feat(eval): add extremely basic evaluation

added a basic `eval_expr` function that evaluates an expression.
it still does not reduce/simplify anything, and it cannot evaluate to fractions yet

Cargo.toml

@@ -4,4 +4,4 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-rust_decimal = { version = "1.43.0", features = ["macros"] }
+rust_decimal = { version = "1.43.0", features = ["macros", "maths"] }

src/main.rs

@@ -13,7 +13,7 @@ fn main() -> io::Result<()> {
 
         let line = line.unwrap();
         let parsed = math::parse(line.trim());
-        println!("{}", parsed);
+        println!("{}", math::eval(parsed));
 
         prompt()?;
     }

src/math/eval.rs

@@ -0,0 +1,37 @@
+use rust_decimal::MathematicalOps;
+
+use crate::math::types::{Expression, Operator};
+
+pub fn eval_expr(expr: Expression) -> Expression {
+    match expr {
+        Expression::Number(_) => expr,
+        Expression::Variable(_) => expr,
+        Expression::Fraction(numberator, denominator) => {
+            // TODO: implement proper fraction handling
+            eval_expr(Expression::Operation(
+                Operator::Divide,
+                numberator,
+                denominator,
+            ))
+        }
+        Expression::Operation(operator, left, right) => eval_operation(operator, *left, *right),
+        Expression::Function(_name, _params) => todo!(),
+    }
+}
+
+fn eval_operation(operator: Operator, left: Expression, right: Expression) -> Expression {
+    let left = eval_expr(left);
+    let right = eval_expr(right);
+    match (&left, &right) {
+        (Expression::Number(left), Expression::Number(right)) => match operator {
+            Operator::Add => Expression::Number(left + right),
+            Operator::Subtract => Expression::Number(left - right),
+            Operator::Multiply => Expression::Number(left * right),
+            Operator::Divide => Expression::Number(left / right),
+            Operator::Modulo => Expression::Number(left % right),
+            Operator::Power => Expression::Number(left.powd(*right)),
+        },
+        // TODO: implement handling for variables
+        _ => Expression::Operation(operator, Box::new(left), Box::new(right)),
+    }
+}

src/math/mod.rs

@@ -1,6 +1,7 @@
 use crate::math::types::Expression;
 
 mod display;
+mod eval;
 mod lexer;
 mod parser;
 
@@ -11,3 +12,7 @@ pub fn parse(source: &str) -> Expression {
     let mut parser = parser::from(lexer.collect());
     parser.parse()
 }
+
+pub fn eval(expr: Expression) -> Expression {
+    eval::eval_expr(expr)
+}