[Hs-Generics] MultiRec generic producer.

[Sebastiaan Visser]

Hey guys,

While playing around with MultiRec most things are pretty  
straightforward and most usage can be derived from the examples. The  
only thing that seems tricky to do, and of which I cannot find any  
examples, are generic producers.

Any examples of generic producers that take no values as input but do  
produce values as output? E.g. generic parsers (read), binary get,  
arbitrary?

I have some `SingleRec' producers that are fairly trivial to port  
except for the `Tag' case. Any clues?

Thanks,

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Sebastiaan Visser