Maybe
(特别是Maybe链)。例如,从客户那里收到提款请求并获得客户ID...在缓存中查找客户...如果找到客户...查找她的储蓄账户...如果有账户...提款...在此链中的任何时刻,如果查找失败,则不执行任何操作并返回失败。我的链很长...有时长达6个...因此这是我在C++0x中尝试实现
Haskell.Data.Maybe
的方式...(请注意...如果我停止使用可变参数模板,则应该可以在C++中工作)。我已经为接受一个参数的自由函数和不接受参数的成员函数解决了链接问题,并且我对接口感到满意。但是,对于接受多个参数的函数...我必须编写lambda函数来模拟部分应用程序。是否有避免它的方法?请参见main()
的最后一行。即使取消注释,由于const/non-const混合,它也无法编译。但问题仍然存在。抱歉代码很长...我希望这不会让本来可能对此感兴趣的人望而却步...
#include <iostream>
#include <map>
#include <deque>
#include <algorithm>
#include <type_traits>
typedef long long int int64;
namespace monad { namespace maybe {
struct Nothing {};
template < typename T >
struct Maybe {
template < typename U, typename Enable = void >
struct ValueType {
typedef U * const type;
};
template < typename U >
struct ValueType < U, typename std::enable_if < std::is_reference < U >::value >::type > {
typedef typename std::remove_reference < T >::type * const type;
};
typedef typename ValueType < T >::type value_type;
value_type m_v;
Maybe(Nothing const &) : m_v(0) {}
struct Just {
value_type m_v;
Just() = delete;
explicit Just(T &v) : m_v(&v) {
}
};
Maybe(Just const &just) : m_v(just.m_v) {
}
};
Nothing nothing() {
return Nothing();
}
template < typename T >
Maybe < T > just(T &v) {
return typename Maybe < T >::Just(v);
}
template < typename T >
Maybe < T const > just(T const &v) {
return typename Maybe < T const >::Just(v);
}
template < typename T, typename R, typename A >
Maybe < R > operator | (Maybe < T > const &t, R (*f)(A const &)) {
if (t.m_v)
return just < R >(f(*t.m_v));
else
return nothing();
}
template < typename T, typename R, typename A >
Maybe < R > operator | (Maybe < T > const &t, Maybe < R > (*f)(A const &)) {
if (t.m_v)
return f(*t.m_v);
else
return nothing();
}
template < typename T, typename R, typename A >
Maybe < R > operator | (Maybe < T > const &t, R (*f)(A &)) {
if (t.m_v)
return just < R >(f(*t.m_v));
else
return nothing();
}
template < typename T, typename R, typename A >
Maybe < R > operator | (Maybe < T > const &t, Maybe < R > (*f)(A &)) {
if (t.m_v)
return f(*t.m_v);
else
return nothing();
}
template < typename T, typename R, typename... A >
Maybe < R > operator | (Maybe < T const > const &t, R (T::*f)(A const &...) const) {
if (t.m_v)
return just < R >(((*t.m_v).*f)());
else
return nothing();
}
template < typename T, typename R, typename... A >
Maybe < R > operator | (Maybe < T const > const &t, Maybe < R > (T::*f)(A const &...) const) {
if (t.m_v)
return just < R >((t.m_v->*f)());
else
return nothing();
}
template < typename T, typename R, typename... A >
Maybe < R > operator | (Maybe < T const > const &t, R (T::*f)(A const &...)) {
if (t.m_v)
return just < R >(((*t.m_v).*f)());
else
return nothing();
}
template < typename T, typename R, typename... A >
Maybe < R > operator | (Maybe < T const > const &t, Maybe < R > (T::*f)(A const &...)) {
if (t.m_v)
return just < R >((t.m_v->*f)());
else
return nothing();
}
template < typename T, typename A >
void operator | (Maybe < T > const &t, void (*f)(A const &)) {
if (t.m_v)
f(*t.m_v);
}
}}
struct Account {
std::string const m_id;
enum Type { CHECKING, SAVINGS } m_type;
int64 m_balance;
int64 withdraw(int64 const amt) {
if (m_balance < amt)
m_balance -= amt;
return m_balance;
}
std::string const &getId() const {
return m_id;
}
};
std::ostream &operator << (std::ostream &os, Account const &acct) {
os << "{" << acct.m_id << ", "
<< (acct.m_type == Account::CHECKING ? "Checking" : "Savings")
<< ", " << acct.m_balance << "}";
}
struct Customer {
std::string const m_id;
std::deque < Account > const m_accounts;
};
typedef std::map < std::string, Customer > Customers;
using namespace monad::maybe;
Maybe < Customer const > getCustomer(Customers const &customers, std::string const &id) {
auto customer = customers.find(id);
if (customer == customers.end())
return nothing();
else
return just(customer->second);
};
Maybe < Account const > getAccountByType(Customer const &customer, Account::Type const type) {
auto const &accounts = customer.m_accounts;
auto account = std::find_if(accounts.begin(), accounts.end(), [type](Account const &account) -> bool { return account.m_type == type; });
if (account == accounts.end())
return nothing();
else
return just(*account);
}
Maybe < Account const > getCheckingAccount(Customer const &customer) {
return getAccountByType(customer, Account::CHECKING);
};
Maybe < Account const > getSavingsAccount(Customer const &customer) {
return getAccountByType(customer, Account::SAVINGS);
};
int64 const &getBalance(Account const &acct) {
return acct.m_balance;
}
template < typename T >
void print(T const &v) {
std::cout << v << std::endl;
}
int main(int const argc, char const * const argv[]) {
Customers customers = {
{ "12345", { "12345", { { "12345000", Account::CHECKING, 20000 }, { "12345001", Account::SAVINGS, 117000 } } } }
, { "12346", { "12346", { { "12346000", Account::SAVINGS, 1000000 } } } }
};
getCustomer(customers, "12346") | getCheckingAccount | getBalance | &print < int64 const >;
getCustomer(customers, "12345") | getCheckingAccount | getBalance | &print < int64 const >;
getCustomer(customers, "12345") | getSavingsAccount | &Account::getId | &print < std::string const >;
// getCustomer(customers, "12345") | getSavingsAccount | [](Account &acct){ return acct.withdraw(100); } | &print < std::string const >;
}