From fbab9c521d1a8fb275263056fb1a1244f75c1830 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Sat, 26 Sep 2026 09:11:01 +0100
Subject: [PATCH] Define Expression type

---
 src/expression.rs | 135 ++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs        |   2 +
 2 files changed, 137 insertions(+)
 create mode 100644 src/expression.rs

diff --git a/src/expression.rs b/src/expression.rs
new file mode 100644
index 0000000..6694b08
--- /dev/null
+++ b/src/expression.rs
@@ -0,0 +1,135 @@
+#[derive(Debug, PartialEq)]
+pub enum Expression {
+	Symbol(char),
+	Integer(i32),
+	Sum(std::boxed::Box<Expression>, std::boxed::Box<Expression>),
+	Difference(std::boxed::Box<Expression>, std::boxed::Box<Expression>),
+	Product(std::boxed::Box<Expression>, std::boxed::Box<Expression>),
+	Quotient(std::boxed::Box<Expression>, std::boxed::Box<Expression>),
+	Exponent(std::boxed::Box<Expression>, std::boxed::Box<Expression>),
+}
+
+impl Expression {
+	pub fn sum(left: Expression, right: Expression) -> Expression {
+		Expression::Sum(
+			std::boxed::Box::new(left), std::boxed::Box::new(right))
+	}
+
+	pub fn difference(left: Expression, right: Expression) -> Expression {
+		Expression::Difference(
+			std::boxed::Box::new(left), std::boxed::Box::new(right))
+	}
+
+	pub fn product(left: Expression, right: Expression) -> Expression {
+		Expression::Product(
+			std::boxed::Box::new(left), std::boxed::Box::new(right))
+	}
+
+	pub fn quotient(left: Expression, right: Expression) -> Expression {
+		Expression::Quotient(
+			std::boxed::Box::new(left), std::boxed::Box::new(right))
+	}
+
+	pub fn exponent(left: Expression, right: Expression) -> Expression {
+		Expression::Exponent(
+			std::boxed::Box::new(left), std::boxed::Box::new(right))
+	}
+
+	pub fn is_constant(&self) -> bool {
+		match self {
+			Expression::Integer(_) => true,
+			Expression::Symbol(_) => false,
+			Expression::Sum(x, y) => x.is_constant() && y.is_constant(),
+			Expression::Difference(x, y) =>
+				x.is_constant() && y.is_constant(),
+			Expression::Product(x, y) => x.is_constant() && y.is_constant(),
+			Expression::Quotient(x, y) => x.is_constant() && y.is_constant(),
+			Expression::Exponent(x, y) => x.is_constant() && y.is_constant(),
+		}
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn integer_is_constant() {
+		assert!(Expression::Integer(1).is_constant());
+	}
+
+	#[test]
+	fn symbol_is_not_constant() {
+		assert!(!Expression::Symbol('x').is_constant());
+	}
+
+	#[test]
+	fn sum_of_integers_is_constant() {
+		assert!(Expression::sum(
+			Expression::Integer(1),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn sum_of_integer_and_symbol_is_not_constant() {
+		assert!(!Expression::sum(
+			Expression::Symbol('x'),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn difference_of_integers_is_constant() {
+		assert!(Expression::difference(
+			Expression::Integer(1),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn difference_of_integer_and_symbol_is_not_constant() {
+		assert!(!Expression::difference(
+			Expression::Symbol('x'),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn product_of_integers_is_constant() {
+		assert!(Expression::product(
+			Expression::Integer(1),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn product_of_integer_and_symbol_is_not_constant() {
+		assert!(!Expression::product(
+			Expression::Symbol('x'),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn quotient_of_integers_is_constant() {
+		assert!(Expression::quotient(
+			Expression::Integer(1),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn quotient_of_integer_and_symbol_is_not_constant() {
+		assert!(!Expression::quotient(
+			Expression::Symbol('x'),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn exponent_of_integers_is_constant() {
+		assert!(Expression::exponent(
+			Expression::Integer(1),
+			Expression::Integer(1)).is_constant());
+	}
+
+	#[test]
+	fn exponent_of_integer_and_symbol_is_not_constant() {
+		assert!(!Expression::exponent(
+			Expression::Symbol('x'),
+			Expression::Integer(1)).is_constant());
+	}
+}
diff --git a/src/lib.rs b/src/lib.rs
index f793d1a..6769acd 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,2 +1,4 @@
 pub mod token;
+pub mod expression;
+
 pub mod lex;