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

From: Thomas Wenisch (twenisch_at_[hidden])
Date: 2002-07-18 16:12:05

On Thu, 18 Jul 2002, David Abrahams wrote:

> From: "Thomas Wenisch" <twenisch_at_[hidden]>
> 
> > Here are my comments on the MPL paper:
> >
> > General Comments
> > ================
> >
> > Permutations Example
> >
> > The permutations example used in section 1.3 of the paper is described as
> > a "simple" example.  However, the code is fairly complex, and uses a
> > number of tricks which I would describe as advanced uses of STL.  For
> > example, the use of &*in to obtain the parameter for std::copy.
> > Additionally, the comment reads
> >
> >   // return an array of the N! permutations of x
> >
> > However, the function returns all N! permutations of the array in, x does
> > not appear in all_permutations.
> 
> I don't understand that sentence. What does it mean?
> 

What I mean is:  I believe "x" is a typo in the comment.  The name of the
variable passed as a parameter to all_permutations is "in".  The function
returns a permutation_holder<> which contains all N! permutations of
"in".  I believe the correct comment should read:

 // return an array of the N! permutations of the array "in".

My apologies for not saying it more clearly.

> 
> > One thing I would suggest, though, is to mention the FSM example from
> > section 5 early on in the introductory material.  Section 5 is a great
> > demonstration of the power of MPL-enabled code generation.  I think
> > mentioning this problem as motivation in section 1 would be a good way to
> > make the problems that metaprogramming solve more concrete for the
> > uninitiated.
> 
> IMO that example is also fairly esoteric and complicated. Even mentioning
> it might not be much of a help in an intro section. It would be much better
> to be able to motivate metaprogramming with something more straightforward,
> if possible.
> 

Yes, perhaps this is true.  I personally found the FSM example more
readable because I have had to solve that very problem myself in the past 
(try to autogenerate code for an FSM from a table), and have never come up
with a solution as concise as what appears in the paper.  Since the
problem was more familiar, I liked the example better :)  However, my
opinion might not be representative of the majority of people new to
metaprogramming that are reading the paper, and that is the audience the
paper should target.

Best Regards,
-Tom Wenisch
Computer Architecure Lab
Carnegie Mellon University