Arrows that are also Functors

Henning Thielemann lemming at
Tue Apr 19 17:29:34 CEST 2011

Is it common to make a type an instance of both Arrow and Functor type 
class? If a type is both instance of Arrow and Functor, would you expect 
that fmap = (^<<) ? If yes, how about adding this as expected law to the 
Control.Arrow documentation? Same question for Applicative functors and 
liftA2 (,) = (&&&). (Btw. Control.Arrow haddock documentation does not 
mention any Arrow law so far.)

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