Writer Monad

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import Data.Monoid 
newtype Writer w a = Writer { runWriter :: (a, w) } deriving Show
 
instance Functor (Writer w) where
    fmap f (Writer (a, w)) = Writer (f a, w)
 
instance Monoid w => Applicative (Writer w) where
    pure a = Writer (a, mempty)
    Writer (f, w) <*> Writer (a, w') = Writer (f a, w' <> w)
 
 
 
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Snippet Metadata
Owner	homam
Created	6 years ago
Modified	6 years ago
Visibility	Public