add evaluator

src/eval.rs

@@ -0,0 +1,38 @@
+use crate::parser::{BinaryOp, Expr, UnaryOp};
+
+pub struct Evaluator {
+    ast: Expr
+}
+
+impl Evaluator {
+    pub fn new(ast: Expr) -> Self {
+        Self {
+            ast
+        }
+    }
+
+    fn eval_expr(&self, expr: Expr) -> i64 {
+        match expr {
+            Expr::Number(n) => n,
+            Expr::Unary { op, right } => {
+                match op {
+                    UnaryOp::Neg => -self.eval_expr(*right)
+                }
+            },
+            Expr::Binary { op, left, right } => {
+                match op {
+                    BinaryOp::Add => self.eval_expr(*left) + self.eval_expr(*right),
+                    BinaryOp::Sub => self.eval_expr(*left) - self.eval_expr(*right),
+                    BinaryOp::Mul => self.eval_expr(*left) * self.eval_expr(*right),
+                    BinaryOp::Div => self.eval_expr(*left) / self.eval_expr(*right),
+                    BinaryOp::Mod => self.eval_expr(*left) % self.eval_expr(*right)
+                }
+            },
+            expr => panic!("can't eval expression: {expr:?}")
+        }
+    }
+
+    pub fn eval(&self) -> i64 {
+        self.eval_expr(self.ast.clone())
+    }
+}