[Haskell-cafe] Mutually recursive modules and google protocol-buffers

[Henning Thielemann]

On Tue, 15 Jul 2008, Chris Kuklewicz wrote:

> Consider these 3 files:
>
> A.hs:
>> module A(A) where
>> import B(B)
>> data A = A B
>
> B.hs
>> module B(B) where
>> import A(A)
>> data B = B A
>
> Main.hs
>> module Main where
>> import A
>> import B
>> main = return ()


Sooner or later you want generalize your datatypes. Then you can define
    data A b = A b
   and you do not need to import B any longer. I do not know if this is a 
generally applicable approach, but it helped me in some cases.
  There is still a problem with mutually recursive classes. In the one case 
where I had this problem, I could solve it the opposite way, namely by 
turning one type variable into a concrete type, which could represent all 
values one could represent with the variable type.