[Haskell-beginners] Beginners Digest, Vol 122, Issue 6

[Anthony Lee]

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

On Fri, Aug 17, 2018 at 8:41 AM <beginners-request at haskell.org> 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 <anthonynlee at gmail.com>
> To: beginners at haskell.org
> Subject: [Haskell-beginners] Bound library questions
> Message-ID:
>         <CA+pBo5HVraf_8jbs15bR6QAy1S7=z_u-3KBwT=
> c3sqkAgc0m5Q at mail.gmail.com>
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>
> 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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