Add onError interface to Expected type (#1104)

Summary:
So that we can do some thing in the middle or chaining, for example log the error.

While using `Expected`, we found that It's nice to have the chaining operations, but one downside is  there is no place to log each possible error here and there.

So I added this `onError` interface, expecting usage like:

  Expected<D, Error> maybeD = someFunc()
      .then([](A a){return B(a);})
      .onError([](Error e) { LOG_DEBUG("error happened while executing function A->B");})
      .then([](B b){return C(b);})
      .then([](C c){return D(c);});

Pull Request resolved: #1104

Reviewed By: dmm-fb

Differential Revision: D15120090

Pulled By: yfeldblum

fbshipit-source-id: 07bc30c96ede25ef415ff921592f1d8676489b17

folly/Expected.h

@@ -810,6 +810,19 @@ struct ExpectedHelper {
     return makeUnexpected(static_cast<decltype(ex)&&>(ex).error());
   }
 
+  template <
+      class This,
+      class OnError,
+      class Err =
+          decltype(std::declval<OnError>()(std::declval<This>().error()))
+              FOLLY_REQUIRES_TRAILING(std::is_void<Err>::value)>
+  static This onError_(This&& ex, OnError&& onError) {
+    if (UNLIKELY(ex.which_ == expected_detail::Which::eError)) {
+      static_cast<OnError&&>(onError)(static_cast<This&&>(ex).error());
+    }
+    return ex;
+  }
+
   FOLLY_POP_WARNING
 };
 } // namespace expected_detail_ExpectedHelper
@@ -1354,6 +1367,37 @@ class Expected final : expected_detail::ExpectedStorage<Value, Error> {
         std::move(base()), static_cast<Yes&&>(yes), static_cast<No&&>(no)));
   }
 
+  /**
+   * onError
+   */
+  template <class OnError>
+  auto onError(OnError&& onError) const& {
+    if (this->uninitializedByException()) {
+      throw_exception<BadExpectedAccess<void>>();
+    }
+
+    return expected_detail::ExpectedHelper::onError_(
+        *this, static_cast<OnError&&>(onError));
+  }
+
+  template <class OnError>
+  auto onError(OnError&& onError) & {
+    if (this->uninitializedByException()) {
+      throw_exception<BadExpectedAccess<void>>();
+    }
+    return expected_detail::ExpectedHelper::onError_(
+        *this, static_cast<OnError&&>(onError));
+  }
+
+  template <class OnError>
+  auto onError(OnError&& onError) && {
+    if (this->uninitializedByException()) {
+      throw_exception<BadExpectedAccess<void>>();
+    }
+    return expected_detail::ExpectedHelper::onError_(
+        std::move(*this), static_cast<OnError&&>(onError));
+  }
+
   // Quasi-private, exposed only for implementing efficient short-circuiting
   // coroutines on top of `Expected`.  Do NOT use this instead of
   // `optional<Expected<>>`, for these reasons: