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

From: David Abrahams (david.abrahams_at_[hidden])
Date: 2002-07-18 15:17:22

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?

> More importantly, the example of permuting an array strikes me as fairly
> esoteric, and does not motivate template metaprogramming very
> strongly.   I understand the pedagogical advantage of starting with
> factorial as the canonical example of functional programming, and then
> building from there throughout section 1.  However, an example that
> accomplishes something more familiar to "everyday" programming would be
> stronger.

I wrote that part of the paper, and I agree that it's too esoteric, but
like you, I couldn't come up with a better simple motivator.

> Unfortunately, I don't have anything better to suggest right now.  Anyone
> else have any ideas?  I will think about this some more and see if I can
> give a concrete suggestion, but I think a better example would make the
> paper more accessible.

I totally agree.

> 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.

-Dave