Add rnfFoldableLTR and rnfFoldableRTL

[treeowl]

If a type has a Foldable instance, then we can force it from
left to right or from right to left.

Fixes #17

[RyanGlScott]

For context: Haskell libraries mailing list discussion: https://mail.haskell.org/pipermail/libraries/2016-July/027164.html

[rwbarton]

This should carry a huge warning that it is normally not a sensible definition for Foldable instances where f a contains fields of types other than a, such as a pair.

[treeowl]

It has such a warning, specifically mentioning Either a and (,) a. Is
the warning insufficient?

On Jul 27, 2016 5:03 PM, "Reid Barton" notifications@github.com wrote:

This should carry a huge warning that it is normally not a sensible
definition for Foldable instances where f a contains fields of types
other than a, such as a pair.

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[rwbarton]

Oh, so it does. There is such a lot of noise in the diff that I missed it.

[RyanGlScott]

To avoid invoking the wrath of -Wnoncanonical-monoid-instances in later GHCs, it'd be prudent to define mappend like this:

instance Monoid UnitLTR where
  mempty = UnitLTR ()
#if MIN_VERSION_base(4,9,0)
  mappend = (<>)
#else
  UnitLTR () `mappend` y = y
#endif

And similarly for UnitRTL.

[treeowl]

Sure.

[chessai]

This seems useful, can this be worked in to the modern deepseq?

[cartazio]

Perhaps I’m overlooking something obvious : but can’t we tease out assoxiativity of a structure from its foldmap?

…

On Fri, May 3, 2019 at 6:56 PM chessai ***@***.***> wrote: This seems useful, can this be worked in to the modern deepseq? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#18 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAABBQXQJ6QXWWMAR4BP2TDPTS7JJANCNFSM4CJYQ6WQ> .

[chessai]

Are we worried at all about the loss of Safe?

[mixphix]

foldMap can be more efficient than either foldl or foldr, so Unit was added by #106 (same as UnitLTR).