[Haskell-cafe] Re: some ideas for Haskell', from Python

ChrisK haskell at list.mightyreason.com
Mon Jan 19 14:00:52 EST 2009

Manlio Perillo wrote:
> Brandon S. Allbery KF8NH ha scritto:
>  >
>> ...in theory. In practice GHC needs help with circular imports, and 
>> some cycles might be impossible to resolve.
> This is interesting.
> Where can I find some examples?
> Is this explained in the Real World Haskell book?

I have no idea about RWH, but there are certainly mutual import cycles that 
cannot be resolved by using hs-boot files with GHC.

Consider three modules A and B and C, which are A-B-C permutations of

> module A(A,AKBC, AKCB)
> import B(B,BKAC)
> import C(C,CKAB)
> data A
> AKBC :: Either B C
> AKCB :: Either C B
 > instance Show (A,BKAC,CKAB) where ...

There is no way to break the ?K?? import cycle with just hs-boot files.  I had 
to solve this by generating "helper" modules.

Call the "data A" the rank-1 declarations.  Then the ?K?? are built on rank-1 
types such as "B" and "C" and are rank-2 declarations.  The rank-1 declarations 
can all be put in hs-boot files but the rank-2 declaration import cycle cannot 
be broken with the same hs-boot files.  Some of these need to be put in separate 

It may be possible to make a useful definition of rank-3 and higher declarations.


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