Now I understand it. Thanks a lot! Anthony On Sat, Aug 18, 2018 at 5:34 AM wrote:

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Today's Topics:

1. Re: Beginners Digest, Vol 122, Issue 6 (Anthony Lee) 2. Re: Bound library questions (David McBride) 3. Empty list and null (trent shipley) 4. Re: Empty list and null (Ut Primum)

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Message: 1 Date: Fri, 17 Aug 2018 09:21:47 -0400 From: Anthony Lee To: beginners@haskell.org Subject: Re: [Haskell-beginners] Beginners Digest, Vol 122, Issue 6 Message-ID: Content-Type: text/plain; charset="utf-8"

The source code is here: https://github.com/ekmett/bound/

On Fri, Aug 17, 2018 at 8:41 AM wrote:

...

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1. Bound library questions (Anthony Lee)

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Message: 1 Date: Fri, 17 Aug 2018 06:46:52 -0400 From: Anthony Lee To: beginners@haskell.org Subject: [Haskell-beginners] Bound library questions Message-ID: 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 #-}