uncurry

Summary: Extending ApplyTuple to support this common functional construct, with good forwarding semantics.

Reviewed By: yfeldblum

Differential Revision: D4787560

fbshipit-source-id: 2c740e448e0cb916abe948b79709d5ecd8ba54bb

folly/ApplyTuple.h

@@ -113,5 +113,52 @@ inline constexpr auto applyTuple(F&& f, Tuples&&... t)
       detail::apply_tuple::MakeIndexSequenceFromTuple<Tuples...>{});
 }
 
+namespace detail {
+namespace apply_tuple {
+
+template <class F>
+class Uncurry {
+ public:
+  explicit Uncurry(F&& func) : func_(std::move(func)) {}
+  explicit Uncurry(const F& func) : func_(func) {}
+
+  template <class Tuple>
+  auto operator()(Tuple&& tuple) const
+      -> decltype(applyTuple(std::declval<F>(), std::forward<Tuple>(tuple))) {
+    return applyTuple(func_, std::forward<Tuple>(tuple));
+  }
+
+ private:
+  F func_;
+};
+} // namespace apply_tuple
+} // namespace detail
+
+/**
+ * Wraps a function taking N arguments into a function which accepts a tuple of
+ * N arguments. Note: This function will also accept an std::pair if N == 2.
+ *
+ * For example, given the below code:
+ *
+ *    std::vector<std::tuple<int, int, int>> rows = ...;
+ *    auto test = [](std::tuple<int, int, int>& row) {
+ *      return std::get<0>(row) * std::get<1>(row) * std::get<2>(row) == 24;
+ *    };
+ *    auto found = std::find_if(rows.begin(), rows.end(), test);
+ *
+ *
+ * 'test' could be rewritten as:
+ *
+ *    auto test =
+ *        folly::uncurry([](int a, int b, int c) { return a * b * c == 24; });
+ *
+ */
+template <class F>
+auto uncurry(F&& f)
+    -> detail::apply_tuple::Uncurry<typename std::decay<F>::type> {
+  return detail::apply_tuple::Uncurry<typename std::decay<F>::type>(
+      std::forward<F>(f));
+}
+
 //////////////////////////////////////////////////////////////////////
 }

folly/gen/test/StringTest.cpp

@@ -18,6 +18,7 @@
 #include <map>
 #include <vector>
 
+#include <folly/ApplyTuple.h>
 #include <folly/gen/String.h>
 #include <folly/portability/GTest.h>
 
@@ -360,3 +361,19 @@ TEST(StringGen, Batch) {
   EXPECT_EQ(lines, from(chunks) | resplit('\n') | eachTo<std::string>() |
                        batch(3) | rconcat | as<vector>());
 }
+
+TEST(StringGen, UncurryTuple) {
+  folly::StringPiece file = "1\t2\t3\n1\t4\t9";
+  auto rows = split(file, '\n') | eachToTuple<int, int, int>('\t');
+  auto productSum =
+      rows | map(uncurry([](int x, int y, int z) { return x * y * z; })) | sum;
+  EXPECT_EQ(42, productSum);
+}
+
+TEST(StringGen, UncurryPair) {
+  folly::StringPiece file = "2\t3\n4\t9";
+  auto rows = split(file, '\n') | eachToPair<int, int>('\t');
+  auto productSum =
+      rows | map(uncurry([](int x, int y) { return x * y; })) | sum;
+  EXPECT_EQ(42, productSum);
+}

folly/test/ApplyTupleTest.cpp

@@ -314,3 +314,59 @@ TEST(ApplyTuple, MultipleTuples) {
       folly::applyTuple(
           add, std::make_tuple(1), std::make_tuple(), std::make_tuple(2, 3)));
 }
+
+TEST(ApplyTuple, UncurryCopyMove) {
+  std::string separator = "================================\n";
+  auto formatRow = folly::uncurry([=](std::string a, std::string b) {
+    // capture separator by copy
+    return separator + a + "\n" + b + "\n" + separator;
+  });
+  auto row = std::make_tuple("hello", "world");
+  auto expected = separator + "hello\nworld\n" + separator;
+  EXPECT_EQ(expected, formatRow(row));
+  auto formatRowCopy = formatRow;
+  EXPECT_EQ(expected, formatRowCopy(row));
+  auto formatRowMove = std::move(formatRow);
+  EXPECT_EQ(expected, formatRowMove(row));
+
+  // capture value moved out from formatRow
+  EXPECT_NE(expected, formatRow(row));
+}
+
+TEST(ApplyTuple, Uncurry) {
+  EXPECT_EQ(42, folly::uncurry([](int x, int y) {
+              return x * y;
+            })(std::pair<int, int>(6, 7)));
+  EXPECT_EQ(42, folly::uncurry([](int&& x, int&& y) {
+              return x * y;
+            })(std::pair<int&&, int&&>(6, 7)));
+  EXPECT_EQ(42, folly::uncurry([](int&& x, int&& y) {
+              return x * y;
+            })(std::pair<int&&, int&&>(6, 7)));
+
+  std::string long1 = "a long string exceeding small string size";
+  std::string long2 = "and here is another one!";
+  std::string expected = long1 + long2;
+
+  auto cat = folly::uncurry(
+      [](std::string a, std::string b) { return std::move(a) + std::move(b); });
+
+  EXPECT_EQ(expected, cat(std::make_pair(long1, long2)));
+  EXPECT_FALSE(long1.empty());
+  EXPECT_FALSE(long2.empty());
+  EXPECT_EQ(expected, cat(std::tie(long1, long2)));
+  EXPECT_FALSE(long1.empty());
+  EXPECT_FALSE(long2.empty());
+  EXPECT_EQ(
+      expected, cat(std::forward_as_tuple(std::move(long1), std::move(long2))));
+  EXPECT_TRUE(long1.empty());
+  EXPECT_TRUE(long2.empty());
+}
+
+TEST(ApplyTuple, UncurryStdFind) {
+  std::vector<std::pair<int, int>> v{{1, 9}, {2, 8}, {3, 7}, {4, 6}, {5, 5}};
+  EXPECT_EQ(
+      3, std::count_if(v.begin(), v.end(), folly::uncurry([](int a, int b) {
+                         return b % a == 0;
+                       })));
+}