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.

[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 notifications@github.com wrote:

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

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[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).