Create a Dictionary abstraction over the raw Cons list. (#327)

* Create a Dictionary abstraction over the raw Cons list.

* renamed Cons and some methods of Dictionary, various other cleanup

executable_semantics/interpreter/dictionary.h

@@ -0,0 +1,60 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_INTERPRETER_DICTIONARY_H_
+#define EXECUTABLE_SEMANTICS_INTERPRETER_DICTIONARY_H_
+
+#include <iostream>
+#include <list>
+#include <optional>
+#include <string>
+
+#include "executable_semantics/interpreter/list_node.h"
+
+namespace Carbon {
+
+// A persistent dictionary with a simple implementation.
+// Copying the dictionary is O(1) time.
+template <class K, class V>
+class Dictionary {
+ public:
+  // Create an empty dictionary.
+  Dictionary() { head = nullptr; }
+
+  // Return the value associated with the given key.
+  // Time complexity: O(n) where n is the number of times
+  //    any value has been set across all keys.
+  auto Get(const K& key) -> std::optional<V> {
+    for (auto kv : *this) {
+      if (kv.first == key) {
+        return kv.second;
+      }
+    }
+    return std::nullopt;
+  }
+
+  // Associate the value v with key k in the dictionary.
+  // Time complexity: O(1).
+  auto Set(const K& k, const V& v) -> void {
+    head = new ListNode<std::pair<K, V>>(std::make_pair(k, v), head);
+  }
+
+  typedef ListNodeIterator<std::pair<K, V>> Iterator;
+
+  // The position of the first element of the dictionary
+  // or `end()` if the dictionary is empty.
+  auto begin() const -> Iterator { return Iterator(head); }
+
+  // The position one past that of the last element.
+  auto end() const -> Iterator { return Iterator(nullptr); }
+
+ private:
+  Dictionary(ListNode<std::pair<K, V>>* h) : head(h) {}
+
+  ListNode<std::pair<K, V>>* head;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_INTERPRETER_DICTIONARY_H_