1 files changed, 51 insertions, 0 deletions
diff --git a/dhall/src/context.rs b/dhall/src/context.rs
new file mode 100644
index 0000000..c2e1913
--- /dev/null
+++ b/dhall/src/context.rs
@@ -0,0 +1,51 @@
+use std::collections::HashMap;
+
+/// A `(Context a)` associates `Text` labels with values of type `a`
+///
+/// The `Context` is used for type-checking when `(a = Expr X)`
+///
+/// * You create a `Context` using `empty` and `insert`
+/// * You transform a `Context` using `fmap`
+/// * You consume a `Context` using `lookup` and `toList`
+///
+/// The difference between a `Context` and a `Map` is that a `Context` lets you
+/// have multiple ordered occurrences of the same key and you can query for the
+/// `n`th occurrence of a given key.
+///
+#[derive(Debug, Clone)]
+pub struct Context<'i, T>(HashMap<&'i str, Vec<T>>);
+
+impl<'i, T> Context<'i, T> {
+    /// An empty context with no key-value pairs
+    pub fn new() -> Self {
+        Context(HashMap::new())
+    }
+
+    /// Look up a key by name and index
+    ///
+    /// ```c
+    /// lookup _ _         empty  = Nothing
+    /// lookup k 0 (insert k v c) = Just v
+    /// lookup k n (insert k v c) = lookup k (n - 1) c  -- 1 <= n
+    /// lookup k n (insert j v c) = lookup k  n      c  -- k /= j
+    /// ```
+    pub fn lookup<'a>(&'a self, k: &str, n: usize) -> Option<&'a T> {
+        self.0.get(k).and_then(|v| v.get(v.len() - 1 - n))
+    }
+
+    pub fn map<U, F: Fn(&T) -> U>(&self, f: F) -> Context<'i, U> {
+        Context(self.0.iter().map(|(k, v)| (*k, v.iter().map(&f).collect())).collect())
+    }
+}
+
+impl<'i, T: Clone> Context<'i, T> {
+    /// Add a key-value pair to the `Context`
+    pub fn insert(&self, k: &'i str, v: T) -> Self {
+        let mut ctx = (*self).clone();
+        {
+            let m = ctx.0.entry(k).or_insert_with(Vec::new);
+            m.push(v);
+        }
+        ctx
+    }
+}