notes/foldFinal.hs

   1 {-# LANGUAGE ExistentialQuantification #-}
   2 {-# LANGUAGE RankNTypes                #-}
   3
   4 import Control.Lens (Getting, foldMapOf)
   5
   6 data Fold i o = forall m . Monoid m => Fold (i -> m) (m -> o)
   7
   8
   9 instance Functor (Fold i) where
  10     fmap k (Fold tally summarize) = Fold tally (k . summarize)
  11
  12 instance Applicative (Fold i) where
  13     pure o = Fold (\_ -> ()) (\_ -> o)
  14
  15     Fold tallyF summarizeF <*> Fold tallyX summarizeX = Fold tally summarize
  16         where
  17             tally i = (tallyF i, tallyX i)
  18             summarize (nF, nX) = summarizeF nF (summarizeX nX)
  19
  20 focus :: (forall m . Monoid m => Getting m b a) -> Fold a o -> Fold b o
  21 focus lens (Fold tally summarize) = Fold (foldMapOf lens tally) summarize
  22
  23
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  25