fix: resolve lambda infinite loop

src/bin/eval.rs

@@ -19,6 +19,7 @@ fn main() {
 
     match result {
         Node::String(s) => println!("{}", s),
+        Node::Integer(n) => println!("{}", n),
         _ => panic!("Unexpected result: {:?}", result),
     }
 }

src/icfp/evaluator.rs

@@ -22,7 +22,7 @@ impl Evaluator {
             Node::String(_) => node.clone(),
             Node::Boolean(_) => node.clone(),
             Node::Variable(_) => node.clone(),
-            Node::Lambda(arity, body) => Node::Lambda(*arity, Box::new(self.evaluate_node(body))),
+            Node::Lambda(arity, body) => Node::Lambda(*arity, body.clone()),
             Node::UnaryOperator(operator, operand) => {
                 self.evaluate_unary_operator(operator, *operand.clone())
             }
@@ -180,6 +180,16 @@ impl Evaluator {
                 let new_right = self.replace_variable(right, variables);
                 Node::BinaryOperator(operator.clone(), Box::new(new_left), Box::new(new_right))
             }
+            Node::If(condition, then_branch, else_branch) => {
+                let new_condition = self.replace_variable(condition, variables);
+                let new_then_branch = self.replace_variable(then_branch, variables);
+                let new_else_branch = self.replace_variable(else_branch, variables);
+                Node::If(
+                    Box::new(new_condition),
+                    Box::new(new_then_branch),
+                    Box::new(new_else_branch),
+                )
+            }
             _ => panic!("Unsupported node: {:?}", node),
         }
     }