RE: [boost] Formal Review: Template Metaprogramming Library (MPL)

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-07-29 22:18:42

Eric Friedman wrote:
> Below is my formal review for the Boost MPL Library:
> 
> 
> __Library Design__
> 
> On the whole, I find MPL to exhibit an astonishingly high 
> level of design; from lambda/bind expressions to familiar 
> STL-like algorithms, MPL exposes a great amount of power 
> through a consistent and highly reusable interface. 
> Overally, I applaud Aleksey for his crafting of MPL's 
> interface.

Thank you!

> 
> One facility sorely missing is an mpl::is_sequence query 
> metafunction. The ability to query an arbitrary type T to 
> determine whether it is an MPL-compatible sequence will be 
> of utmost importance in creating flexible class templates. 
> For example, the desired semantics for the under-development 
> boost::variant is to allow a simulated variadic template 
> parameter list *or* a single MPL-sequence. That is to allow 
> both of the following to exist as valid declarations,
> 
>   typedef boost::variant<int,double,std::string> my_variant;
> 
> or
> 
>   typedef mpl::list<int,double,std::string> types;
>   typedef boost::variant< types > my_variant;
> 
> Such an 'overloaded' interface cannot be implemented without 
> a mpl::is_sequence facility. I discussed the inclusion of 
> this facility with Aleksey some weeks ago, but I have not 
> yet seen its appearance in the library; it is vital, 
> however, that it eventually appears.

In one or another form, it will. I just didn't have time to think it
through.

[...]

> __Library Documentation__
> 
> MPL's documentation seems to be the shoddiest part of the 
> library. This is not to say that it is *bad* -- as that is 
> far from being the case -- but I feel the following changes 
> should be made.
> 
> MPL has a steep learning curve. I have been using various 
> version of MPL for the past 6 months, so I am largely past 
> this curve, but I fear for new users of the library.
> Primarily, I think more examples should be provided in the 
> tutorial, paper, and reference documentation. Currently the 
> first is limited to simple explanations of mpl::if_ and 
> mpl::apply_if, the second to somewhat obscure applications 
> of MPL, 

I protest, FSMs are not obscure! ;).

> and the last to near-trivial uses of the various 
> algorithms, metafunctions, etc. While I understand that good 
> documentation is hard to write, and I therefore understand 
> while it has been long in the coming, I think it will be 
> important for MPL's ultimate success in Boost.

I am determined to improve the volume and quality of the documentation over
time. However, I think that current state is up to the Boost standards
(among the official, relatively large libraries, there are very few that
cover things more thoroughly). It's not an excuse, just a reminder ;).

[...]
> 
> For those who need to do template metaprogramming, MPL will 
> serve invaluable. Before using MPL, I did extensive 
> metaprogramming work without it. There is no way I would 
> consider going back to my pre-MPL days <g>.
> 
> I wholeheartedly vote for formal acceptance of MPL into 
> Boost (with the hope that my nits, particularly regarding 
> mpl::is_sequence, will be heard and eventually addressed).

Thank you for your comments,
Aleksey