folly/folly/gen/Base-inl.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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 FOLLY_GEN_BASE_H_
#error This file may only be included from folly/gen/Base.h
#endif

#include <folly/Function.h>
#include <folly/Portability.h>
#include <folly/container/F14Map.h>
#include <folly/container/F14Set.h>
#include <folly/functional/Invoke.h>

// inner condition from:
// https://github.com/ericniebler/range-v3/blob/0.11.0/include/range/v3/detail/config.hpp#L222
#define FOLLY_DETAIL_GEN_BASE_HAS_RANGEV3 …

#if FOLLY_DETAIL_GEN_BASE_HAS_RANGEV3
#include <range/v3/view/filter.hpp>
#include <range/v3/view/transform.hpp>
#endif

// Ignore shadowing warnings within this file, so includers can use -Wshadow.
FOLLY_PUSH_WARNING
FOLLY_GNU_DISABLE_WARNING(…)

namespace folly {
namespace gen {

/**
 * ArgumentReference - For determining ideal argument type to receive a value.
 */
template <class T>
struct ArgumentReference : public std::conditional<
                               std::is_reference<T>::value,
                               T, // T& -> T&, T&& -> T&&, const T& -> const T&
                               typename std::conditional<
                                   std::is_const<T>::value,
                                   T&, // const int -> const int&
                                   T&& // int -> int&&
                                   >::type> { … };

/**
 * Group - The output objects from the GroupBy operator
 */
template <class Key, class Value>
class Group : public GenImpl<Value&&, Group<Key, Value>> { … };

namespace detail {

// Classes used for the implementation of Sources, Operators, and Sinks

/*
 ******************************* Sources ***************************************
 */

/*
 * ReferencedSource - Generate values from an STL-like container using
 * iterators from .begin() until .end(). Value type defaults to the type of
 * *container->begin(). For std::vector<int>, this would be int&. Note that the
 * value here is a reference, so the values in the vector will be passed by
 * reference to downstream operators.
 *
 * This type is primarily used through the 'from' helper method, like:
 *
 *   string& longestName = from(names)
 *                       | maxBy([](string& s) { return s.size() });
 */
template <class Container, class Value>
class ReferencedSource
    : public GenImpl<Value, ReferencedSource<Container, Value>> { … };

/**
 * CopiedSource - For producing values from eagerly from a sequence of values
 * whose storage is owned by this class. Useful for preparing a generator for
 * use after a source collection will no longer be available, or for when the
 * values are specified literally with an initializer list.
 *
 * This type is primarily used through the 'fromCopy' function, like:
 *
 *   auto sourceCopy = fromCopy(makeAVector());
 *   auto sum = sourceCopy | sum;
 *   auto max = sourceCopy | max;
 *
 * Though it is also used for the initializer_list specialization of from().
 */
template <class StorageType, class Container>
class CopiedSource
    : public GenImpl<const StorageType&, CopiedSource<StorageType, Container>> { … };

/**
 * RangeSource - For producing values from a folly::Range. Useful for referring
 * to a slice of some container.
 *
 * This type is primarily used through the 'from' function, like:
 *
 *   auto rangeSource = from(folly::range(v.begin(), v.end()));
 *   auto sum = rangeSource | sum;
 *
 * Reminder: Be careful not to invalidate iterators when using ranges like this.
 */
template <class Iterator>
class RangeSource : public GenImpl<
                        typename Range<Iterator>::reference,
                        RangeSource<Iterator>> { … };

/**
 * Sequence - For generating values from beginning value, incremented along the
 * way with the ++ and += operators. Iteration may continue indefinitely.
 * Value type specified explicitly.
 *
 * This type is primarily used through the 'seq' and 'range' function, like:
 *
 *   int total = seq(1, 10) | sum;
 *   auto indexes = range(0, 10);
 *   auto endless = seq(0); // 0, 1, 2, 3, ...
 */
template <class Value, class SequenceImpl>
class Sequence : public GenImpl<const Value&, Sequence<Value, SequenceImpl>> { … };

/**
 * Sequence implementations (range, sequence, infinite, with/without step)
 **/
template <class Value>
class RangeImpl { … };

template <class Value, class Distance>
class RangeWithStepImpl { … };

template <class Value>
class SeqImpl { … };

template <class Value, class Distance>
class SeqWithStepImpl { … };

template <class Value>
class InfiniteImpl { … };

/**
 * GenratorBuilder - Helper for GENERTATOR macro.
 **/
template <class Value>
struct GeneratorBuilder { … };

/**
 * Yield - For producing values from a user-defined generator by way of a
 * 'yield' function.
 **/
template <class Value, class Source>
class Yield : public GenImpl<Value, Yield<Value, Source>> { … };

template <class Value>
class Empty : public GenImpl<Value, Empty<Value>> { … };

template <class Value>
class SingleReference : public GenImpl<Value&, SingleReference<Value>> { … };

template <class Value>
class SingleCopy : public GenImpl<const Value&, SingleCopy<Value>> { … };

/*
 ***************************** Operators ***************************************
 */

/**
 * Map - For producing a sequence of values by passing each value from a source
 * collection through a predicate.
 *
 * This type is usually used through the 'map' or 'mapped' helper function:
 *
 *   auto squares = seq(1, 10) | map(square) | as<std::vector>();
 */
template <class Predicate>
class Map : public Operator<Map<Predicate>> { … };

/**
 * Filter - For filtering values from a source sequence by a predicate.
 *
 * This type is usually used through the 'filter' helper function, like:
 *
 *   auto nonEmpty = from(strings)
 *                 | filter([](const string& str) -> bool {
 *                     return !str.empty();
 *                   });
 *
 * Note that if no predicate is provided, the values are casted to bool and
 * filtered based on that. So if pointers is a vector of pointers,
 *
 *   auto nonNull = from(pointers) | filter();
 *
 * will give a vector of all the pointers != nullptr.
 */
template <class Predicate>
class Filter : public Operator<Filter<Predicate>> { … };

/**
 * Until - For producing values from a source until a predicate is satisfied.
 *
 * This type is usually used through the 'until' helper function, like:
 *
 *   auto best = from(sortedItems)
 *             | until([](Item& item) { return item.score > 100; })
 *             | as<std::vector>();
 */
template <class Predicate>
class Until : public Operator<Until<Predicate>> { … };

/**
 * Take - For producing up to N values from a source.
 *
 * This type is usually used through the 'take' helper function, like:
 *
 *   auto best = from(docs)
 *             | orderByDescending(scoreDoc)
 *             | take(10);
 */
class Take : public Operator<Take> { … };

/**
 * Visit - For calling a function on each item before passing it down the
 * pipeline.
 *
 * This type is usually used through the 'visit' helper function:
 *
 *   auto printedValues = seq(1) | visit(debugPrint);
 *   // nothing printed yet
 *   auto results = take(10) | as<std::vector>();
 *   // results now populated, 10 values printed
 */
template <class Visitor>
class Visit : public Operator<Visit<Visitor>> { … };

/**
 * Stride - For producing every Nth value from a source.
 *
 * This type is usually used through the 'stride' helper function, like:
 *
 *   auto half = from(samples)
 *             | stride(2);
 */
class Stride : public Operator<Stride> { … };

/**
 * Sample - For taking a random sample of N elements from a sequence
 * (without replacement).
 */
template <class Random>
class Sample : public Operator<Sample<Random>> { … };

/**
 * Skip - For skipping N items from the beginning of a source generator.
 *
 * This type is usually used through the 'skip' helper function, like:
 *
 *   auto page = from(results)
 *             | skip(pageSize * startPage)
 *             | take(10);
 */
class Skip : public Operator<Skip> { … };

/**
 * Order - For ordering a sequence of values from a source by key.
 * The key is extracted by the given selector functor, and this key is then
 * compared using the specified comparator.
 *
 * This type is usually used through the 'order' helper function, like:
 *
 *   auto closest = from(places)
 *                | orderBy([](Place& p) {
 *                    return -distance(p.location, here);
 *                  })
 *                | take(10);
 */
template <class Selector, class Comparer>
class Order : public Operator<Order<Selector, Comparer>> { … };

/**
 * GroupBy - Group values by a given key selector, producing a sequence of
 * groups.
 *
 * This type is usually used through the 'groupBy' helper function, like:
 *
 *   auto bests
 *     = from(places)
 *     | groupBy([](const Place& p) {
 *         return p.city;
 *       })
 *     | [](Group<std::string, Place>&& g) {
 *         cout << g.key() << ": " << (g | first).description;
 *       };
 */
template <class Selector>
class GroupBy : public Operator<GroupBy<Selector>> { … };

/**
 * GroupByAdjacent - Group adjacent values by a given key selector, producing a
 * sequence of groups. This differs from GroupBy in that only contiguous sets
 * of values with the same key are considered part of the same group. Unlike
 * GroupBy, this can be used on infinite sequences.
 *
 * This type is usually used through the 'groupByAdjacent' helper function:
 *
 *   auto tens
 *     = seq(0)
 *     | groupByAdjacent([](int i){ return (i / 10) % 2; })
 *
 * This example results in a list like [ 0:[0-9], 1:[10-19], 0:[20-29], ... ]
 */
template <class Selector>
class GroupByAdjacent : public Operator<GroupByAdjacent<Selector>> { … };

/*
 * TypeAssertion - For verifying the exact type of the value produced by a
 * generator. Useful for testing and debugging, and acts as a no-op at runtime.
 * Pass-through at runtime. Used through the 'assert_type<>()' factory method
 * like so:
 *
 *   auto c =  from(vector) | assert_type<int&>() | sum;
 *
 */
template <class Expected>
class TypeAssertion : public Operator<TypeAssertion<Expected>> { … };

/**
 * Distinct - For filtering duplicates out of a sequence. A selector may be
 * provided to generate a key to uniquify for each value.
 *
 * This type is usually used through the 'distinct' helper function, like:
 *
 *   auto closest = from(results)
 *                | distinctBy([](Item& i) {
 *                    return i.target;
 *                  })
 *                | take(10);
 */
template <class Selector>
class Distinct : public Operator<Distinct<Selector>> { … };

/**
 * Composer - Helper class for adapting pipelines into functors. Primarily used
 * for 'mapOp'.
 */
template <class Operators>
class Composer { … };

/**
 * Batch - For producing fixed-size batches of each value from a source.
 *
 * This type is usually used through the 'batch' helper function:
 *
 *   auto batchSums
 *     = seq(1, 10)
 *     | batch(3)
 *     | map([](const std::vector<int>& batch) {
 *         return from(batch) | sum;
 *       })
 *     | as<vector>();
 */
class Batch : public Operator<Batch> { … };

/**
 * Window - For overlapping the lifetimes of pipeline values, especially with
 * Futures.
 *
 * This type is usually used through the 'window' helper function:
 *
 *   auto responses
 *     = byLine(STDIN)
 *     | map(makeRequestFuture)
 *     | window(1000)
 *     | map(waitFuture)
 *     | as<vector>();
 */
class Window : public Operator<Window> { … };

/**
 * Concat - For flattening generators of generators.
 *
 * This type is usually used through the 'concat' static value, like:
 *
 *   auto edges =
 *       from(nodes)
 *     | map([](Node& x) {
 *           return from(x.neighbors)
 *                | map([&](Node& y) {
 *                    return Edge(x, y);
 *                  });
 *         })
 *     | concat
 *     | as<std::set>();
 */
class Concat : public Operator<Concat> { … };

/**
 * RangeConcat - For flattening generators of iterables.
 *
 * This type is usually used through the 'rconcat' static value, like:
 *
 *   map<int, vector<int>> adjacency;
 *   auto sinks =
 *       from(adjacency)
 *     | get<1>()
 *     | rconcat()
 *     | as<std::set>();
 */
class RangeConcat : public Operator<RangeConcat> { … };

/**
 * GuardImpl - For handling exceptions from downstream computation. Requires the
 * type of exception to catch, and handler function to invoke in the event of
 * the exception. Note that the handler may:
 *   1) return true to continue processing the sequence
 *   2) return false to end the sequence immediately
 *   3) throw, to pass the exception to the next catch
 * The handler must match the signature 'bool(Exception&, Value)'.
 *
 * This type is used through the `guard` helper, like so:
 *
 *  auto indexes
 *    = byLine(STDIN_FILENO)
 *    | guard<std::runtime_error>([](std::runtime_error& e,
 *                                   StringPiece sp) {
 *        LOG(ERROR) << sp << ": " << e.str();
 *        return true; // continue processing subsequent lines
 *      })
 *    | eachTo<int>()
 *    | as<vector>();
 *
 *  KNOWN ISSUE: This only guards pipelines through operators which do not
 *  retain resulting values. Exceptions thrown after operators like pmap, order,
 *  batch, cannot be caught from here.
 **/
template <class Exception, class ErrorHandler>
class GuardImpl : public Operator<GuardImpl<Exception, ErrorHandler>> { … };

/**
 * Dereference - For dereferencing a sequence of pointers while filtering out
 * null pointers.
 *
 * This type is usually used through the 'dereference' static value, like:
 *
 *   auto refs = from(ptrs) | dereference;
 */
class Dereference : public Operator<Dereference> { … };

/**
 * Indirect - For producing a sequence of the addresses of the values in the
 * input.
 *
 * This type is usually used through the 'indirect' static value, like:
 *
 *   auto ptrs = from(refs) | indirect;
 */
class Indirect : public Operator<Indirect> { … };

/**
 * Cycle - For repeating a sequence forever.
 *
 * This type is usually used through the 'cycle' static value, like:
 *
 *   auto tests
 *     = from(samples)
 *     | cycle
 *     | take(100);
 *
 * or in the finite case:
 *
 *   auto thrice = g | cycle(3);
 */
template <bool forever>
class Cycle : public Operator<Cycle<forever>> { … };

/*
 ******************************* Sinks *****************************************
 */

/**
 * FoldLeft - Left-associative functional fold. For producing an aggregate value
 * from a seed and a folder function. Useful for custom aggregators on a
 * sequence.
 *
 * This type is primarily used through the 'foldl' helper method, like:
 *
 *   double movingAverage = from(values)
 *                        | foldl(0.0, [](double avg, double sample) {
 *                            return sample * 0.1 + avg * 0.9;
 *                          });
 */
template <class Seed, class Fold>
class FoldLeft : public Operator<FoldLeft<Seed, Fold>> { … };

/**
 * First - For finding the first value in a sequence.
 *
 * This type is primarily used through the 'first' static value, like:
 *
 *   int firstThreeDigitPrime = seq(100) | filter(isPrime) | first;
 */
class First : public Operator<First> { … };

/**
 * IsEmpty - a helper class for isEmpty and notEmpty
 *
 * Essentially returns 'result' if the source is empty. Note that this cannot be
 * called on an infinite source, because then there is only one possible return
 * value.
 *
 *
 *  Used primarily through 'isEmpty' and 'notEmpty' static values
 *
 *  bool hasPrimes = g | filter(prime) | notEmpty;
 *  bool lacksEvens = g | filter(even) | isEmpty;
 *
 *  Also used in the implementation of 'any' and 'all'
 */
template <bool emptyResult>
class IsEmpty : public Operator<IsEmpty<emptyResult>> { … };

/**
 * Reduce - Functional reduce, for recursively combining values from a source
 * using a reducer function until there is only one item left. Useful for
 * combining values when an empty sequence doesn't make sense.
 *
 * This type is primarily used through the 'reduce' helper method, like:
 *
 *   sring longest = from(names)
 *                 | reduce([](string&& best, string& current) {
 *                     return best.size() >= current.size() ? best : current;
 *                   });
 */
template <class Reducer>
class Reduce : public Operator<Reduce<Reducer>> { … };

/**
 * Count - for simply counting the items in a collection.
 *
 * This type is usually used through its singleton, 'count':
 *
 *   auto shortPrimes = seq(1, 100) | filter(isPrime) | count;
 */
class Count : public Operator<Count> { … };

/**
 * Sum - For simply summing up all the values from a source.
 *
 * This type is usually used through its singleton, 'sum':
 *
 *   auto gaussSum = seq(1, 100) | sum;
 */
class Sum : public Operator<Sum> { … };

/**
 * Contains - For testing whether a value matching the given value is contained
 * in a sequence.
 *
 * This type should be used through the 'contains' helper method, like:
 *
 *   bool contained = seq(1, 10) | map(square) | contains(49);
 */
template <class Needle>
class Contains : public Operator<Contains<Needle>> { … };

/**
 * Min - For a value which minimizes a key, where the key is determined by a
 * given selector, and compared by given comparer.
 *
 * This type is usually used through the singletone 'min' or through the helper
 * functions 'minBy' and 'maxBy'.
 *
 *   auto oldest = from(people)
 *               | minBy([](Person& p) {
 *                   return p.dateOfBirth;
 *                 });
 */
template <class Selector, class Comparer>
class Min : public Operator<Min<Selector, Comparer>> { … };

/**
 * Append - For collecting values from a source into a given output container
 * by appending.
 *
 * This type is usually used through the helper function 'appendTo', like:
 *
 *   vector<int64_t> ids;
 *   from(results) | map([](Person& p) { return p.id })
 *                 | appendTo(ids);
 */
template <class Collection>
class Append : public Operator<Append<Collection>> { … };

/**
 * Collect - For collecting values from a source in a collection of the desired
 * type.
 *
 * This type is usually used through the helper function 'as', like:
 *
 *   std::string upper = from(stringPiece)
 *                     | map(&toupper)
 *                     | as<std::string>();
 */
template <class Collection>
class Collect : public Operator<Collect<Collection>> { … };

/**
 * CollectTemplate - For collecting values from a source in a collection
 * constructed using the specified template type. Given the type of values
 * produced by the given generator, the collection type will be:
 *   Container<Value, Allocator<Value>>
 *
 * The allocator defaults to std::allocator, so this may be used for the STL
 * containers by simply using operators like 'as<set>', 'as<deque>',
 * 'as<vector>'. 'as', here is the helper method which is the usual means of
 * constructing this operator.
 *
 * Example:
 *
 *   set<string> uniqueNames = from(names) | as<set>();
 */
template <
    template <class, class>
    class Container,
    template <class>
    class Allocator>
class CollectTemplate : public Operator<CollectTemplate<Container, Allocator>> { … };

/**
 * UnwrapOr - For unwrapping folly::Optional values, or providing the given
 * fallback value. Usually used through the 'unwrapOr' helper like so:
 *
 *   auto best = from(scores) | max | unwrapOr(-1);
 *
 * Note that the fallback value needn't match the value in the Optional it is
 * unwrapping. If mis-matched types are supported, the common type of the two is
 * returned by value. If the types match, a reference (T&& > T& > const T&) is
 * returned.
 */
template <class T>
class UnwrapOr { … };

template <class T>
T&& operator|(Optional<T>&& opt, UnwrapOr<T>&& fallback) { … }

template <class T>
T& operator|(Optional<T>& opt, UnwrapOr<T>& fallback) { … }

template <class T>
const T& operator|(const Optional<T>& opt, const UnwrapOr<T>& fallback) { … }

// Mixed type unwrapping always returns values, moving where possible
template <
    class T,
    class U,
    class R = typename std::enable_if<
        !std::is_same<T, U>::value,
        typename std::common_type<T, U>::type>::type>
R operator|(Optional<T>&& opt, UnwrapOr<U>&& fallback) { … }

template <
    class T,
    class U,
    class R = typename std::enable_if<
        !std::is_same<T, U>::value,
        typename std::common_type<T, U>::type>::type>
R operator|(const Optional<T>& opt, UnwrapOr<U>&& fallback) { … }

template <
    class T,
    class U,
    class R = typename std::enable_if<
        !std::is_same<T, U>::value,
        typename std::common_type<T, U>::type>::type>
R operator|(Optional<T>&& opt, const UnwrapOr<U>& fallback) { … }

template <
    class T,
    class U,
    class R = typename std::enable_if<
        !std::is_same<T, U>::value,
        typename std::common_type<T, U>::type>::type>
R operator|(const Optional<T>& opt, const UnwrapOr<U>& fallback) { … }

/**
 * Unwrap - For unwrapping folly::Optional values in a folly::gen style. Usually
 * used through the 'unwrap' instace like so:
 *
 *   auto best = from(scores) | max | unwrap; // may throw
 */
class Unwrap { … };

template <class T>
T&& operator|(Optional<T>&& opt, const Unwrap&) { … }

template <class T>
T& operator|(Optional<T>& opt, const Unwrap&) { … }

template <class T>
const T& operator|(const Optional<T>& opt, const Unwrap&) { … }

class ToVirtualGen : public Operator<ToVirtualGen> { … };

#if FOLLY_DETAIL_GEN_BASE_HAS_RANGEV3
template <class RangeV3, class Value>
class RangeV3Source
    : public gen::GenImpl<Value, RangeV3Source<RangeV3, Value>> {
  mutable RangeV3 r_; // mutable since some ranges are not const-iteratable

 public:
  explicit RangeV3Source(RangeV3 const& r) : r_(r) {}

  template <class Body>
  void foreach(Body&& body) const {
    for (auto const& value : r_) {
      body(value);
    }
  }

  template <class Handler>
  bool apply(Handler&& handler) const {
    for (auto const& value : r_) {
      if (!handler(value)) {
        return false;
      }
    }
    return true;
  }

  static constexpr bool infinite = false;
};

template <class RangeV3, class Value>
class RangeV3CopySource
    : public gen::GenImpl<Value, RangeV3CopySource<RangeV3, Value>> {
  mutable RangeV3 r_; // mutable since some ranges are not const-iteratable

 public:
  explicit RangeV3CopySource(RangeV3&& r) : r_(std::move(r)) {}

  template <class Body>
  void foreach(Body&& body) const {
    for (auto const& value : r_) {
      body(value);
    }
  }

  template <class Handler>
  bool apply(Handler&& handler) const {
    for (auto const& value : r_) {
      if (!handler(value)) {
        return false;
      }
    }
    return true;
  }

  static constexpr bool infinite = false;
};

struct from_container_fn {
  template <typename Container>
  friend auto operator|(Container&& c, from_container_fn) {
    return gen::from(std::forward<Container>(c));
  }
};

struct from_rangev3_fn {
  template <typename Range>
  friend auto operator|(Range&& r, from_rangev3_fn) {
    using DecayedRange = std::decay_t<Range>;
    using DecayedValue = std::decay_t<decltype(*r.begin())>;
    return RangeV3Source<DecayedRange, DecayedValue>(r);
  }
};

struct from_rangev3_copy_fn {
  template <typename Range>
  friend auto operator|(Range&& r, from_rangev3_copy_fn) {
    using RangeDecay = std::decay_t<Range>;
    using Value = std::decay_t<decltype(*r.begin())>;
    return RangeV3CopySource<RangeDecay, Value>(std::move(r));
  }
};
#endif // FOLLY_DETAIL_GEN_BASE_HAS_RANGEV3
} // namespace detail

#if FOLLY_DETAIL_GEN_BASE_HAS_RANGEV3
/*
 ******************************************************************************
 * Pipe fittings between a container/range-v3 and a folly::gen.
 * Example: vec | gen::from_container | folly::gen::filter(...);
 * Example: vec | ranges::views::filter(...) | gen::from_rangev3 | gen::xxx;
 ******************************************************************************
 */
constexpr detail::from_container_fn from_container;
constexpr detail::from_rangev3_fn from_rangev3;
constexpr detail::from_rangev3_copy_fn from_rangev3_copy;

template <typename Range>
auto from_rangev3_call(Range&& r) {
  using Value = std::decay_t<decltype(*r.begin())>;
  return detail::RangeV3Source<Range, Value>(r);
}

// it is safe to pipe an rvalue into a range-v3 view if the rest of the pipeline
// will finish its traversal within the current full-expr, a condition provided
// by folly::gen.
template <typename Range>
auto rangev3_will_be_consumed(Range&& r) {
  // intentionally use `r` instead of `std::forward<Range>(r)`; see above.
  // range-v3 ranges copy in O(1) so it is appropriate.
  return ::ranges::views::all(r);
}
#endif // FOLLY_DETAIL_GEN_BASE_HAS_RANGEV3

/**
 * VirtualGen<T> - For wrapping template types in simple polymorphic wrapper.
 **/
template <class Value, class Self>
class VirtualGenBase : public GenImpl<Value, Self> { … };

template <class Value>
class VirtualGen : public VirtualGenBase<Value, VirtualGen<Value>> { … };

template <class Value>
class VirtualGenMoveOnly
    : public VirtualGenBase<Value, VirtualGenMoveOnly<Value>> { … };

/**
 * non-template operators, statically defined to avoid the need for anything but
 * the header.
 */
constexpr detail::Sum sum{ … };

constexpr detail::Count count{ … };

constexpr detail::First first{ … };

constexpr detail::IsEmpty<true> isEmpty{ … };

constexpr detail::IsEmpty<false> notEmpty{ … };

constexpr detail::Min<Identity, Less> min{ … };

constexpr detail::Min<Identity, Greater> max{ … };

constexpr detail::Order<Identity> order{ … };

constexpr detail::Distinct<Identity> distinct{ … };

constexpr detail::Map<Move> move{ … };

constexpr detail::Concat concat{ … };

constexpr detail::RangeConcat rconcat{ … };

constexpr detail::Cycle<true> cycle{ … };

constexpr detail::Dereference dereference{ … };

constexpr detail::Indirect indirect{ … };

constexpr detail::Unwrap unwrap{ … };

constexpr detail::ToVirtualGen virtualize{ … };

template <class Number>
inline detail::Take take(Number count) { … }

inline detail::Stride stride(size_t s) { … }

template <class Random = std::default_random_engine>
inline detail::Sample<Random> sample(size_t count, Random rng = Random()) { … }

inline detail::Skip skip(size_t count) { … }

inline detail::Batch batch(size_t batchSize) { … }

inline detail::Window window(size_t windowSize) { … }

} // namespace gen
} // namespace folly

FOLLY_POP_WARNING