Remember that constructors are functions, except that you can't pattern match against them.
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The downside is that you can't pattern-match against these functions.
The thing is that I need pattern matching, just functions won't do.
It's "only" a preprocessor, but Conor's "she" allows pattern synonyms: http://personal.cis.strath.ac.uk/~conor/pub/she
Anyway, a new question arose. If I have already declared a type, can I add new constructors to it from other modules?
Again, not within Haskell itself. "she" also has a feature that allows something like this. There are various other techniques or proposals. For example: http://www.cs.ru.nl/~wouters/Publications/DataTypesALaCarte.pdf http://people.cs.uu.nl/andres/OpenDatatypes.html Cheers, Andres