Re: [boost] Template metaprogramming libraries

Subject: Re: [boost] Template metaprogramming libraries
From: Larry Evans (cppljevans_at_[hidden])
Date: 2011-09-11 09:22:26

On 09/11/11 02:30, Jeffrey Lee Hellrung, Jr. wrote:
> On Sun, Sep 11, 2011 at 12:01 AM, Ábel Sinkovics <abel_at_[hidden]> wrote:
> 
>> Hi Larry,
>>
>>> Are you saying that using the existing mpl, one cannot embed
>>> lambda expressions inside other lambda expressions?  Is that
>>> mpl problem an example of name capture:
>>>
>>>   http://dictionary.reference.com/browse/name+capture
>>>
>>> ?
>>
>> You can embed MPL lambda expressions inside other ones. I couldn't find
>> a way of accessing the arguments of the outer lambda expression from the
>> inner one, because the names _1, _2, etc were referring to the arguments
>> of the inner expression, not the outer one (they were shadowing the
>> outer ones).
>>
>> I'd express it with "\x.\x.x" in lambda calculus. Inside the inner
>> lambda abstraction "x" refers to the argument of the inner, not the
>> outer one.
>>
> 
> A while ago someone suggested (on the developers' list) adding de Bruijn
> indices [1] to Boost.Bind and/or Boost.MPL (and may have provided at least a
> sample implementation for one or the other, I'm not sure),

I attempted an implementation, but was not successful, as indicated in
this post:

  http://lists.boost.org/Archives/boost/2010/09/171123.php

> which ( I think) would allow you to do what you want to do.
> 
> There might be some protect and bind combination of contortions to do what
> you want, but I'd certainly agree that, even if it could be done, it
> probably wouldn't be pretty.

I remember doing something with those templates to workaround a problem
that *seemed* like it was a name capture problem; so, I'd agree with
Jeffrey here that it might work with some combination of bind protect
(I also think I used lambda).

> 
> [1] http://en.wikipedia.org/wiki/De_Bruijn_indices
> 
> - Jeff
> 
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