Re: [boost] [Optional] Monadic bind

Subject: Re: [boost] [Optional] Monadic bind
From: Tobias Loew (Tobias.Loew_at_[hidden])
Date: 2015-06-22 10:17:13

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);

}

Von: Andrzej Krzemienski [via Boost] [mailto:ml-node+s2283326n4677432h98_at_[hidden]]
Gesendet: Montag, 22. Juni 2015 16:02
An: Löw, Tobias (STEAG Energy Services GmbH)
Betreff: Re: [Optional] Monadic bind

2015-06-22 15:49 GMT+02:00 Tobias Loew <[hidden email]</user/SendEmail.jtp?type=node&node=4677432&i=0>>:

> While playing around with boost::optionals I had code like the following
>
> struct A {
> ...
> int i;
> ...
> };
>
> boost::optional  a;
> // do something with 'a'
>
> // optional-projection to a::i
> boost::optional<int> a_i;
> if(a)
>     a_i = a->i;
>
>
Could you give us an example here that would compile?

boost::optional  a;
The above is not a valid variable declaration (template argument missing),
so it is difficult for me to gather what you are trying to do.

Regards,
&rzej

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