FTL: Add Optional<T>::ok_or and FTL_TRY

Optional<T>::ok_or maps to Expected<T, E> where nullopt becomes E.

FTL_TRY unwraps T for Expected<T, E> or does an early out on error.

Bug: 185536303
Test: ftl_test
Change-Id: Ia03f7e3d8773878db1c493b62772ab2c2b7a4fed

6 files changed, 132 insertions, 1 deletions

diff --git a/include/ftl/expected.h b/include/ftl/expected.h
index 12b6102b6f..57448dc1e0 100644
--- a/include/ftl/expected.h
+++ b/include/ftl/expected.h
@@ -18,9 +18,57 @@
 
 #include <android-base/expected.h>
 #include <ftl/optional.h>
+#include <ftl/unit.h>
 
 #include <utility>
 
+// Given an expression `expr` that evaluates to an ftl::Expected<T, E> result (R for short), FTL_TRY
+// unwraps T out of R, or bails out of the enclosing function F if R has an error E. The return type
+// of F must be R, since FTL_TRY propagates R in the error case. As a special case, ftl::Unit may be
+// used as the error E to allow FTL_TRY expressions when F returns `void`.
+//
+// The non-standard syntax requires `-Wno-gnu-statement-expression-from-macro-expansion` to compile.
+// The UnitToVoid conversion allows the macro to be used for early exit from a function that returns
+// `void`.
+//
+// Example usage:
+//
+//   using StringExp = ftl::Expected<std::string, std::errc>;
+//
+//   StringExp repeat(StringExp exp) {
+//     const std::string str = FTL_TRY(exp);
+//     return StringExp(str + str);
+//   }
+//
+//   assert(StringExp("haha"s) == repeat(StringExp("ha"s)));
+//   assert(repeat(ftl::Unexpected(std::errc::bad_message)).has_error([](std::errc e) {
+//     return e == std::errc::bad_message;
+//   }));
+//
+//
+// FTL_TRY may be used in void-returning functions by using ftl::Unit as the error type:
+//
+//   void uppercase(char& c, ftl::Optional<char> opt) {
+//     c = std::toupper(FTL_TRY(std::move(opt).ok_or(ftl::Unit())));
+//   }
+//
+//   char c = '?';
+//   uppercase(c, std::nullopt);
+//   assert(c == '?');
+//
+//   uppercase(c, 'a');
+//   assert(c == 'A');
+//
+#define FTL_TRY(expr)                                                     \
+  ({                                                                      \
+    auto exp_ = (expr);                                                   \
+    if (!exp_.has_value()) {                                              \
+      using E = decltype(exp_)::error_type;                               \
+      return android::ftl::details::UnitToVoid<E>::from(std::move(exp_)); \
+    }                                                                     \
+    exp_.value();                                                         \
+  })
+
 namespace android::ftl {
 
 // Superset of base::expected<T, E> with monadic operations.
diff --git a/include/ftl/optional.h b/include/ftl/optional.h
index 94d8e3d7cb..e245d88c1c 100644
--- a/include/ftl/optional.h
+++ b/include/ftl/optional.h
@@ -20,13 +20,14 @@
 #include <optional>
 #include <utility>
 
+#include <android-base/expected.h>
 #include <ftl/details/optional.h>
 
 namespace android::ftl {
 
 // Superset of std::optional<T> with monadic operations, as proposed in https://wg21.link/P0798R8.
 //
-// TODO: Remove in C++23.
+// TODO: Remove standard APIs in C++23.
 //
 template <typename T>
 struct Optional final : std::optional<T> {
@@ -109,6 +110,13 @@ struct Optional final : std::optional<T> {
     return std::forward<F>(f)();
   }
 
+  // Maps this Optional<T> to expected<T, E> where nullopt becomes E.
+  template <typename E>
+  constexpr auto ok_or(E&& e) && -> base::expected<T, E> {
+    if (has_value()) return std::move(value());
+    return base::unexpected(std::forward<E>(e));
+  }
+
   // Delete new for this class. Its base doesn't have a virtual destructor, and
   // if it got deleted via base class pointer, it would cause undefined
   // behavior. There's not a good reason to allocate this object on the heap
diff --git a/include/ftl/unit.h b/include/ftl/unit.h
index e38230b976..62549a3f9d 100644
--- a/include/ftl/unit.h
+++ b/include/ftl/unit.h
@@ -58,4 +58,22 @@ constexpr auto unit_fn(F&& f) -> UnitFn<std::decay_t<F>> {
   return {std::forward<F>(f)};
 }
 
+namespace details {
+
+// Identity function for all T except Unit, which maps to void.
+template <typename T>
+struct UnitToVoid {
+  template <typename U>
+  static auto from(U&& value) {
+    return value;
+  }
+};
+
+template <>
+struct UnitToVoid<Unit> {
+  template <typename U>
+  static void from(U&&) {}
+};
+
+}  // namespace details
 }  // namespace android::ftl
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 4b41152c12..368f5e079c 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -41,5 +41,6 @@ cc_test {
         "-Wextra",
         "-Wpedantic",
         "-Wthread-safety",
+        "-Wno-gnu-statement-expression-from-macro-expansion",
     ],
 }
diff --git a/libs/ftl/expected_test.cpp b/libs/ftl/expected_test.cpp
index 8cb07e4696..9b7f017df3 100644
--- a/libs/ftl/expected_test.cpp
+++ b/libs/ftl/expected_test.cpp
@@ -15,8 +15,11 @@
  */
 
 #include <ftl/expected.h>
+#include <ftl/optional.h>
+#include <ftl/unit.h>
 #include <gtest/gtest.h>
 
+#include <cctype>
 #include <string>
 #include <system_error>
 
@@ -74,4 +77,42 @@ TEST(Expected, ValueOpt) {
   }
 }
 
+namespace {
+
+IntExp increment(IntExp exp) {
+  const int i = FTL_TRY(exp);
+  return IntExp(i + 1);
+}
+
+StringExp repeat(StringExp exp) {
+  const std::string str = FTL_TRY(exp);
+  return StringExp(str + str);
+}
+
+void uppercase(char& c, ftl::Optional<char> opt) {
+  c = std::toupper(FTL_TRY(std::move(opt).ok_or(ftl::Unit())));
+}
+
+}  // namespace
+
+// Keep in sync with example usage in header file.
+TEST(Expected, Try) {
+  EXPECT_EQ(IntExp(100), increment(IntExp(99)));
+  EXPECT_TRUE(repeat(ftl::Unexpected(std::errc::value_too_large)).has_error([](std::errc e) {
+    return e == std::errc::value_too_large;
+  }));
+
+  EXPECT_EQ(StringExp("haha"s), repeat(StringExp("ha"s)));
+  EXPECT_TRUE(repeat(ftl::Unexpected(std::errc::bad_message)).has_error([](std::errc e) {
+    return e == std::errc::bad_message;
+  }));
+
+  char c = '?';
+  uppercase(c, std::nullopt);
+  EXPECT_EQ(c, '?');
+
+  uppercase(c, 'a');
+  EXPECT_EQ(c, 'A');
+}
+
 }  // namespace android::test
diff --git a/libs/ftl/optional_test.cpp b/libs/ftl/optional_test.cpp
index 91bf7bc5b6..e7f1f14ad9 100644
--- a/libs/ftl/optional_test.cpp
+++ b/libs/ftl/optional_test.cpp
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <ftl/expected.h>
 #include <ftl/optional.h>
 #include <ftl/static_vector.h>
 #include <ftl/string.h>
@@ -23,6 +24,7 @@
 #include <cstdlib>
 #include <functional>
 #include <numeric>
+#include <system_error>
 #include <utility>
 
 using namespace std::placeholders;
@@ -204,6 +206,19 @@ TEST(Optional, OrElse) {
                      .or_else([] { return Optional(-1); }));
 }
 
+TEST(Optional, OkOr) {
+  using CharExp = ftl::Expected<char, std::errc>;
+  using StringExp = ftl::Expected<std::string, std::errc>;
+
+  EXPECT_EQ(CharExp('z'), Optional('z').ok_or(std::errc::broken_pipe));
+  EXPECT_EQ(CharExp(ftl::Unexpected(std::errc::broken_pipe)),
+            Optional<char>().ok_or(std::errc::broken_pipe));
+
+  EXPECT_EQ(StringExp("abc"s), Optional("abc"s).ok_or(std::errc::protocol_error));
+  EXPECT_EQ(StringExp(ftl::Unexpected(std::errc::protocol_error)),
+            Optional<std::string>().ok_or(std::errc::protocol_error));
+}
+
 // Comparison.
 namespace {