file separation

src/context.rs

@@ -0,0 +1,31 @@
+use super::value::Value;
+use std::collections::HashMap;
+
+#[derive(Clone)]
+pub struct Context {
+    dict: HashMap<String, Value>,
+    parent: Option<Box<Context>>,
+}
+
+impl Context {
+    pub fn new(parent: Option<Box<Context>>) -> Context {
+        Context {
+            dict: HashMap::new(),
+            parent,
+        }
+    }
+
+    pub fn assign(&mut self, key: String, value: Value) {
+        self.dict.insert(key, value);
+    }
+
+    pub fn value(&self, key: &String) -> Option<&Value> {
+        self.dict
+            .get(key)
+            .or(self.parent.as_ref().and_then(|p| p.value(key)))
+    }
+
+    pub fn take(&mut self, key: &String) -> Option<Value> {
+        self.dict.remove(key)
+    }
+}