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これはどうにかなりませんかねえ

Haskell

Last updated at 2012-12-26Posted at 2012-03-16

Monad m => Maybe (m (Maybe b))という型がついている物体の、二つのMaybeをエレガントに統合する方法もとむ

MainRand.hs

recMapM :: forall a b m. (Data a, Data b, Monad m) => (b -> m b) -> a -> m a
recMapM f x = do
  let recurse = gmapM (recMapM f) x
  case cast x of
    Nothing -> recurse
    Just bx -> do
      z <- f bx
      case cast z of
        Nothing -> recurse
        Just az -> return az

c.f.
http://d.hatena.ne.jp/nushio/20120104
http://d.hatena.ne.jp/nushio/20120105

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