hphp/util/match.h

   1 /*
   2    +----------------------------------------------------------------------+
   3    | HipHop for PHP                                                       |
   4    +----------------------------------------------------------------------+
   5    | Copyright (c) 2010-present Facebook, Inc. (http://www.facebook.com)  |
   6    +----------------------------------------------------------------------+
   7    | This source file is subject to version 3.01 of the PHP license,      |
   8    | that is bundled with this package in the file LICENSE, and is        |
   9    | available through the world-wide-web at the following url:           |
  10    | http://www.php.net/license/3_01.txt                                  |
  11    | If you did not receive a copy of the PHP license and are unable to   |
  12    | obtain it through the world-wide-web, please send a note to          |
  13    | license@php.net so we can mail you a copy immediately.               |
  14    +----------------------------------------------------------------------+
  15 */
  16 #pragma once
  17
  18 #include <type_traits>
  19 #include <utility>
  20
  21 #include <boost/variant.hpp>
  22
  23 #include <folly/DiscriminatedPtr.h>
  24
  25 namespace HPHP {
  26
  27 //////////////////////////////////////////////////////////////////////
  28
  29 /*
  30  * This is a utility for short-hand visitors (using lambdas) with
  31  * boost::apply_visitor.
  32  *
  33  * Usage e.g.:
  34  *
  35  *   match<return_type>(
  36  *     thing,
  37  *     [&] (TypeA a) { ... },
  38  *     [&] (TypeB b) { ... }
  39  *   );
  40  */
  41
  42 //////////////////////////////////////////////////////////////////////
  43
  44 namespace match_detail {
  45
  46 template<class Ret, class... Lambdas> struct visitor;
  47
  48 template<class Ret, class L, class... Lambdas>
  49 struct visitor<Ret,L,Lambdas...> : L, visitor<Ret,Lambdas...> {
  50   using L::operator();
  51   using visitor<Ret,Lambdas...>::operator();
  52   visitor(L l, Lambdas&&... lambdas)
  53     : L(l)
  54     , visitor<Ret,Lambdas...>(std::forward<Lambdas>(lambdas)...)
  55   {}
  56 };
  57
  58 template<class Ret, class L>
  59 struct visitor<Ret,L> : L {
  60   typedef Ret result_type;
  61   using L::operator();
  62   /* implicit */ visitor(L l) : L(l) {}
  63 };
  64
  65 template<class Ret> struct visitor<Ret> {
  66   typedef Ret result_type;
  67   visitor() {}
  68 };
  69
  70 template<class Ret, class... Funcs>
  71 visitor<Ret,Funcs...> make_visitor(Funcs&&... funcs) {
  72   return { std::forward<Funcs>(funcs)... };
  73 }
  74
  75 template <typename T> struct is_variant {
  76   static constexpr bool value = false;
  77 };
  78 template <typename... T> struct is_variant<boost::variant<T...>> {
  79   static constexpr bool value = true;
  80 };
  81
  82 template <typename T> struct is_discriminated_ptr {
  83   static constexpr bool value = false;
  84 };
  85 template <typename... T>
  86 struct is_discriminated_ptr<folly::DiscriminatedPtr<T...>> {
  87   static constexpr bool value = true;
  88 };
  89
  90 }
  91
  92 template<class Ret, class Var, class... Funcs>
  93 typename std::enable_if<
  94   match_detail::is_variant<
  95     typename std::remove_cv<Var>::type
  96     >::value,
  97   Ret>::type
  98 match(Var& v, Funcs&&... funcs) {
  99   return boost::apply_visitor(
 100     match_detail::make_visitor<Ret>(std::forward<Funcs>(funcs)...),
 101     v
 102   );
 103 }
 104
 105 template<class Ret, class Var, class... Funcs>
 106 typename std::enable_if<
 107   match_detail::is_discriminated_ptr<
 108     typename std::remove_cv<Var>::type
 109     >::value,
 110   Ret>::type
 111 match(Var& v, Funcs&&... funcs) {
 112   return v.apply(
 113     match_detail::make_visitor<Ret>(std::forward<Funcs>(funcs)...)
 114   );
 115 }
 116
 117 //////////////////////////////////////////////////////////////////////
 118
 119 }
 120