recursive import

[Serge D. Mechveliani]

On Tue, Jun 06, 2006 at 02:39:00PM +0200, Johannes Waldmann wrote:
> As for recursive module imports,
> could you perhaps show an example where this occurs.
> I found that in my code, I can re-arrange things
> so as to remove the cyclic dependency. Best regards,


This is somewhat like this:

--------------------------------------------
module DInteger 
where
import Polynomial
... various classes

class ... => WithFactorization a where gcD     :: ...
                                       isPrime :: ...
                                       factor  :: ...
                                       ...
instance WithFactorization Integer 
  where 
  gcD = gcd
  isPrime n =  ... complex implementation using instances for 
                   Polynomial Integer
  factor n = ... 
  ...

... various instances for Integer. 

--------------------------------------------
module Polynomial 
where
import DInteger
...
various instances for  Polynomial,  which use instances for  Integer.
---------------------------------------------

`Integer' is a basic data used by many "higher level" data.
On the other hand, some advanced method implementations for Integer 
need operating with Polynomials, Matrices, and so on.

If I join the above two modules, this will cause joining into one 
module about all (numerous and large) modules of the project.

Of course, one can also avoid import recursion by adding intermediate
classes and by copying parts of implementation. But this 
complicates the global algorithm presentation, and it is better to 
use recursive import.

>From scientific point of view, modules should express the 
classification of notions (or algorithm parts) by  relevance.
This often leads to cycles by the operation import.

Also note that the GHC implementation is said to use recursive 
import.

-----------------
Serge Mechveliani
mechvel at botik.ru