C++17 zero-overhead syntactic sugar for variant and optional.

• Overview
• Installation/usage
• Documentation
• Resources

std::variant and std::optional were introduced to C++17's Standard Library. They are sum types that can greatly improve type safety and performance.

However, there are some problems with them:

• The syntax of some common operations such as visitation is not as nice as it could be, and requires a significant amount of boilerplate.

• Defining and using recursive variant or optional types is not trivial and requires a lot of boilerplate.

• std::optional doesn't support visitation.

• The interface of std::variant and std::optional is different from some other commonly used ADT implementations - interoperability requires significant boilerplate.

scelta aims to fix all the aformenetioned problems by providing zero-overhead syntactic sugar that:

• Automatically detects and homogenizes all available variant and optional implementations, providing a single implementation-independent interface.

• Provides "pattern matching"-like syntax for visitation and recursive visitation which works both for variant and optional.

• Provides an intuitive placeholder-based recursive variant and optional type definition.

• Provides monadic operations such as map and and_then for optional types, including infix syntax.

scelta detects and works out-of-the-box with:

• std::variant
• boost::variant
• mpark::variant
• eggs::variant
• type_safe::variant
• std::optional
• boost::optional
• type_safe::optional
• tl::optional

Other implementation can be easily adapted by providing specializations of the helper traits structs. PRs are welcome!

scelta provides curried, constexpr-friendly, and SFINAE-friendly visitation utilities both for variant and optional. The final user syntax resembles pattern matching. Recursive data structures are supported.

using shape = std::variant<circle, box>;

shape s0{circle{/*...*/}};
shape s1{box{/*...*/}};

// In place `match` visitation.
scelta::match([](circle, circle){ /* ... */ },
              [](circle, box)   { /* ... */ },
              [](box,    circle){ /* ... */ },
              [](box,    box)   { /* ... */ })(s0, s1);

The match function is intentionally curried in order to allow reuse of a particular visitor in a scope, even on different implementations of variant/optional.

using boost_optstr = boost::optional<std::string>;
using std_optstr = std::optional<std::string>;

// Curried `match` usage.
auto print = scelta::match([](std::string s)    { cout << s;       },
                           [](scelta::nullopt_t){ cout << "empty"; });

boost_optstr s0{/*...*/};
std_optstr s1{/*...*/};

// Implementation-independent visitation.
print(s0);
print(s1);

Recursive variant and optional data structures can be easily created through the use of placeholders.

namespace impl
{
    namespace sr = scelta::recursive;

    // `placeholder` and `builder` can be used to define recursive
    // sum types.
    using _ = sr::placeholder;
    using builder = sr::builder<std::variant<int, std::vector<_>>>;

    // `type` evaluates to the final recursive data structure type.
    using type = sr::type<builder>;

    // `resolve` completely evaluates one of the alternatives.
    // (In this case, even the `Allocator` template parameter is
    // resolved!)
    using vector_type = sr::resolve<builder, std::vector<_>>;
}

using int_tree = impl::type;
using int_tree_vector = impl::vector_type;

After defining recursive structures, in place recursive visitation is also possible. scelta provides two ways of performing recursive visitation:

• scelta::match(/* base cases */)(/* recursive cases */)(/* visitables */)

  This is an "homogeneous" match function that works for both non-recursive and recursive visitation. The first invocation always takes an arbitrary amount of base cases. If recursive cases are provided to the second invocation, then a third invocation with visitables is expected. Unless explicitly provided, the return type is deduced from the base cases.

  The base cases must have arity N, the recursive cases must have arity N + 1. N is the number of visitables that will be provided.

• scelta::recursive::match</* return type */>(/* recursive cases */)(/* visitables */)

  This version always requires an explicit return type and an arbitrary amount of recursive cases with arity N + 1, where N is the number of visitables that will be provided.

int_tree t0{/*...*/};

scelta::match(
    // Base case.
    [](int x){ cout << x; }
)(
    // Recursive case.
    [](auto recurse, int_tree_vector v){ for(auto x : v) recurse(v); }
)(t0);

// ... or ...

scelta::recursive::match<return_type>(
    // Base case.
    [](auto, int x){ cout << x; },

    // Recursive case.
    [](auto recurse, int_tree_vector v){ for(auto x : v) recurse(v); }
)(t0);

scelta provides various monadic operations that work on any supported optional type. Here's an example inspired by Simon Brand's "Functional exceptionless error-handling with optional and expected" article:

optional<image_view> crop_to_cat(image_view);
optional<image_view> add_bow_tie(image_view);
optional<image_view> make_eyes_sparkle(image_view);
image_view make_smaller(image_view);
image_view add_rainbow(image_view);

optional<image_view> get_cute_cat(image_view img)
{
    using namespace scelta::infix;
    return crop_to_cat(img)
         | and_then(add_bow_tie)
         | and_then(make_eyes_sparkle)
         | map(make_smaller)
         | map(add_rainbow);
}

scelta is an header-only library. It is sufficient to include it.

// main.cpp
#include <scelta.hpp>

int main() { return 0; }

g++ -std=c++1z main.cpp -Isome_path/scelta/include

Tests can be easily built and run using CMake.

git clone https://github.com/SuperV1234/scelta && cd scelta
./init-repository.sh # get `vrm_cmake` dependency
mkdir build && cd build

cmake ..
make check # build and run tests

make example_error_handling # error handling via pattern matching
make example_expression     # recursive expression evaluation
make example_optional_cat   # monadic optional operations

All tests currently pass on Arch Linux x64 with:

• g++ (GCC) 8.0.0 20170514 (experimental)

• clang version 5.0.0 (trunk 303617)

1. Add this repository and SuperV1234/vrm_cmake as submodules of your project, in subfolders inside your_project/extlibs/:

   git submodule add   https://github.com/SuperV1234/vrm_cmake.git   your_project/extlibs/vrm_cmake
   git submodule add   https://github.com/SuperV1234/scelta.git      your_project/extlibs/scelta

2. Include vrm_cmake in your project's CMakeLists.txt and look for the scelta extlib:

   # Include `vrm_cmake`:
   list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/extlibs/vrm_cmake/cmake/")
   include(vrm_cmake)
   
   # Find `scelta`:
   vrm_cmake_find_extlib(scelta)

Executes non-recursive visitation.

• Interface:

  template <typename Visitor, typename... Visitables>
  constexpr /*deduced*/ visit(Visitor&& visitor, Visitables&&... visitables)
      noexcept(/*deduced*/);

  • visitables... must all be the same type. (i.e. different implementations of variant/optional currently cannot be mixed together)

  • visitor must be invocable with all the alternatives of the passed visitables.

• Examples:

  struct visitor
  {
      auto operator()(int) { return 0; }
      auto operator()(char){ return 1; }
  };
  
  variant<int, char> v0{'a'};
  assert(
      scelta::nonrecursive::visit(visitor{}, v0) == 1
  );

  struct visitor
  {
      auto operator()(int)              { return 0; }
      auto operator()(scelta::nullopt_t){ return 1; }
  };
  
  optional<int> o0{0};
  assert(
      scelta::nonrecursive::visit(visitor{}, o0) == 0
  );

Executes non-recursive in-place visitation.

• Interface:

  template <typename... FunctionObjects>
  constexpr /*deduced*/ match(FunctionObjects&&... functionObjects)
      noexcept(/*deduced*/)
  {
      return [o = overload(functionObjects...)](auto&&... visitables)
          noexcept(/*deduced*/)
          -> /*deduced*/
      {
          // ... perform visitation with `scelta::nonrecursive::visit` ...
      };
  };

  • Invoking match takes a number of functionObjects... and returns a new function which takes a number of visitables....

  • visitables... must all be the same type. (i.e. different implementations of variant/optional currently cannot be mixed together)

  • o must be invocable with all the alternatives of the passed visitables. (i.e. the overload of all functionObjects... must produce an exhaustive visitor)

• Examples:

  variant<int, char> v0{'a'};
  assert(
      scelta::nonrecursive::match([](int) { return 0; }
                                  [](char){ return 1; })(v0) == 1
  );

  optional<int> o0{0};
  assert(
      scelta::nonrecursive::match([](int)              { return 0; }
                                  [](scelta::nullopt_t){ return 1; })(o0) == 1
  );

Allows placeholder-based definition of recursive ADTs.

• Interface:

  template <typename ADT>
  class builder;
  
  struct placeholder;
  
  template <typename Builder>
  using type = /* ... recursive ADT type wrapper ... */;
  
  template <typename Builder, typename T>
  using resolve = /* ... resolved ADT alternative ... */;

  • builder takes any ADT containing zero or more placeholder alternatives. (i.e. both optional and variant)

  • placeholder is replaced with the recursive ADT itself when using type or resolve.

  • type returns a wrapper around a fully-resolved recursive ADT.

  • resolve returns a fully-resolved alternative contained in ADT.

• Examples:

  using _ = scelta::recursive::placeholder;
  using b = scelta::recursive::builder<variant<int, _*>>;
  
  using recursive_adt = scelta::recursive::type<b>;
  using ptr_alternative = scelta::recursive::resolve<b, _*>;
  
  recursive_adt v0{0};
  recursive_adt v1{&v0};

Executes recursive visitation.

• Interface:

  template <typename Return, typename Visitor, typename... Visitables>
  constexpr Return visit(Visitor&& visitor, Visitables&&... visitables)
      noexcept(false);

  • Similar to scelta::nonrecursive::visit, but requires an explicit return type and is not noexcept-friendly.

  • The operator() overloads of visitor... must take one extra generic argument to receive the recurse helper.

• Examples:

  using _ = scelta::recursive::placeholder;
  using b = scelta::recursive::builder<variant<int, std::vector<_>>>;
  
  using recursive_adt = scelta::recursive::type<b>;
  using rvec = scelta::recursive::resolve<b, std::vector<_>>;
  
  struct visitor
  {
      auto operator()(auto,         int x)  { /* base case */ },
      auto operator()(auto recurse, rvec& v){ for(auto& x : v) recurse(x); }
  };
  
  recursive_adt v0{rvec{recursive_adt{0}, recursive_adt{1}}};
  scelta::recursive::visit(visitor{}, v0};

Executes recursive visitation.

• Interface:

  template <typename Return, typename... FunctionObjects>
  constexpr auto match(FunctionObjects&&... functionObjects)
      noexcept(false)
  {
      return [o = overload(functionObjects...)](auto&&... visitables)
          noexcept(false)
          -> Return
      {
          // ... perform visitation with `scelta::recursive::visit` ...
      };
  };

  • Similar to scelta::nonrecursive::match, but requires an explicit return type and is not noexcept-friendly.

  • The passed functionObjects... must take one extra generic argument to receive the recurse helper.

• Examples:

  using _ = scelta::recursive::placeholder;
  using b = scelta::recursive::builder<variant<int, std::vector<_>>>;
  
  using recursive_adt = scelta::recursive::type<b>;
  using rvec = scelta::recursive::resolve<b, std::vector<_>>;
  
  recursive_adt v0{rvec{recursive_adt{0}, recursive_adt{1}}};
  scelta::recursive::match(
      [](auto,         int x)  { /* base case */ },
      [](auto recurse, rvec& v){ for(auto& x : v) recurse(x); }
  )(v0);

Executes visitation (both non-recursive and recursive). Attempts to deduce the return type from the base cases, optionally supports user-provided explicit return type.

• Interface:

  template <typename Return = impl::deduce_t, typename... BaseCases>
  constexpr auto match(BaseCases&&... baseCases)
  {
      return [bco = overload(adapt(baseCases)...)](auto... xs)
      {
          if constexpr(are_visitables<decltype(xs)...>())
          {
              // ... perform visitation with `scelta::nonrecursive::visit` ...
          }
          else
          {
              return [o = overload(bco, xs...)](auto&&... visitables)
              {
                  // ... perform visitation with `scelta::recursive::visit` ...
              };
          }
      };
  };

  • The first invocation of scelta::match takes one or more base cases. A base case is a function object with the same arity as the number of objects that will be visited.

  • The function returned by the first invocation takes either a number of recursive cases or a number of visitables.

    • Recursive cases are function objects with arity equal to the number of objects that will be visited plus one (the +1 is for the recurse argument).

    • Visitables are variants or optionals. If visitables are passed here, non-recursive visitation will be performed immediately.

  • If recursive cases were passed, the last returned function takes any number of visitables. Recursive visitation will then be performed immediately.

• Examples:

  variant<int, char> v0{'a'};
  assert(
      scelta::match([](int) { return 0; }
                    [](char){ return 1; })(v0) == 1
  );

  using _ = scelta::recursive::placeholder;
  using b = scelta::recursive::builder<variant<int, std::vector<_>>>;
  
  using recursive_adt = scelta::recursive::type<b>;
  using rvec = scelta::recursive::resolve<b, std::vector<_>>;
  
  recursive_adt v0{rvec{recursive_adt{0}, recursive_adt{1}}};
  scelta::match(
      [](int x){ /* base case */ }
  )(
      [](auto recurse, rvec& v){ for(auto& x : v) recurse(x); }
  )(v0);

scelta provides various monadic optional operations. They can be used in two different ways:

optional<int> o{/* ... */};

// Free function syntax:
scelta::map(o, [](int x){ return x + 1; });

// Infix syntax:
o | scelta::infix::map([](int x){ return x + 1; });

These are the available operations:

• map_or_else(o, f_def, f)

  • Returns f(*o) if o is set, f_def() otherwise.

• map_or(o, def, f)

  • Returns f(*o) if o is set, def otherwise.

• map(o, f)

  • Returns optional{f(*o)} if o is set, an empty optional otherwise.

• and_then(o, f)

  • Returns f(*o) if o is set, an empty optional otherwise.

• and_(o, ob)

  • Returns ob if o is set, an empty ob otherwise.

• or_else(o, f)

  • Returns o if o is set, f() otherwise.

• or_(o, def)

  • Returns o if o is set, def otherwise.

The example file example/optional_cat.cpp shows usage of map and and_then using scelta::infix syntax.

• ACCU 2017 talk: "Implementing variant Visitation Using Lambdas"

• C++Now 2017 talk: "Implementing variant visitation using lambdas"

• C++::London (May 2017) talk: "Implementing variant visitation using lambdas"