What sorts of things cause cyclic dependencies?

[Jonathan M Davis]

On Wednesday 13 October 2010 23:03:05 Lars T. Kyllingstad wrote:
> On Thu, 14 Oct 2010 05:53:51 +0000, Lars T. Kyllingstad wrote:
> > On Wed, 13 Oct 2010 21:25:15 -0700, Jonathan M Davis wrote:
> >> Okay. in the code that I'm working on at the moment, I get an exception
> >> saying that a cyclic dependency was detected but no information
> >> whatsoever as to where it is or what it means. I haven't been able to
> >> find much information on them other than some discussions of making it
> >> so that the compiler detects them at compile time. I have yet to figure
> >> out _what_ it is that is causes such errors. Could someone clue me in?
> >> It's really hard to track down a bug when the error only tells you that
> >> there's a bug and doesn't say anything about where it happens, and I
> >> have no clue what sort of things can be cyclically dependent.
> > 
> > Say you have two modules, a and b.  Both have static constructors, and a
> > 
> > imports b:
> >   // a.d
> >   module a;
> >   import b;
> >   static this() { ... }
> >   
> >   // b.d
> >   module b;
> >   static this() { ... }
> > 
> > The language is then defined so that b's constructor is run before a's,
> > since a depends on b.  But what happens if b imports a as well?  There
> > is no way to determine which constructor to run first, so the runtime
> > throws a "cyclic dependency" exception.
> 
> I should mention that this can happen with larger cycles as well, i.e. "a
> imports b, b imports c, c imports ..., ... imports a".  This can make
> some cyclic dependencies very hard to track down.
> 
> There was a discussion about this on the Phobos mailing list a few months
> ago:
> 
>   http://lists.puremagic.com/pipermail/phobos/2010-June/thread.html#949
> 
> -Lars

Well, just knowing that it involves module constructors is definitely useful, 
though the error message really needs to be improved. Thanks.

- Jonathan M Davis