Re: [boost] [optional] Safe optional

Vicente J. Botet Escriba writes:

Le 17/11/14 18:14, Joaquin M Lopez Munoz a écrit :

...

Just to throw some idea in, there's an altenative based on the following approach:[...] we could *lift* f so that we write this instead:

auto ret=lift<f>(arg1,...,argn);

with the same semantics. This is a case of *monadic lifting* as explained (and implemented) at

http://bannalia.blogspot.com/2014/03/monadic-lifting-in-c.html

Hi Joaquin, I missed your post.

I like this, of course :)

If I understood your blog, this should be something like

auto ret = lift(a1, ..., an);

Or could the optional template be deduced?

It would be more like lift<optional>(f)(a1,...,an) but in any case the particular monad being lifted to (here optional) must be specified.

doesn't scales to expected. Instead of a template <class> parameter, lift should take a type constructor, In Boost.Expected we would do this as

auto ret = lift, f>(a1, ..., an);

where _holder is a holder type that makes expected<_holder, error_code> to behave as a type constructor.

Absolutely, additional template parameters like in expected would call for a type constructor.

This lift operation is a good thing when you have a function. The problem is that people don't have always a function to call, they have just code to execute. The closer we have are lambdas

auto res = fmap(o1, ..., on , [](a1, ..., an) { // the code });

You can lift lambdas as well: auto res=lift<optional>([](int x,int y){ return ...; })(o1,o2); Joaquín M López Muñoz Telefónica