Elke Kasimir wrote:
No Problem at all for Haskell, but a problem for certain often-used Haskell compilers and interpreters...
Factoring out "the common part" does not work in examples like above, so the only way is to collapse everything, and to reduce class constraints as much as is possible - something which hurts deep inside! That the problems appear step-by-step in the course of implementation or even debugging, adds to the pain.
So I have put onto my personal Haskell whish-list that certain compilers and interpreters shall either ultimately converged against the language they intend to support, or else provide some suitable alternative. Thinking in
the first direction, a precompiler appears before my inner eye which
is able to collapse mutually dependent modules into one bigger module. Even if this led to incorrect line numbers in error messages and the like,
I would definitely use such a precompiler. A step in the second direction was a slight modification of Haskell such that instance declaration and instance
implementation can be provided in separate modules.
there is no need to use such hacks. it is not dificult to add suport for mutually recursive modules to an implementation directly.
unfortunatley none of the working haskell implementations support recursive modules, i guess they are probably not considered important enough. and the haskell report does not say how they should work (ergh why?), but if you are interested you can take a look at "A formal specification for the Haskel 98 module system" a paper we wrote to remedy this situation.
in the mean time you can work around the problem in GHC, by using the hi-boot files take a look at the documentation "How to compile mutually recursive modules".
bye iavor
-- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================
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