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Today's Topics:

   1.  Bound library questions (Anthony Lee)


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Message: 1
Date: Fri, 17 Aug 2018 06:46:52 -0400
From: Anthony Lee <anthonynlee@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] Bound library questions
Message-ID:
        <CA+pBo5HVraf_8jbs15bR6QAy1S7=z_u-3KBwT=c3sqkAgc0m5Q@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

In Scope.hs there are some functions I feel difficult to understand,
Why fmap/foldmap/traverse is applied three times?

instance Functor f => Functor (Scope b f) where
fmap f (Scope a) = Scope (fmap (fmap (fmap f)) a)
{-# INLINE fmap #-}

-- | @'toList'@ is provides a list (with duplicates) of the free variables
instance Foldable f => Foldable (Scope b f) where
foldMap f (Scope a) = foldMap (foldMap (foldMap f)) a
{-# INLINE foldMap #-}

instance Traversable f => Traversable (Scope b f) where
traverse f (Scope a) = Scope <$> traverse (traverse (traverse f)) a
{-# INLINE traverse #-}
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