1 files changed, 80 insertions, 0 deletions
diff --git a/dhall/src/syntax/core/context.rs b/dhall/src/syntax/core/context.rs
new file mode 100644
index 0000000..6844baa
--- /dev/null
+++ b/dhall/src/syntax/core/context.rs
@@ -0,0 +1,80 @@
+use std::cmp::Eq;
+use std::collections::HashMap;
+use std::hash::Hash;
+
+/// A `(Context a)` associates `Text` labels with values of type `a`
+///
+/// The `Context` is used for type-checking when `(a = Expr)`
+///
+/// * 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<K: Eq + Hash, T>(HashMap<K, Vec<T>>);
+
+impl<K: Hash + Eq + Clone, T> Context<K, 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: &K, n: usize) -> Option<&'a T> {
+        self.0.get(k).and_then(|v| {
+            if n < v.len() {
+                v.get(v.len() - 1 - n)
+            } else {
+                None
+            }
+        })
+    }
+
+    pub fn map<U, F: Fn(&K, &T) -> U>(&self, f: F) -> Context<K, U> {
+        Context(
+            self.0
+                .iter()
+                .map(|(k, vs)| {
+                    ((*k).clone(), vs.iter().map(|v| f(k, v)).collect())
+                })
+                .collect(),
+        )
+    }
+
+    pub fn lookup_all<'a>(&'a self, k: &K) -> impl Iterator<Item = &T> {
+        self.0.get(k).into_iter().flat_map(|v| v.iter())
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (&K, &T)> {
+        self.0
+            .iter()
+            .flat_map(|(k, vs)| vs.iter().map(move |v| (k, v)))
+    }
+
+    pub fn iter_keys(&self) -> impl Iterator<Item = (&K, &Vec<T>)> {
+        self.0.iter()
+    }
+}
+
+impl<K: Hash + Eq + Clone, T: Clone> Context<K, T> {
+    /// Add a key-value pair to the `Context`
+    pub fn insert(&self, k: K, v: T) -> Self {
+        let mut ctx = (*self).clone();
+        {
+            let m = ctx.0.entry(k).or_insert_with(Vec::new);
+            m.push(v);
+        }
+        ctx
+    }
+}