1 files changed, 139 insertions, 0 deletions
diff --git a/src/eval/class.rs b/src/eval/class.rs
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+++ b/src/eval/class.rs
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+use std::fmt::{self, Debug, Formatter, Write};
+use std::marker::PhantomData;
+use std::rc::Rc;
+
+use super::{Args, EvalContext, Node, Styles};
+use crate::diag::TypResult;
+use crate::util::EcoString;
+
+/// A class of [nodes](Node).
+///
+/// You can [construct] an instance of a class in Typst code by invoking the
+/// class as a callable. This always produces some node, but not necessarily one
+/// of fixed type. For example, the `text` constructor does not actually create
+/// a [`TextNode`]. Instead it applies styling to whatever node you pass in and
+/// returns it structurally unchanged.
+///
+/// The arguments you can pass to a class constructor fall into two categories:
+/// Data that is inherent to the instance (e.g. the text of a heading) and style
+/// properties (e.g. the fill color of a heading). As the latter are often
+/// shared by many instances throughout a document, they can also be
+/// conveniently configured through class's [`set`] rule. Then, they apply to
+/// all nodes that are instantiated into the template where the `set` was
+/// executed.
+///
+/// ```typst
+/// This is normal.
+/// [
+///   #set text(weight: "bold")
+///   #set heading(fill: blue)
+///   = A blue & bold heading
+/// ]
+/// Normal again.
+/// ```
+///
+/// [construct]: Self::construct
+/// [`TextNode`]: crate::library::TextNode
+/// [`set`]: Self::set
+#[derive(Clone)]
+pub struct Class(Rc<Inner<dyn Bounds>>);
+
+/// The unsized structure behind the [`Rc`].
+struct Inner<T: ?Sized> {
+    name: EcoString,
+    shim: T,
+}
+
+impl Class {
+    /// Create a new class.
+    pub fn new<T>(name: EcoString) -> Self
+    where
+        T: Construct + Set + 'static,
+    {
+        // By specializing the shim to `T`, its vtable will contain T's
+        // `Construct` and `Set` impls (through the `Bounds` trait), enabling us
+        // to use them in the class's methods.
+        Self(Rc::new(Inner { name, shim: Shim::<T>(PhantomData) }))
+    }
+
+    /// The name of the class.
+    pub fn name(&self) -> &EcoString {
+        &self.0.name
+    }
+
+    /// Construct an instance of the class.
+    ///
+    /// This parses both property and data arguments (in this order) and styles
+    /// the node constructed from the data with the style properties.
+    pub fn construct(&self, ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
+        let mut styles = Styles::new();
+        self.set(args, &mut styles)?;
+        let node = self.0.shim.construct(ctx, args)?;
+        Ok(node.styled(styles))
+    }
+
+    /// Execute the class's set rule.
+    ///
+    /// This parses property arguments and writes the resulting styles into the
+    /// given style map. There are no further side effects.
+    pub fn set(&self, args: &mut Args, styles: &mut Styles) -> TypResult<()> {
+        self.0.shim.set(args, styles)
+    }
+}
+
+impl Debug for Class {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.write_str("<class ")?;
+        f.write_str(&self.0.name)?;
+        f.write_char('>')
+    }
+}
+
+impl PartialEq for Class {
+    fn eq(&self, other: &Self) -> bool {
+        // We cast to thin pointers for comparison because we don't want to
+        // compare vtables (there can be duplicate vtables across codegen units).
+        std::ptr::eq(
+            Rc::as_ptr(&self.0) as *const (),
+            Rc::as_ptr(&other.0) as *const (),
+        )
+    }
+}
+
+/// Construct an instance of a class.
+pub trait Construct {
+    /// Construct an instance of this class from the arguments.
+    ///
+    /// This is passed only the arguments that remain after execution of the
+    /// class's set rule.
+    fn construct(ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node>;
+}
+
+/// Set style properties of a class.
+pub trait Set {
+    /// Parse the arguments and insert style properties of this class into the
+    /// given style map.
+    fn set(args: &mut Args, styles: &mut Styles) -> TypResult<()>;
+}
+
+/// Rewires the operations available on a class in an object-safe way. This is
+/// only implemented by the zero-sized `Shim` struct.
+trait Bounds {
+    fn construct(&self, ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node>;
+    fn set(&self, args: &mut Args, styles: &mut Styles) -> TypResult<()>;
+}
+
+struct Shim<T>(PhantomData<T>);
+
+impl<T> Bounds for Shim<T>
+where
+    T: Construct + Set,
+{
+    fn construct(&self, ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
+        T::construct(ctx, args)
+    }
+
+    fn set(&self, args: &mut Args, styles: &mut Styles) -> TypResult<()> {
+        T::set(args, styles)
+    }
+}