diff --git a/readme.md b/readme.md
new file mode 100644
index 0000000..a555d29
--- /dev/null
+++ b/readme.md
@@ -0,0 +1,4 @@
+# Monkey Interpreter
+
+This project is my attempt at writing an interpreter while working through the book, [Writing an Interpreter in Go](https://interpreterbook.com/).
+It supports all the features from the book.
diff --git a/src/evaluator/mod.rs b/src/evaluator/mod.rs
index 7774a33..2054912 100644
--- a/src/evaluator/mod.rs
+++ b/src/evaluator/mod.rs
@@ -346,4 +346,38 @@ mod tests {
 
         run_test_cases(&test_cases);
     }
+
+    #[test]
+    fn test_closure() {
+        let test_cases = [(
+            "let newAdder = fn(x) {
+                fn(y) { x + y ; } ;
+            };
+            let adder5 = newAdder(5);
+            adder5(10);
+            ",
+            Some(Object::Integer(15)),
+        )];
+
+        run_test_cases(&test_cases);
+    }
+
+    #[test]
+    fn memory_test() {
+        let test_cases = [(
+            "let counter = fn(x) {
+                if (x > 100 ) {
+                    return true;
+                } else {
+                    let foobar = 9999;
+                    counter(x+1);
+                }
+            };
+            counter(0);
+            ",
+            Some(Object::Integer(15)),
+        )];
+
+        run_test_cases(&test_cases);
+    }
 }
diff --git a/src/evaluator/tree_walker/mod.rs b/src/evaluator/tree_walker/mod.rs
index 1039621..29e3381 100644
--- a/src/evaluator/tree_walker/mod.rs
+++ b/src/evaluator/tree_walker/mod.rs
@@ -84,7 +84,6 @@ impl Evaluator for TreeWalker {
                         Some(NULL)
                     }
                 }
-                _ => None,
             },
         }
     }
@@ -239,13 +238,16 @@ impl TreeWalker {
     fn apply_function(&self, function: Object, args: Vec<Object>) -> Option<Object> {
         let function = match function {
             Object::Function(f) => f,
+            Object::Error(e) => {
+                return Some(Object::Error(format!("not a function: {}", e.to_string())));
+            }
             v => return Some(Object::Error(format!("not a function: {}", v.to_string()))),
         };
+        println!("{:?}", function.env);
 
         let mut enclosed_env = Environment::new_enclosed(function.env);
         for (i, parameter) in function.parameters.iter().enumerate() {
             if args.len() <= i {
-                println!("{:?} {}", args, i);
                 return Some(Object::Error(format!("incorrect number of arguments")));
             }
 
diff --git a/src/lexer/mod.rs b/src/lexer/mod.rs
index d23f618..9640ec4 100644
--- a/src/lexer/mod.rs
+++ b/src/lexer/mod.rs
@@ -158,7 +158,7 @@ impl<'a> Lexer<'a> {
     fn read_identifier(&mut self, first: char) -> String {
         let mut ident = String::new();
         ident.push(first);
-        while self.peek_is_letter() {
+        while self.peek_is_letter() || self.peek_is_ascii_digit() {
             ident.push(self.read_char().unwrap());
         }
         ident