Re: [boost] [guidelines] why template errors suck

Subject: Re: [boost] [guidelines] why template errors suck
From: David Abrahams (dave_at_[hidden])
Date: 2010-09-27 18:15:27

At Mon, 27 Sep 2010 14:13:26 -0700,
Steven Watanabe wrote:
> 
> IMHO, the fundamental problem is that concepts guarantee
> that if the template type-checks against the concept definition,
> and the template arguments are models of the concept, then
> there will be no type errors when the template is instantiated.

Right in principle about how concepts work.  [Full disclosure: there
are some corner cases where you can still have a type error but those
are waaaaaaaay off in the obscure corners.  Concepts catch all the
basic cases where you try to use an operation that's not guaranteed by
the constraints.  However, I don't yet see what makes that the
*fundamental* problem.]...

...but I'm not even sure there is a problem yet, even though I was one
of the first people to recognize that concepts clash with TMP as
practiced today.  The question is whether a different approach can
accomplish the same ends.  As far as I'm concerned, that's still an
open question.

> This is a nice property, but the problem is that for template
> metaprogramming, the type /is/ the data.  The things that would
> normally be runtime errors become type errors.  Thus, once
> you actually instantiate the template, there can be no errors
> whatsoever.  We don't apply this level of type-checking to
> runtime code, because it's simply impractical to guarantee.

I think you mean we don't apply this level of runtime checking to
runtime code?  

But isn't that exactly what contract programming is?
(http://en.wikipedia.org/wiki/Design_by_contract#Languages_with_native_support)?

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com