply nested the current layout tree position is.
     #[cfg(feature = "layout-cache")]
@@ -60,29 +100,22 @@ impl<'a> LayoutContext<'a> {
     }
 }
 
-/// A node that can be layouted into a sequence of regions.
+/// A layout node that produces an empty frame.
 ///
-/// Layout return one frame per used region alongside constraints that define
-/// whether the result is reusable in other regions.
-pub trait Layout {
-    /// Layout the node into the given regions, producing constrained frames.
+/// The packed version of this is returned by [`PackedNode::default`].
+#[derive(Debug, Hash)]
+pub struct EmptyNode;
+
+impl Layout for EmptyNode {
     fn layout(
         &self,
-        ctx: &mut LayoutContext,
+        _: &mut LayoutContext,
         regions: &Regions,
-    ) -> Vec<Constrained<Rc<Frame>>>;
-
-    /// Convert to a packed node.
-    fn pack(self) -> PackedNode
-    where
-        Self: Debug + Hash + Sized + 'static,
-    {
-        PackedNode {
-            #[cfg(feature = "layout-cache")]
-            hash: self.hash64(),
-            node: Rc::new(self),
-            styles: Styles::new(),
-        }
+    ) -> Vec<Constrained<Rc<Frame>>> {
+        let size = regions.expand.select(regions.current, Size::zero());
+        let mut cts = Constraints::new(regions.expand);
+        cts.exact = regions.current.filter(regions.expand);
+        vec![Frame::new(size).constrain(cts)]
     }
 }
 
@@ -288,22 +321,3 @@ where
         state.finish()
     }
 }
-
-/// A layout node that produces an empty frame.
-///
-/// The packed version of this is returned by [`PackedNode::default`].
-#[derive(Debug, Hash)]
-pub struct EmptyNode;
-
-impl Layout for EmptyNode {
-    fn layout(
-        &self,
-        _: &mut LayoutContext,
-        regions: &Regions,
-    ) -> Vec<Constrained<Rc<Frame>>> {
-        let size = regions.expand.select(regions.current, Size::zero());
-        let mut cts = Constraints::new(regions.expand);
-        cts.exact = regions.current.filter(regions.expand);
-        vec![Frame::new(size).constrain(cts)]
-    }
-}
diff --git a/src/lib.rs b/src/lib.rs
index 62452655..79b1a8f2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,30 +2,32 @@
 //!
 //! # Steps
 //! - **Parsing:** The parsing step first transforms a plain string into an
-//!   [iterator of tokens][tokens]. This token stream is [parsed] into [markup].
-//!   The syntactical structures describing markup and embedded code can be
-//!   found in the [syntax] module.
+//!   [iterator of tokens][tokens]. This token stream is [parsed] into a
+//!   [green tree]. The green tree itself is untyped, but a typed layer over it
+//!   is provided in the [AST] module.
 //! - **Evaluation:** The next step is to [evaluate] the markup. This produces a
 //!   [module], consisting of a scope of values that were exported by the code
-//!   and a template with the contents of the module. This template can be
-//!   instantiated with a style to produce a layout tree, a high-level, fully
-//!   styled representation, rooted in the [document node]. The nodes of this
-//!   tree are self-contained and order-independent and thus much better suited
-//!   for layouting than the raw markup.
+//!   and a [node] with the contents of the module. This node can be converted
+//!   into a [layout tree], a hierarchical, styled representation of the
+//!   document. The nodes of this tree are well structured and order-independent
+//!   and thus much better suited for layouting than the raw markup.
 //! - **Layouting:** Next, the tree is [layouted] into a portable version of the
 //!   typeset document. The output of this is a collection of [`Frame`]s (one
-//!   per page), ready for exporting.
+//!   per page), ready for exporting. This step is supported by an incremental
+//!   [cache] that enables reuse of intermediate layouting results.
 //! - **Exporting:** The finished layout can be exported into a supported
 //!   format. Currently, the only supported output format is [PDF].
 //!
 //! [tokens]: parse::Tokens
 //! [parsed]: parse::parse
-//! [markup]: syntax::ast::Markup
-//! [evaluate]: eval::eval
+//! [green tree]: syntax::GreenNode
+//! [AST]: syntax::ast
+//! [evaluate]: Context::evaluate
 //! [module]: eval::Module
-//! [layout tree]: layout::LayoutTree
-//! [document node]: library::DocumentNode
-//! [layouted]: layout::layout
+//! [node]: eval::Node
+//! [layout tree]: layout::RootNode
+//! [layouted]: layout::RootNode::layout
+//! [cache]: layout::LayoutCache
 //! [PDF]: export::pdf
 
 #[macro_use]
@@ -49,13 +51,12 @@ pub mod syntax;
 use std::rc::Rc;
 
 use crate::diag::TypResult;
-use crate::eval::{Module, Scope, Styles};
+use crate::eval::{Eval, EvalContext, Module, Scope, Styles};
 use crate::font::FontStore;
 use crate::frame::Frame;
 use crate::image::ImageStore;
 #[cfg(feature = "layout-cache")]
 use crate::layout::{EvictionPolicy, LayoutCache};
-use crate::library::DocumentNode;
 use crate::loading::Loader;
 use crate::source::{SourceId, SourceStore};
 
@@ -100,15 +101,15 @@ impl Context {
     }
 
     /// Evaluate a source file and return the resulting module.
+    ///
+    /// Returns either a module containing a scope with top-level bindings and a
+    /// layoutable node or diagnostics in the form of a vector of error message
+    /// with file and span information.
     pub fn evaluate(&mut self, id: SourceId) -> TypResult<Module> {
-        let ast = self.sources.get(id).ast()?;
-        eval::eval(self, id, &ast)
-    }
-
-    /// Execute a source file and produce the resulting page nodes.
-    pub fn execute(&mut self, id: SourceId) -> TypResult<DocumentNode> {
-        let module = self.evaluate(id)?;
-        Ok(module.node.into_document())
+        let markup = self.sources.get(id).ast()?;
+        let mut ctx = EvalContext::new(self, id);
+        let node = markup.eval(&mut ctx)?;
+        Ok(Module { scope: ctx.scopes.top, node })
     }
 
     /// Typeset a source file into a collection of layouted frames.
@@ -117,8 +118,9 @@ impl Context {
     /// diagnostics in the form of a vector of error message with file and span
     /// information.
     pub fn typeset(&mut self, id: SourceId) -> TypResult<Vec<Rc<Frame>>> {
-        let tree = self.execute(id)?;
-        let frames = layout::layout(self, &tree);
+        let module = self.evaluate(id)?;
+        let tree = module.into_root();
+        let frames = tree.layout(self);
         Ok(frames)
     }
 
diff --git a/src/library/document.rs b/src/library/document.rs
deleted file mode 100644
index 84673270..00000000
--- a/src/library/document.rs
+++ /dev/null
@@ -1,20 +0,0 @@
-use super::prelude::*;
-use super::PageNode;
-
-/// The root layout node, a document consisting of top-level page runs.
-#[derive(Hash)]
-pub struct DocumentNode(pub Vec<PageNode>);
-
-impl DocumentNode {
-    /// Layout the document into a sequence of frames, one per page.
-    pub fn layout(&self, ctx: &mut LayoutContext) -> Vec<Rc<Frame>> {
-        self.0.iter().flat_map(|node| node.layout(ctx)).collect()
-    }
-}
-
-impl Debug for DocumentNode {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.write_str("Document ")?;
-        f.debug_list().entries(&self.0).finish()
-    }
-}
diff --git a/src/library/grid.rs b/src/library/grid.rs
index e82f09ef..d341cf5b 100644
--- a/src/library/grid.rs
+++ b/src/library/grid.rs
@@ -10,7 +10,7 @@ pub fn grid(_: &mut EvalContext, args: &mut Args) -> TypResult<Value> {
         Value::Relative(v) => vec![TrackSizing::Linear(v.into())],
         Value::Linear(v) => vec![TrackSizing::Linear(v)],
         Value::Fractional(v) => vec![TrackSizing::Fractional(v)],
-        Value::Int(count) => vec![TrackSizing::Auto; count.max(0) as usize],
+        Value::Int(v) => vec![TrackSizing::Auto; Value::Int(v).cast()?],
         Value::Array(values) => values
             .into_iter()
             .filter_map(|v| v.cast().ok())
diff --git a/src/library/heading.rs b/src/library/heading.rs
new file mode 100644
index 00000000..c9777577
--- /dev/null
+++ b/src/library/heading.rs
@@ -0,0 +1,63 @@
+use super::prelude::*;
+use super::{FontFamily, TextNode};
+
+/// A section heading.
+#[derive(Debug, Hash)]
+pub struct HeadingNode {
+    /// The node that produces the heading's contents.
+    pub child: PackedNode,
+    /// The logical nesting depth of the section, starting from one. In the
+    /// default style, this controls the text size of the heading.
+    pub level: usize,
+}
+
+#[properties]
+impl HeadingNode {
+    /// The heading's font family.
+    pub const FAMILY: Smart<String> = Smart::Auto;
+    /// The fill color of heading in the text. Just the surrounding text color
+    /// if `auto`.
+    pub const FILL: Smart<Paint> = Smart::Auto;
+}
+
+impl Construct for HeadingNode {
+    fn construct(_: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
+        Ok(Node::block(Self {
+            child: args.expect::<Node>("body")?.into_block(),
+            level: args.named("level")?.unwrap_or(1),
+        }))
+    }
+}
+
+impl Set for HeadingNode {
+    fn set(styles: &mut Styles, args: &mut Args) -> TypResult<()> {
+        styles.set_opt(Self::FAMILY, args.named("family")?);
+        styles.set_opt(Self::FILL, args.named("fill")?);
+        Ok(())
+    }
+}
+
+impl Layout for HeadingNode {
+    fn layout(
+        &self,
+        ctx: &mut LayoutContext,
+        regions: &Regions,
+    ) -> Vec<Constrained<Rc<Frame>>> {
+        let upscale = (1.6 - 0.1 * self.level as f64).max(0.75);
+        ctx.styles.set(TextNode::STRONG, true);
+        ctx.styles.set(TextNode::SIZE, Relative::new(upscale).into());
+
+        if let Smart::Custom(family) = ctx.styles.get_ref(Self::FAMILY) {
+            let list: Vec<_> = std::iter::once(FontFamily::named(family))
+                .chain(ctx.styles.get_ref(TextNode::FAMILY_LIST).iter().cloned())
+                .collect();
+            ctx.styles.set(TextNode::FAMILY_LIST, list);
+        }
+
+        if let Smart::Custom(fill) = ctx.styles.get(Self::FILL) {
+            ctx.styles.set(TextNode::FILL, fill);
+        }
+
+        self.child.layout(ctx, regions)
+    }
+}
diff --git a/src/library/list.rs b/src/library/list.rs
new file mode 100644
index 00000000..74f0abe8
--- /dev/null
+++ b/src/library/list.rs
@@ -0,0 +1,102 @@
+use std::hash::Hash;
+
+use super::prelude::*;
+use super::{GridNode, TextNode, TrackSizing};
+
+/// An unordered or ordered list.
+#[derive(Debug, Hash)]
+pub struct ListNode<L> {
+    /// The node that produces the item's body.
+    pub child: PackedNode,
+    /// The list labelling style -- unordered or ordered.
+    pub labelling: L,
+}
+
+#[properties]
+impl<L: Labelling> ListNode<L> {
+    /// The indentation of each item's label.
+    pub const LABEL_INDENT: Linear = Relative::new(0.0).into();
+    /// The space between the label and the body of each item.
+    pub const BODY_INDENT: Linear = Relative::new(0.5).into();
+}
+
+impl<L: Labelling> Construct for ListNode<L> {
+    fn construct(_: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
+        Ok(args
+            .all()
+            .map(|node: Node| {
+                Node::block(Self {
+                    child: node.into_block(),
+                    labelling: L::default(),
+                })
+            })
+            .sum())
+    }
+}
+
+impl<L: Labelling> Set for ListNode<L> {
+    fn set(styles: &mut Styles, args: &mut Args) -> TypResult<()> {
+        styles.set_opt(Self::LABEL_INDENT, args.named("label-indent")?);
+        styles.set_opt(Self::BODY_INDENT, args.named("body-indent")?);
+        Ok(())
+    }
+}
+
+impl<L: Labelling> Layout for ListNode<L> {
+    fn layout(
+        &self,
+        ctx: &mut LayoutContext,
+        regions: &Regions,
+    ) -> Vec<Constrained<Rc<Frame>>> {
+        let em = ctx.styles.get(TextNode::SIZE).abs;
+        let label_indent = ctx.styles.get(Self::LABEL_INDENT).resolve(em);
+        let body_indent = ctx.styles.get(Self::BODY_INDENT).resolve(em);
+
+        let columns = vec![
+            TrackSizing::Linear(label_indent.into()),
+            TrackSizing::Auto,
+            TrackSizing::Linear(body_indent.into()),
+            TrackSizing::Auto,
+        ];
+
+        let children = vec![
+            PackedNode::default(),
+            Node::Text(self.labelling.label()).into_block(),
+            PackedNode::default(),
+            self.child.clone(),
+        ];
+
+        GridNode {
+            tracks: Spec::new(columns, vec![]),
+            gutter: Spec::default(),
+            children,
+        }
+        .layout(ctx, regions)
+    }
+}
+
+/// How to label a list.
+pub trait Labelling: Debug + Default + Hash + 'static {
+    /// Return the item's label.
+    fn label(&self) -> EcoString;
+}
+
+/// Unordered list labelling style.
+#[derive(Debug, Default, Hash)]
+pub struct Unordered;
+
+impl Labelling for Unordered {
+    fn label(&self) -> EcoString {
+        '•'.into()
+    }
+}
+
+/// Ordered list labelling style.
+#[derive(Debug, Default, Hash)]
+pub struct Ordered(pub Option<usize>);
+
+impl Labelling for Ordered {
+    fn label(&self) -> EcoString {
+        format_eco!("{}.", self.0.unwrap_or(1))
+    }
+}
diff --git a/src/library/mod.rs b/src/library/mod.rs
index 5852f2bb..b2dd0dbe 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -4,11 +4,12 @@
 //! definitions.
 
 mod align;
-mod document;
 mod flow;
 mod grid;
+mod heading;
 mod image;
 mod link;
+mod list;
 mod pad;
 mod page;
 mod par;
@@ -41,10 +42,11 @@ mod prelude {
 
 pub use self::image::*;
 pub use align::*;
-pub use document::*;
 pub use flow::*;
 pub use grid::*;
+pub use heading::*;
 pub use link::*;
+pub use list::*;
 pub use pad::*;
 pub use page::*;
 pub use par::*;
@@ -68,21 +70,29 @@ pub fn new() -> Scope {
     std.def_class::<PageNode>("page");
     std.def_class::<ParNode>("par");
     std.def_class::<TextNode>("text");
+    std.def_class::<HeadingNode>("heading");
+    std.def_class::<ListNode<Unordered>>("list");
+    std.def_class::<ListNode<Ordered>>("enum");
 
     // Text functions.
+    // TODO(style): These should be classes, once that works for inline nodes.
     std.def_func("strike", strike);
     std.def_func("underline", underline);
     std.def_func("overline", overline);
     std.def_func("link", link);
 
-    // Layout functions.
+    // Break and spacing functions.
+    std.def_func("pagebreak", pagebreak);
+    std.def_func("parbreak", parbreak);
+    std.def_func("linebreak", linebreak);
     std.def_func("h", h);
     std.def_func("v", v);
+
+    // Layout functions.
+    // TODO(style): Decide which of these should be classes
+    // (and which of their properties should be settable).
     std.def_func("box", box_);
     std.def_func("block", block);
-    std.def_func("pagebreak", pagebreak);
-    std.def_func("parbreak", parbreak);
-    std.def_func("linebreak", linebreak);
     std.def_func("stack", stack);
     std.def_func("grid", grid);
     std.def_func("pad", pad);
@@ -91,8 +101,6 @@ pub fn new() -> Scope {
     std.def_func("move", move_);
     std.def_func("scale", scale);
     std.def_func("rotate", rotate);
-
-    // Element functions.
     std.def_func("image", image);
     std.def_func("rect", rect);
     std.def_func("square", square);
@@ -118,6 +126,7 @@ pub fn new() -> Scope {
     std.def_func("sorted", sorted);
 
     // Predefined colors.
+    // TODO: More colors.
     std.def_const("white", RgbaColor::WHITE);
     std.def_const("black", RgbaColor::BLACK);
     std.def_const("eastern", RgbaColor::new(0x23, 0x9D, 0xAD, 0xFF));
@@ -151,3 +160,15 @@ castable! {
     Expected: "color",
     Value::Color(color) => Paint::Solid(color),
 }
+
+castable! {
+    usize,
+    Expected: "non-negative integer",
+    Value::Int(int) => int.try_into().map_err(|_| "must be at least zero")?,
+}
+
+castable! {
+    String,
+    Expected: "string",
+    Value::Str(string) => string.into(),
+}
diff --git a/src/library/page.rs b/src/library/page.rs
index 3bb5cbd3..7fbcd058 100644
--- a/src/library/page.rs
+++ b/src/library/page.rs
@@ -44,8 +44,6 @@ impl PageNode {
 
 impl Construct for PageNode {
     fn construct(_: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
-        // TODO(set): Make sure it's really a page so that it doesn't merge
-        // with adjacent pages.
         Ok(Node::Page(args.expect::<Node>("body")?.into_block()))
     }
 }
@@ -69,13 +67,12 @@ impl Set for PageNode {
         }
 
         let margins = args.named("margins")?;
-
-        set!(styles, Self::FLIPPED => args.named("flipped")?);
-        set!(styles, Self::LEFT => args.named("left")?.or(margins));
-        set!(styles, Self::TOP => args.named("top")?.or(margins));
-        set!(styles, Self::RIGHT => args.named("right")?.or(margins));
-        set!(styles, Self::BOTTOM => args.named("bottom")?.or(margins));
-        set!(styles, Self::FILL => args.named("fill")?);
+        styles.set_opt(Self::FLIPPED, args.named("flipped")?);
+        styles.set_opt(Self::LEFT, args.named("left")?.or(margins));
+        styles.set_opt(Self::TOP, args.named("top")?.or(margins));
+        styles.set_opt(Self::RIGHT, args.named("right")?.or(margins));
+        styles.set_opt(Self::BOTTOM, args.named("bottom")?.or(margins));
+        styles.set_opt(Self::FILL, args.named("fill")?);
 
         Ok(())
     }
diff --git a/src/library/par.rs b/src/library/par.rs
index 9a70b2c7..5dffd1c0 100644
--- a/src/library/par.rs
+++ b/src/library/par.rs
@@ -74,10 +74,10 @@ impl Set for ParNode {
             align = Some(if dir == Dir::LTR { Align::Left } else { Align::Right });
         }
 
-        set!(styles, Self::DIR => dir);
-        set!(styles, Self::ALIGN => align);
-        set!(styles, Self::LEADING => leading);
-        set!(styles, Self::SPACING => spacing);
+        styles.set_opt(Self::DIR, dir);
+        styles.set_opt(Self::ALIGN, align);
+        styles.set_opt(Self::LEADING, leading);
+        styles.set_opt(Self::SPACING, spacing);
 
         Ok(())
     }
@@ -93,8 +93,7 @@ impl Layout for ParNode {
         let text = self.collect_text();
 
         // Find out the BiDi embedding levels.
-        let default_level = Level::from_dir(ctx.styles.get(Self::DIR));
-        let bidi = BidiInfo::new(&text, default_level);
+        let bidi = BidiInfo::new(&text, Level::from_dir(ctx.styles.get(Self::DIR)));
 
         // Prepare paragraph layout by building a representation on which we can
         // do line breaking without layouting each and every line from scratch.
diff --git a/src/library/text.rs b/src/library/text.rs
index e0cbb1ad..4ff9b5cd 100644
--- a/src/library/text.rs
+++ b/src/library/text.rs
@@ -56,11 +56,11 @@ impl TextNode {
     /// A prioritized sequence of font families.
     pub const FAMILY_LIST: Vec<FontFamily> = vec![FontFamily::SansSerif];
     /// The serif font family/families.
-    pub const SERIF_LIST: Vec<String> = vec!["ibm plex serif".into()];
+    pub const SERIF_LIST: Vec<NamedFamily> = vec![NamedFamily::new("IBM Plex Serif")];
     /// The sans-serif font family/families.
-    pub const SANS_SERIF_LIST: Vec<String> = vec!["ibm plex sans".into()];
+    pub const SANS_SERIF_LIST: Vec<NamedFamily> = vec![NamedFamily::new("IBM Plex Sans")];
     /// The monospace font family/families.
-    pub const MONOSPACE_LIST: Vec<String> = vec!["ibm plex mono".into()];
+    pub const MONOSPACE_LIST: Vec<NamedFamily> = vec![NamedFamily::new("IBM Plex Mono")];
     /// Whether to allow font fallback when the primary font list contains no
     /// match.
     pub const FALLBACK: bool = true;
@@ -139,32 +139,38 @@ impl Set for TextNode {
             (!families.is_empty()).then(|| families)
         });
 
-        set!(styles, Self::FAMILY_LIST => list);
-        set!(styles, Self::SERIF_LIST => args.named("serif")?);
-        set!(styles, Self::SANS_SERIF_LIST => args.named("sans-serif")?);
-        set!(styles, Self::MONOSPACE_LIST => args.named("monospace")?);
-        set!(styles, Self::FALLBACK => args.named("fallback")?);
-        set!(styles, Self::STYLE => args.named("style")?);
-        set!(styles, Self::WEIGHT => args.named("weight")?);
-        set!(styles, Self::STRETCH => args.named("stretch")?);
-        set!(styles, Self::FILL => args.named("fill")?.or_else(|| args.find()));
-        set!(styles, Self::SIZE => args.named("size")?.or_else(|| args.find()));
-        set!(styles, Self::TRACKING => args.named("tracking")?.map(Em::new));
-        set!(styles, Self::TOP_EDGE => args.named("top-edge")?);
-        set!(styles, Self::BOTTOM_EDGE => args.named("bottom-edge")?);
-        set!(styles, Self::KERNING => args.named("kerning")?);
-        set!(styles, Self::SMALLCAPS => args.named("smallcaps")?);
-        set!(styles, Self::ALTERNATES => args.named("alternates")?);
-        set!(styles, Self::STYLISTIC_SET => args.named("stylistic-set")?);
-        set!(styles, Self::LIGATURES => args.named("ligatures")?);
-        set!(styles, Self::DISCRETIONARY_LIGATURES => args.named("discretionary-ligatures")?);
-        set!(styles, Self::HISTORICAL_LIGATURES => args.named("historical-ligatures")?);
-        set!(styles, Self::NUMBER_TYPE => args.named("number-type")?);
-        set!(styles, Self::NUMBER_WIDTH => args.named("number-width")?);
-        set!(styles, Self::NUMBER_POSITION => args.named("number-position")?);
-        set!(styles, Self::SLASHED_ZERO => args.named("slashed-zero")?);
-        set!(styles, Self::FRACTIONS => args.named("fractions")?);
-        set!(styles, Self::FEATURES => args.named("features")?);
+        styles.set_opt(Self::FAMILY_LIST, list);
+        styles.set_opt(Self::SERIF_LIST, args.named("serif")?);
+        styles.set_opt(Self::SANS_SERIF_LIST, args.named("sans-serif")?);
+        styles.set_opt(Self::MONOSPACE_LIST, args.named("monospace")?);
+        styles.set_opt(Self::FALLBACK, args.named("fallback")?);
+        styles.set_opt(Self::STYLE, args.named("style")?);
+        styles.set_opt(Self::WEIGHT, args.named("weight")?);
+        styles.set_opt(Self::STRETCH, args.named("stretch")?);
+        styles.set_opt(Self::FILL, args.named("fill")?.or_else(|| args.find()));
+        styles.set_opt(Self::SIZE, args.named("size")?.or_else(|| args.find()));
+        styles.set_opt(Self::TRACKING, args.named("tracking")?.map(Em::new));
+        styles.set_opt(Self::TOP_EDGE, args.named("top-edge")?);
+        styles.set_opt(Self::BOTTOM_EDGE, args.named("bottom-edge")?);
+        styles.set_opt(Self::KERNING, args.named("kerning")?);
+        styles.set_opt(Self::SMALLCAPS, args.named("smallcaps")?);
+        styles.set_opt(Self::ALTERNATES, args.named("alternates")?);
+        styles.set_opt(Self::STYLISTIC_SET, args.named("stylistic-set")?);
+        styles.set_opt(Self::LIGATURES, args.named("ligatures")?);
+        styles.set_opt(
+            Self::DISCRETIONARY_LIGATURES,
+            args.named("discretionary-ligatures")?,
+        );
+        styles.set_opt(
+            Self::HISTORICAL_LIGATURES,
+            args.named("historical-ligatures")?,
+        );
+        styles.set_opt(Self::NUMBER_TYPE, args.named("number-type")?);
+        styles.set_opt(Self::NUMBER_WIDTH, args.named("number-width")?);
+        styles.set_opt(Self::NUMBER_POSITION, args.named("number-position")?);
+        styles.set_opt(Self::SLASHED_ZERO, args.named("slashed-zero")?);
+        styles.set_opt(Self::FRACTIONS, args.named("fractions")?);
+        styles.set_opt(Self::FEATURES, args.named("features")?);
 
         Ok(())
     }
@@ -188,8 +194,15 @@ pub enum FontFamily {
     SansSerif,
     /// A family in which (almost) all glyphs are of equal width.
     Monospace,
-    /// A specific family with a name.
-    Named(String),
+    /// A specific font family like "Arial".
+    Named(NamedFamily),
+}
+
+impl FontFamily {
+    /// Create a named font family variant, directly from a string.
+    pub fn named(string: &str) -> Self {
+        Self::Named(NamedFamily::new(string))
+    }
 }
 
 impl Debug for FontFamily {
@@ -203,15 +216,37 @@ impl Debug for FontFamily {
     }
 }
 
+/// A specific font family like "Arial".
+#[derive(Clone, Eq, PartialEq, Hash)]
+pub struct NamedFamily(String);
+
+impl NamedFamily {
+    /// Create a named font family variant.
+    pub fn new(string: &str) -> Self {
+        Self(string.to_lowercase())
+    }
+
+    /// The lowercased family name.
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl Debug for NamedFamily {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 dynamic! {
     FontFamily: "font family",
-    Value::Str(string) => Self::Named(string.to_lowercase().into()),
+    Value::Str(string) => Self::named(&string),
 }
 
 castable! {
     Vec<FontFamily>,
     Expected: "string, generic family or array thereof",
-    Value::Str(string) => vec![FontFamily::Named(string.to_lowercase().into())],
+    Value::Str(string) => vec![FontFamily::named(&string)],
     Value::Array(values) => {
         values.into_iter().filter_map(|v| v.cast().ok()).collect()
     },
@@ -219,13 +254,13 @@ castable! {
 }
 
 castable! {
-    Vec<String>,
+    Vec<NamedFamily>,
     Expected: "string or array of strings",
-    Value::Str(string) => vec![string.to_lowercase().into()],
+    Value::Str(string) => vec![NamedFamily::new(&string)],
     Value::Array(values) => values
         .into_iter()
         .filter_map(|v| v.cast().ok())
-        .map(|string: EcoString| string.to_lowercase().into())
+        .map(|string: EcoString| NamedFamily::new(&string))
         .collect(),
 }
 
@@ -243,7 +278,10 @@ castable! {
 castable! {
     FontWeight,
     Expected: "integer or string",
-    Value::Int(v) => v.try_into().map_or(Self::BLACK, Self::from_number),
+    Value::Int(v) => Value::Int(v)
+        .cast::<usize>()?
+        .try_into()
+        .map_or(Self::BLACK, Self::from_number),
     Value::Str(string) => match string.as_str() {
         "thin" => Self::THIN,
         "extralight" => Self::EXTRALIGHT,
@@ -681,7 +719,7 @@ fn families(styles: &Styles) -> impl Iterator<Item = &str> + Clone {
 
     head.iter()
         .chain(core)
-        .map(String::as_str)
+        .map(|named| named.as_str())
         .chain(tail.iter().copied())
 }
 
@@ -770,7 +808,7 @@ pub struct ShapedText<'a> {
     /// The text direction.
     pub dir: Dir,
     /// The text's style properties.
-    // TODO(set): Go back to reference.
+    // TODO(style): Go back to reference.
     pub styles: Styles,
     /// The font size.
     pub size: Size,
diff --git a/src/source.rs b/src/source.rs
index 5fd85ed9..432688a0 100644
--- a/src/source.rs
+++ b/src/source.rs
@@ -149,12 +149,12 @@ impl SourceFile {
         Self::new(SourceId(0), Path::new(""), src.into())
     }
 
-    /// The root node of the untyped green tree.
+    /// The root node of the file's untyped green tree.
     pub fn root(&self) -> &Rc<GreenNode> {
         &self.root
     }
 
-    /// The file's abstract syntax tree.
+    /// The root node of the file's typed abstract syntax tree.
     pub fn ast(&self) -> TypResult<Markup> {
         let red = RedNode::from_root(self.root.clone(), self.id);
         let errors = red.errors();
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
index 9190953f..ae8ecdc9 100644
--- a/src/syntax/ast.rs
+++ b/src/syntax/ast.rs
@@ -1,4 +1,6 @@
 //! A typed layer over the red-green tree.
+//!
+//! The AST is rooted in the [`Markup`] node.
 
 use std::ops::Deref;
 
@@ -283,6 +285,7 @@ impl Expr {
             Self::Ident(_)
                 | Self::Call(_)
                 | Self::Let(_)
+                | Self::Set(_)
                 | Self::If(_)
                 | Self::While(_)
                 | Self::For(_)
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index b9b00487..d9ad42a8 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -187,7 +187,7 @@ impl From<GreenData> for Green {
 
 impl Debug for GreenData {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "{:?}: {}", self.kind, self.len())
+        write!(f, "{:?}: {}", self.kind, self.len)
     }
 }