Data.Data provides an gmapM as an analogue to mapM but not one to traverse.

The following definition from lens fills in this gap, but it probably belongs in Data.Data alongside its kin. 

gtraverse :: (Applicative f, Data a) => (forall d. Data d => d -> f d) -> a -> f a
gtraverse f = gfoldl (\x y -> x <*> f y) pure

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-Edward Kmett