opDispatch, duck typing, and error messages

[Jonathan M Davis]

> Jonathan M Davis wrote:
> > It _does_ generally make sense for the file and line number to be
> > from the throw point rather than the point where you called the
> > function (especially when the throw point could be several function
> > calls away in the stack), but without a proper stack trace, you
> > have no way of knowing where in your code the problem is.
> 
> I believe I agree completely.
> 
> > Now, in the case of something like to, the types are known at
> > compile time, so in many cases, it should be able to give a
> > compile time error via a template constraint, but it's not able
> > to do that in all cases.
> 
> Indeed. And it usually does. The problem is that the error can be
> pretty hard to read.
> 
> Here's one simple case:
> 
> ===
> 
> import std.conv;
> 
> struct Test {}
> 
> void main() {
> Test t;
> int a = to!int(t);
> }
> 
> ===
> 
> /home/me/d/dmd2/linux/bin/../../src/phobos/std/conv.d(95): Error: template
> std.conv.toImpl(T,S) if (!implicitlyConverts!(S,T) && isSomeString!(T) &&
> isInputRange!(Unqual!(S)) && isSomeChar!(ElementType!(S))) does not match
> any function template declaration
> /home/me/d/dmd2/linux/bin/../../src/phobos/std/conv.d(95): Error: template
> std.conv.toImpl(T,S) if (!implicitlyConverts!(S,T) && isSomeString!(T) &&
> isInputRange!(Unqual!(S)) && isSomeChar!(ElementType!(S))) cannot deduce
> template function from argument types !(int)(Test)
> /home/me/d/dmd2/linux/bin/../../src/phobos/std/conv.d(95): Error: template
> instance errors instantiating template
> /home/me/d/dmd2/linux/bin/../../src/phobos/std/conv.d(7): Error: template
> instance std.conv.to!(int).to!(Test) error instantiating
> 
> 
> Holy crap!
> 
> But, if you know the secret there - to look at the last line -
> it does tell you enough to do some useful stuff. Alas, it doesn't
> tell where in *your* code the problem is, but it does tell the
> types you asked for, so it's pretty useful.
> 
> Still, I'd like it if that was formatted nicer, and it went the
> final step of telling me where in my code the problem is too.
> 
> 
> Then, you have the problem if your type matches two templates.
> Then you have the vomit in my opening post, where it doesn't even
> tell you what types it's having trouble with!

Template errors tend to be fairly good (especially if you're used to reading 
them) when you're only dealing with one template, but as soon as the template 
is overloaded, you're screwed. It generally just gives the first template and 
says that you didn't match it, and often the template that you're actually 
trying to use is quite different. But short of giving _all_ of the possible 
template declarations that it failed, I'm not quite sure what we could do to 
fix that.

- Jonathan M Davis