Henning Thielemann
a situation which occurs only very rarely, and for which there is a relatively easy workaround.
Namely? ...
See below.
It's interesting how other languages solve this problem. In Modula-3 it is solved by explicit module interfaces and partial revelation.
Essentially this is how ghc and nhc98 solve it too. You must write an explicit module interface (the .hi-boot file). This is the easy workaround. One benefit of these systems over Modula is that you /only/ need to write explicit interfaces where there is module recursion - in all non-recursive contexts, the interface generation is automatic.
I'm curious how Oberon solves it, because it doesn't need explicit interfaces but derives them from the implementation files. Probably one can say that Oberon extracts something like a .hiboot file from every module file automatically. Why can't GHC and Hugs go this way?
The main obstacle is that Haskell systems generally process one file/module at a time. To extract an interface from each module first, before further processing, would not only require a second pass over the source files, but to load all of them simultaneously, which can be rather space-hungry. Anyway, as Diatchki, Jones, and Hallgren [1] have demonstrated, it is certainly possible to build a Haskell compiler that deals properly with recursive modules. Regards, Malcolm [1] Iavor S. Diatchki, Mark P. Jones, and Thomas Hallgren "A Formal Specification for the Haskell 98 Module System" ACM Haskell Workshop, 2002, Baltimore http://www.cse.ogi.edu/~diatchki/hsmod/