Optics?

[Alan & Kim Zimmerman]

Hi all

I am working on a variant of the exact printer which updates the annotation
locations from the `EpaSpan` version to the `EpaDelta` version, as the
printing happens

data EpaLocation = EpaSpan RealSrcSpan
                 | EpaDelta DeltaPos

The function doing the work is this

markAnnKw :: (Monad m, Monoid w)
  => EpAnn a -> (a -> EpaLocation) -> (a -> EpaLocation -> a) ->
AnnKeywordId -> EP w m (EpAnn a)

which gets an annotation, a function to pull a specific location out, and
one to update it.

I do not know much about lenses, but have a feeling that I could simplify
things by using one.

Can anyone give me any pointers?

Alan
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