Sorry for that, I've made in implementation with free functions which compiles on VS 2012: #include #include template auto bind(const boost::optional<T>& optional, B&& binder) -> typename std::decay::type { if(optional) { return std::bind(binder, *optional)(); } else { return boost::none; } } template auto map(const boost::optional<T>& optional, B&& binder) -> boost::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@n4.nabble.com] 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]>:

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 _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost ________________________________ If you reply to this email, your message will be added to the discussion below: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677432.h... To unsubscribe from [Optional] Monadic bind, click herehttp://boost.2283326.n4.nabble.com/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=4677431&code=VG9iaWFzLkxvZXdAc3RlYWcuY29tfDQ2Nzc0MzF8MTI1MDAzMDE5Mg==. NAMLhttp://boost.2283326.n4.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml -- View this message in context: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677434.h... Sent from the Boost - Dev mailing list archive at Nabble.com.

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); Furthermore without the std::decay boost::optional will be a boost::optional. Tobias Von: Andrzej Krzemienski [via Boost] [mailto:ml-node+s2283326n4677436h22@n4.nabble.com] 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]>:

Sorry for that,

I've made in implementation with free functions which compiles on VS 2012:

#include #include

template auto bind(const boost::optional<T>& optional, B&& binder) -> typename std::decay::type { if(optional) { return std::bind(binder, *optional)(); } else { return boost::none; } }

template auto map(const boost::optional<T>& optional, B&& binder) -> boost::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 #include #include template auto map2(const boost::optional<T>& o, F f) -> boost::optional { 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 _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost ________________________________ If you reply to this email, your message will be added to the discussion below: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677436.html To unsubscribe from [Optional] Monadic bind, click here. NAML -- View this message in context: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677437.html Sent from the Boost - Dev mailing list archive at Nabble.com.

Von: Andrzej Krzemienski [via Boost] [mailto:ml-node+s2283326n4677451h87@n4.nabble.com] Gesendet: Montag, 22. Juni 2015 20:54 An: Löw, Tobias (STEAG Energy Services GmbH) Betreff: Re: [Optional] Monadic bind 2015-06-22 16:59 GMT+02:00 Tobias Loew <[hidden email]>:

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? I managed to change your map2 to compile in C++03 (on VS 2008) template boost::optional< typename boost::remove_cv< typename boost::remove_reference< typename boost::result_of::type >::type >::type > map2(const boost::optional<T>& o, F f) { if (o) return f(*o); else return boost::none; } I didn’t find an easy way to make my map compile in C++03. Technically, it should be possible by using the same tricks that boost::bind itself uses, but I think that would be by far too much for such a feature. A good solution could be: using a map2-like version (only accepting functions/functors) for C++03 compilers and a the more general version (accepting arbitrary callables) for C++11 compilers. -- View this message in context: http://boost.2283326.n4.nabble.com/Optional-Monadic-bind-tp4677431p4677464.h... Sent from the Boost - Dev mailing list archive at Nabble.com.