• Jonathan M Davis (9/9) Oct 13 2010 Okay. in the code that I'm working on at the moment, I get an exception ...
• Lars T. Kyllingstad (28/37) Oct 13 2010 Say you have two modules, a and b. Both have static constructors, and a...
• Lars T. Kyllingstad (8/37) Oct 13 2010 I should mention that this can happen with larger cycles as well, i.e. "...
• Jonathan M Davis (4/43) Oct 13 2010 Well, just knowing that it involves module constructors is definitely us...
• bearophile (4/6) Oct 14 2010 You may file a bug report, with an example, asking for a better error me...
• Steven Schveighoffer (11/63) Oct 14 2010 I had mostly implemented a cyclic dependency improvement that actually
• spir (30/73) Oct 14 2010 =20

Jonathan M Davis <jmdavisProg gmx.com> writes:

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.

- Jonathan M Davis

"Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:

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.

The way to fix it is to move one of the module constructors into a 
separate module:

  module a;
  import a_init;
  import b;

  module a_init;
  static this() { ... }
  
  module b;
  import a;
  static this() { ... }

For a "real" example of how this is done, check out std.stdio and 
std.stdiobase in Phobos.

-Lars

"Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:

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:



-Lars

Jonathan M Davis <jmdavisProg gmx.com> writes:

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:
 

 
 -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

bearophile <bearophileHUGS lycos.com> writes:

Jonathan M Davis:

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

You may file a bug report, with an example, asking for a better error message
(with a better wording), or a much better error message (that lists the cycle
too).

Bye,
bearophile

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Thu, 14 Oct 2010 02:31:23 -0400, Jonathan M Davis <jmdavisProg gmx.com>  
wrote:

 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:



 -Lars

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

I had mostly implemented a cyclic dependency improvement that actually  
printed the cycle.  However, I never committed it because I found there  
was a huge hole in the algorithm that needed to be addressed.  Essentially  
certain cycles are allowed (even today).

I think it can be done with an O(n^2) space and an O(n) algorithm, but I  
need to work on it.

BTW, there is a bug report for this problem:  
http://d.puremagic.com/issues/show_bug.cgi?id=4384

-Steve

spir <denis.spir gmail.com> writes:

On Thu, 14 Oct 2010 06:03:05 +0000 (UTC)
"Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> wrote:

 On Thu, 14 Oct 2010 05:53:51 +0000, Lars T. Kyllingstad wrote:
=20
 On Wed, 13 Oct 2010 21:25:15 -0700, Jonathan M Davis wrote:
=20
 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.

=20
=20
 Say you have two modules, a and b.  Both have static constructors, and a
 imports b:
=20
   // a.d
   module a;
   import b;
   static this() { ... }
=20
   // b.d
   module b;
   static this() { ... }
=20
 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.

=20
=20
 I should mention that this can happen with larger cycles as well, i.e. "a=

=20
 imports b, b imports c, c imports ..., ... imports a".  This can make=20
 some cyclic dependencies very hard to track down.
=20
 There was a discussion about this on the Phobos mailing list a few months=

=20
 ago:
=20

=20
 -Lars


I'm totally new to D, but have some real-life experience with circular impo=
rts. A typical case is with unit testing. Say you design a parsing lib, wit=
h (among others) a module defining Pattern types and another for (parse tre=
e) Nodes, wich are thee results of successful pattern matches.
To test Patterns, no issue, since it already imports Nodes for normal proce=
ss. But testing Nodes introduces a circular dependance, only for testing, b=
ecause the ordinary way to produce a node, and thus to be able to test its =
functionality, is to create a Pattern and run its match method against sour=
ce text.
2 solutions I know of:
* create fake Pattern types inside the Nodes module, just for testing
* export Nodes' test routines into a third module that imports both Pattern=
s and Nodes, but is not import.
That's probably why many libraries have loads of separated test* unittest m=
odules. But I prefere to have tests locally written in the module itself (w=
hich well seems to be the intention of D's testing features).

Some static languages (eg freepascal) allow mutual dependance as long as at=
 least one module (M1) uses imported elements of the other (M2) on the impl=
ementation side only (not in the interface). This allows the compiler initi=
ally constructing the interface of M1 (without any mention of M2), to be im=
ported by M2. Then, M2 can be fully constructed, and imported into M1 for t=
he needs of its implementation only.


Denis=20
-- -- -- -- -- -- --
vit esse estrany =E2=98=A3

spir.wikidot.com