feat: add monkey-rs

__lang/monkey-rs/src/inbuilt.rs

@@ -0,0 +1,36 @@
+use crate::object::Object;
+
+pub fn get_inbuilt_function(identifier: &str) -> Option<Object> {
+    match identifier {
+        "len" => Some(Object::FunctionInBuilt(String::from("len"))),
+        _ => None
+    }
+}
+
+fn process_len_function(params: &Vec<Object>) -> Object {
+
+    if params.len() != 1 {
+        panic!("Expected a single argument to the len function found {} arguments",
+               params.len())
+    }
+    let argument = &params[0];
+
+    match argument {
+        Object::String(s) => Object::Integer(s.len() as i64),
+        _ => panic!("len expects a string argument")
+    }
+}
+
+pub fn eval_inbuilt_function(func_obj: &Object, params: &Vec<Object>) -> Object {
+    match func_obj {
+        Object::FunctionInBuilt(func_name) => {
+            match func_name.as_str() {
+                "len" => {
+                    return process_len_function(params)
+                }
+                _ => panic!("Invalid inbuild function {}", func_name),
+            }
+        }
+        _ => panic!("Excepted a function object but found {}", func_obj)
+    }
+}