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diff --git a/grpc/src/core/lib/promise/map.h b/grpc/src/core/lib/promise/map.h
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+// Copyright 2021 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GRPC_CORE_LIB_PROMISE_MAP_H
+#define GRPC_CORE_LIB_PROMISE_MAP_H
+
+#include <grpc/support/port_platform.h>
+
+#include <stddef.h>
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/types/variant.h"
+
+#include "src/core/lib/promise/detail/promise_like.h"
+#include "src/core/lib/promise/poll.h"
+
+namespace grpc_core {
+
+namespace promise_detail {
+
+// Implementation of mapping combinator - use this via the free function below!
+// Promise is the type of promise to poll on, Fn is a function that takes the
+// result of Promise and maps it to some new type.
+template <typename Promise, typename Fn>
+class Map {
+ public:
+  Map(Promise promise, Fn fn)
+      : promise_(std::move(promise)), fn_(std::move(fn)) {}
+
+  using PromiseResult = typename PromiseLike<Promise>::Result;
+  using Result =
+      RemoveCVRef<decltype(std::declval<Fn>()(std::declval<PromiseResult>()))>;
+
+  Poll<Result> operator()() {
+    Poll<PromiseResult> r = promise_();
+    if (auto* p = absl::get_if<kPollReadyIdx>(&r)) {
+      return fn_(std::move(*p));
+    }
+    return Pending();
+  }
+
+ private:
+  PromiseLike<Promise> promise_;
+  Fn fn_;
+};
+
+}  // namespace promise_detail
+
+// Mapping combinator.
+// Takes a promise, and a synchronous function to mutate its result, and
+// returns a promise.
+template <typename Promise, typename Fn>
+promise_detail::Map<Promise, Fn> Map(Promise promise, Fn fn) {
+  return promise_detail::Map<Promise, Fn>(std::move(promise), std::move(fn));
+}
+
+// Callable that takes a tuple and returns one element
+template <size_t kElem>
+struct JustElem {
+  template <typename... A>
+  auto operator()(std::tuple<A...>&& t) const
+      -> decltype(std::get<kElem>(std::forward<std::tuple<A...>>(t))) {
+    return std::get<kElem>(std::forward<std::tuple<A...>>(t));
+  }
+  template <typename... A>
+  auto operator()(const std::tuple<A...>& t) const
+      -> decltype(std::get<kElem>(t)) {
+    return std::get<kElem>(t);
+  }
+};
+
+}  // namespace grpc_core
+
+#endif  // GRPC_CORE_LIB_PROMISE_MAP_H