25 lines
665 B
Haskell
25 lines
665 B
Haskell
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module MaybeIO where
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import Control.Applicative
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data MaybeIO a = MaybeIO { runMaybeIO :: IO (Maybe a) }
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instance Functor MaybeIO where
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fmap f = MaybeIO . fmap (fmap f) . runMaybeIO
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instance Applicative MaybeIO where
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pure = MaybeIO . pure . pure
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f <*> x = MaybeIO $ (<*>) <$> f' <*> x'
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where
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f' = runMaybeIO f
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x' = runMaybeIO x
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instance Monad MaybeIO where
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return = pure
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x >>= f = MaybeIO $ x' >>= runMaybeIO . mb . fmap f
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where
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x' = runMaybeIO x
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mb :: Maybe (MaybeIO a) -> MaybeIO a
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mb (Just a) = a
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mb Nothing = MaybeIO $ return Nothing
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