Initial commit

.gitignore

@@ -0,0 +1,2 @@
+/target
+.idea

Cargo.lock

@@ -0,0 +1,16 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "bred"
+version = "0.1.0"
+dependencies = [
+ "take_mut",
+]
+
+[[package]]
+name = "take_mut"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f764005d11ee5f36500a149ace24e00e3da98b0158b3e2d53a7495660d3f4d60"

Cargo.toml

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+[package]
+name = "bred"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+take_mut = "0.2.2"

src/main.rs

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+use std::cell::RefCell;
+use std::rc::Rc;
+
+#[derive(Debug)]
+struct ArgCell(Rc<RefCell<Option<EitherValue>>>);
+
+#[derive(Debug)]
+struct Lambda {
+    arg: ArgCell,
+    expression: Expression
+}
+
+#[derive(Debug)]
+enum EitherValue {
+    Lambda(Lambda),
+    Expression(Expression)
+}
+
+#[derive(Debug, Clone)]
+enum Expression {
+    Valued(ArgCell),
+    Expression(ArgCell, ArgCell)
+}
+
+impl ArgCell {
+    fn lambda(value: Lambda) -> Self {
+        ArgCell(Rc::new(RefCell::new(Some(EitherValue::Lambda(value)))))
+    }
+
+    fn expression(expression: Expression) -> Self {
+        ArgCell(Rc::new(RefCell::new(Some(EitherValue::Expression(expression)))))
+    }
+
+    fn future() -> Self {
+        ArgCell(Rc::new(RefCell::new(None)))
+    }
+
+    fn copy_retain(&self) -> Self {
+        Self(self.0.clone())
+    }
+}
+
+impl Clone for ArgCell {
+    fn clone(&self) -> Self {
+        let cell = self.0.borrow();
+        if cell.is_some() {
+            Self(self.0.clone())
+        } else {
+            Self(Rc::new(RefCell::new(None)))
+        }
+    }
+}
+
+impl Lambda {
+    fn new(expression: impl FnOnce(ArgCell) -> Expression) -> Self {
+        let cell = ArgCell::future();
+        Self {
+            arg: cell.copy_retain(),
+            expression: expression(cell)
+        }
+    }
+
+    fn beta_reduce(self, argument: Option<EitherValue>) -> Expression {
+        *self.arg.0.borrow_mut() = argument;
+        self.expression
+    }
+}
+
+impl Clone for Lambda {
+    fn clone(&self) -> Self {
+        Self::new(|_| self.expression.clone())
+    }
+}
+
+impl EitherValue {
+    fn is_lambda(&self) -> bool {
+        matches!(*self, EitherValue::Lambda(_))
+    }
+
+    fn is_expression(&self) -> bool {
+        matches!(*self, EitherValue::Expression(_))
+    }
+}
+
+impl Expression {
+    fn new_valued(value: ArgCell) -> Self {
+        Self::Valued(value)
+    }
+
+    fn new_expression(callee: ArgCell, argument: ArgCell) -> Self {
+        Self::Expression(callee, argument)
+    }
+
+    fn can_beta_reduce(&self) -> bool {
+        // TODO: Clean this up
+        matches!(self, Self::Expression(callee, _) if callee.0.borrow().is_some() && callee.0.borrow().as_ref().unwrap().is_lambda())
+    }
+}
+
+// (lx.(ly.x(xy))) (lz.z)
+// -> (ly.(lz.z)((lw.w)y))
+// -> (ly.(lz.z)y)
+// -> (ly.y)
+fn main() {
+    // Lambda_x { Expression { Lambda_y { Expression { x(Expression(x, y)) } } } }
+    let func = Lambda::new(|x|
+        Expression::new_valued(ArgCell::lambda(Lambda::new(|y| Expression::new_expression(x.copy_retain(), ArgCell::expression(Expression::Expression(x, y))))))
+    );
+
+    let func_identity = Some(EitherValue::Lambda(Lambda::new(Expression::Valued)));
+    let reduced = func.beta_reduce(func_identity);
+    dbg!(reduced);
+}