Unhelpful error messages

[H. S. Teoh]

On Wed, Jul 10, 2013 at 03:05:25PM +0200, Don wrote:
> On Monday, 8 July 2013 at 20:46:35 UTC, H. S. Teoh wrote:
> >On Mon, Jul 08, 2013 at 09:47:46PM +0200, Peter Alexander wrote:
[...]
> >>Maybe the compiler could just spew out every possible error for
> >>every instantiation, and expect the user to grep, but that's not
> >>going to be a pleasant experience.
> 
> The compiler could join all the constraints and then simplify it to
> create the error message. (eg by creating a Binary Decision Diagram
> (BDD))
> 
> eg given constraints:
> 
> if ( A  && B && C )
> if ( (A  && D) || ( A && E && F) )
> if ( E && G )
> 
> Suppose A is true but the conditions fail. The compiler could then
> write that no templates match because ( B || D || E ) is false.
[...]

Hmm, looking at BDD's again, it appears that the simplification process
may not be quite so simple. Depending on the ordering of constraints,
you may end up with a BDD of exponential length. The bad thing is that
finding the best ordering is NP-hard. There are heuristic algorithms
that work well for "normal" cases, though, so hopefully this won't be
*too* nasty to implement.


T

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Let's not fight disease by killing the patient. -- Sean 'Shaleh' Perry