What sorts of things cause cyclic dependencies?

[Lars T. Kyllingstad]

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