Re: [boost] [Optional] Monadic bind

Subject: Re: [boost] [Optional] Monadic bind
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2015-06-22 15:15:11

2015-06-22 16:59 GMT+02:00 Tobias Loew <Tobias.Loew_at_[hidden]>:

> I deliberately used this std::bind syntax since it accepts any callable
> (including things like pointer-to-member [functions]).
> I tried lambdas with my code and they just work. E.g.
>
>     auto opt_i3 = map(opt_a, [](const A& a){ return a.i;});
>     assert(*opt_i3 == 42);
>

Interesting. Is it implementable in C++03?

> Furthermore without the std::decay boost::optional<decltype(f(*o))> will
> be a boost::optional<T const&>.
>
> Tobias
>
> Von: Andrzej Krzemienski [via Boost] [mailto:
> ml-node+s2283326n4677436h22_at_[hidden]]
> Gesendet: Montag, 22. Juni 2015 16:44
> An: Löw, Tobias (STEAG Energy Services GmbH)
> Betreff: Re: [Optional] Monadic bind
>
> 2015-06-22 16:17 GMT+02:00 Tobias Loew <[hidden
> email]</user/SendEmail.jtp?type=node&node=4677436&i=0>>:
>
> > Sorry for that,
> >
> > I've made in implementation with free functions which compiles on VS
> 2012:
> >
> >
> > #include <assert.h>
> > #include <boost/optional.hpp>
> >
> >
> > template<class T, class B>
> > auto bind(const boost::optional<T>& optional, B&& binder)
> >     -> typename std::decay<decltype(std::bind(binder,
> *optional)())>::type
> > {
> >     if(optional)
> >     {
> >         return std::bind(binder, *optional)();
> >     }
> >     else
> >     {
> >         return boost::none;
> >     }
> > }
> >
> > template<class T, class B>
> > auto map(const boost::optional<T>& optional, B&& binder)
> >     -> boost::optional<typename std::decay<decltype(std::bind(binder,
> > *optional)())>::type>
> > {
> >     if(optional)
> >     {
> >         return std::bind(binder, *optional)();
> >     }
> >     else
> >     {
> >         return boost::none;
> >     }
> > }
> >
> >
> > void foo()
> > {
> >     struct A {
> >         int i;
> >     };
> >     boost::optional<A> opt_a;
> >
> >     auto opt_i = map(opt_a, &A::i);
> >     assert(!opt_i);
> >
> >     A a = {42};
> >     opt_a = a;
> >
> >     auto opt_i2 = map(opt_a, &A::i);
> >     assert(*opt_i2 == 42);
> >
> > }
> >
>
> Would you find the following an acceptable solution?
>
> ```
> #include <assert.h>
> #include <boost/bind.hpp>
> #include <boost/optional.hpp>
>
> template <typename T, typename F>
> auto map2(const boost::optional<T>& o, F f)
>   -> boost::optional<decltype(f(*o))>
> {
>   if (o) return f(*o);
>   else   return boost::none;
> }
>
> void foo()
> {
>     struct A {  int i; };
>     boost::optional<A> opt_a;
>
>     auto opt_i = map2(opt_a, boost::bind(&A::i, _1));
>     assert(!opt_i);
>
>     A a = {42};
>     opt_a = a;
>
>     auto opt_i2 = map2(opt_a, boost::bind(&A::i, _1));
>     assert(opt_i2);
>     assert(*opt_i2 == 42);
> }
>
> int main()
> {
>     foo();
> }
> ```
>
> It requires of you to type a bit more, but at the same time it is more
> flexible: you can type any free function on T, including a lambda.
>
> Regards,
> &rzej
>
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