Set Rules Episode VII: The Set Awakens

1 files changed, 101 insertions, 0 deletions

diff --git a/src/eval/class.rs b/src/eval/class.rs
new file mode 100644
index 00000000..45674933
--- /dev/null
+++ b/src/eval/class.rs
@@ -0,0 +1,101 @@
+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.
+#[derive(Clone)]
+pub struct Class(Rc<Inner<dyn Bounds>>);
+
+/// The unsized structure behind the [`Rc`].
+struct Inner<T: ?Sized> {
+    name: EcoString,
+    dispatch: T,
+}
+
+impl Class {
+    /// Create a new class.
+    pub fn new<T>(name: EcoString) -> Self
+    where
+        T: Construct + Set + 'static,
+    {
+        Self(Rc::new(Inner {
+            name,
+            dispatch: Dispatch::<T>(PhantomData),
+        }))
+    }
+
+    /// The name of the class.
+    pub fn name(&self) -> &EcoString {
+        &self.0.name
+    }
+
+    /// Construct an instance of the class.
+    pub fn construct(&self, ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
+        self.0.dispatch.construct(ctx, args)
+    }
+
+    /// Execute the class's set rule.
+    pub fn set(&self, styles: &mut Styles, args: &mut Args) -> TypResult<()> {
+        self.0.dispatch.set(styles, args)
+    }
+}
+
+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.
+        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(styles: &mut Styles, args: &mut Args) -> TypResult<()>;
+}
+
+/// Zero-sized struct whose vtable contains the constructor and set rule of a
+/// class.
+struct Dispatch<T>(PhantomData<T>);
+
+trait Bounds {
+    fn construct(&self, ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node>;
+    fn set(&self, styles: &mut Styles, args: &mut Args) -> TypResult<()>;
+}
+
+impl<T> Bounds for Dispatch<T>
+where
+    T: Construct + Set,
+{
+    fn construct(&self, ctx: &mut EvalContext, args: &mut Args) -> TypResult<Node> {
+        T::construct(ctx, args)
+    }
+
+    fn set(&self, styles: &mut Styles, args: &mut Args) -> TypResult<()> {
+        T::set(styles, args)
+    }
+}