Migrate to error constants in glacier

lib/glacier/status/error_or.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <glacier/status/error.h>
+
+namespace glcr {
+
+template <typename T>
+class ErrorOr {
+ public:
+  ErrorOr() = delete;
+  ErrorOr(const ErrorOr&) = delete;
+  ErrorOr(ErrorOr&&) = delete;
+
+  ErrorOr(ErrorCode code) : error_(code), ok_(false) {}
+  ErrorOr(T obj) : obj_(obj), ok_(true) {}
+  ErrorOr(T&& obj) : obj_(obj), ok_(true) {}
+
+  bool ok() { return ok_; }
+  operator bool() { return ok_; }
+
+  T& value() { return obj_; }
+
+  ErrorCode error() { return error_; }
+
+ private:
+  union {
+    ErrorCode error_;
+    T obj_;
+  };
+  bool ok_;
+};
+
+#define ASSIGN_OR_RETURN(lhs, rhs) \
+                                   \
+  auto e##__LINE__ = rhs;          \
+  if (!e##__LINE__.ok()) {         \
+    return e##__LINE__.error();    \
+  }                                \
+  lhs = rhs.value();
+
+}  // namespace glcr