nested file-scoped binding environments

1 files changed, 26 insertions, 3 deletions

diff --git a/eval_env.h b/eval_env.h
index 2d1a79e..5d51aa6 100644
--- a/eval_env.h
+++ b/eval_env.h
@@ -1,8 +1,31 @@
 
+// A scope for variable lookups.
+struct Env {
+  virtual string LookupVariable(const string& var) = 0;
+};
+
+// A standard scope, which contains a mapping of variables to values
+// as well as a pointer to a parent scope.
+struct BindingEnv : public Env {
+  virtual string LookupVariable(const string& var) {
+    map<string, string>::iterator i = bindings_.find(var);
+    if (i != bindings_.end())
+      return i->second;
+    if (parent_)
+      return parent_->LookupVariable(var);
+    return "";
+  }
+  void AddBinding(const string& key, const string& val) {
+    bindings_[key] = val;
+  }
+
+  map<string, string> bindings_;
+  Env* parent_;
+};
+
+// A tokenized string that contains variable references.
+// Can be evaluated relative to an Env.
 struct EvalString {
-  struct Env {
-    virtual string Evaluate(const string& var) = 0;
-  };
   bool Parse(const string& input, string* err);
   string Evaluate(Env* env) const;