1 files changed, 59 insertions, 0 deletions
diff --git a/users/aspen/achilles/src/common/env.rs b/users/aspen/achilles/src/common/env.rs
new file mode 100644
index 0000000000..59a5e46c46
--- /dev/null
+++ b/users/aspen/achilles/src/common/env.rs
@@ -0,0 +1,59 @@
+use std::borrow::Borrow;
+use std::collections::HashMap;
+use std::hash::Hash;
+use std::mem;
+
+/// A lexical environment
+#[derive(Debug, PartialEq, Eq)]
+pub struct Env<K: Eq + Hash, V>(Vec<HashMap<K, V>>);
+
+impl<K, V> Default for Env<K, V>
+where
+    K: Eq + Hash,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K, V> Env<K, V>
+where
+    K: Eq + Hash,
+{
+    pub fn new() -> Self {
+        Self(vec![Default::default()])
+    }
+
+    pub fn push(&mut self) {
+        self.0.push(Default::default());
+    }
+
+    pub fn pop(&mut self) {
+        self.0.pop();
+    }
+
+    pub fn save(&mut self) -> Self {
+        mem::take(self)
+    }
+
+    pub fn restore(&mut self, saved: Self) {
+        *self = saved;
+    }
+
+    pub fn set(&mut self, k: K, v: V) {
+        self.0.last_mut().unwrap().insert(k, v);
+    }
+
+    pub fn resolve<'a, Q>(&'a self, k: &Q) -> Option<&'a V>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq + ?Sized,
+    {
+        for ctx in self.0.iter().rev() {
+            if let Some(res) = ctx.get(k) {
+                return Some(res);
+            }
+        }
+        None
+    }
+}