
Boost : 
Subject: Re: [boost] Reimplementation of the MPL for C++11
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 20131020 13:19:41
Abel Sinkovics <abel <at> elte.hu> writes:
> 1)
> I can see that you have this syntax for if_:
>
> if_<CONDITION, THEN>::
> else_if<CONDITION, THEN>::
> ...
> else<ELSE>
>
> which is I think a great DSL for the if structure. Can this be used in a
> lambda expression? For example:
Yes, you can use it with lambda expressions.
This one took a lot of work to get working, though :).
> 2)
> Metafunctions could support lazy evaluation. This makes it possible to
> use lazy evaluation strategy in metaprograms which makes the code easier
> to read. (Even for a factorial or Fibonacci there are huge differences)
I'm aware of this and I'm currently searching for a general solution. I think
I might have a very elegant solution for this; I'll provide an update when/if
I get it working.
> 3)
> Naming the lambda arguments _1, _2, ... has limitations. When you use
> lambda inside lambda, you can not refer to the arguments of the outer
> lambda from the inner one. I have built a lambda implementation where
> you can give the arguments names and use that instead of the MPL one.
Yes, I'm aware of this. I'm currently working on letexpressions. I'm also
working on generalized placeholders: they can perform arbitrary computations
and return sequences of types so that it's possible to use LambdaExpressions
with variadic parameter packs. Example:
mpl11::apply<your_metafunction<mpl11::_args>, ParameterPack...>
The obvious extension to this is to have placeholders like mpl11::_args<i, j>
which returns the arguments in the range [i, j). This is a work in progress.
> You can read about my lambda implementation here:
> http://abel.web.elte.hu/mpllibs/metamonad/manual.html#lambdaexpressions
> It is based on syntaxes and variables, which are described earlier on
> that page.
I'll definitely have a look at this, thanks for the pointer!
> This problem can also be solved by using De Brujin indices, but I find
> using names more readable so I decided to use this in my lambdas.
Same here, I'm not planning on using de Bruijn indices.
Regards,
Louis Dionne
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk