Add generic stack data structure

I'll want to use this soon enough

schala-lang/src/lib.rs

@@ -17,6 +17,7 @@ macro_rules! bx {
   ($e:expr) => { Box::new($e) }
 }
 
+mod util;
 mod builtin;
 mod tokenizing;
 mod parsing;

schala-lang/src/util.rs

@@ -0,0 +1,23 @@
+use std::collections::HashMap;
+use std::hash::Hash;
+use std::cmp::Eq;
+
+#[derive(Default, Debug)]
+pub struct StateStack<'a, T: 'a, V: 'a>  where T: Hash + Eq {
+  parent: Option<&'a StateStack<'a, T, V>>,
+  values: HashMap<T, V>
+}
+
+impl<'a, T, V> StateStack<'a, T, V> where T: Hash + Eq {
+  pub fn insert(&mut self, key: T, value: V) where T: Hash + Eq {
+    self.values.insert(key, value);
+  }
+  pub fn lookup(&self, key: &T) -> Option<&V> where T: Hash + Eq {
+    match (self.values.get(key), self.parent) {
+      (None, None) => None,
+      (None, Some(parent)) => parent.lookup(key),
+      (Some(value), _) => Some(value),
+    }
+  }
+}
+