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Subject: Re: [boost] [guidelines] why template errors suck
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2010-09-28 13:45:23

On 28/09/10 18:33, David Abrahams wrote:
> At Tue, 28 Sep 2010 10:00:54 -0500,
> Andrew Sutton wrote:
>> It seems to me that writing constraints in terms of valid
>> expressions gives you flexibility when you want it, but doesn't
>> preclude the strengthening of requirements when needed.
> It's not that there's anything you *can't* express with valid
> expressions, it's that they're difficult to use correctly, and the
> most natural way of using them creates a big mess for algorithm
> writers. Furthermore, they don't offer any compelling value over the
> alternative.

Please demonstrate how you check that a type models a concept without
using a set of valid expressions to define the concept, but only with
signatures in an archetype-like fashion.

My insight is that you can't.
Therefore what compelling value expressions give over archetypes becomes
fairly obvious: they do what we want, while archetypes don't and do
something different.

You said it yourself: expressions are only half the solution; archetypes
are the other half. They're not the whole and they're no substitute for
using expressions to define and test concepts.

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