Add foldl and foldr aliases to Foldable

[felixmulder]

This PR adds aliases for Foldable's foldRight and foldLeft ops so that the callee can be sure that the correct method is being invoked.

Because of syntax extensions the end signature looks like:

def foldRight[B](b: Eval[B])(f: (Eval[B], A) => Eval[B]): Eval[B]

This can then lead to some confusion when folding over things which already have the foldRight operation, namely pretty much all collections.

Even though we do:

import cats.implicits._

List
  .range(0, 10000000)
  .foldRight(Eval.now(0))((n, e) => e.map(_ + n))

the fold won't be evaluated lazily, using Foldable's definition of foldRight instead the stdlib's version will be chosen.

To avoid this pitfall, this PR introduces these aliases:

• foldr <=> foldRight
• foldl <=> foldLeft

these methods can be used safely with syntax extensions.

[kailuowang]

seems you were on a code base prior to 1.0.0-MF. These alias should probably be added to the type class , but they will break bin compat with 1.0RC1 so it would be helpful if there are more justification in the PR.

[felixmulder]

Sure, I'll update the initial comment with a more detailed explanation.

[kailuowang]

as expected, this fails the binary compat check

Cats core: found 1 potential binary incompatibilities while checking against org.typelevel:cats-core_2.10:1.0.0-RC1

• method catsSyntaxFoldOps(java.lang.Object,cats.Foldable)java.lang.Object in trait cats.syntax.FoldableSyntax is present only in current version
  filter with: ProblemFilters.exclude[ReversedMissingMethodProblem("cats.syntax.FoldableSyntax.catsSyntaxFoldOps")�

we can add an exception filter here
Here is the documentation on how
please also add a comment that this filter shall be removed post 1.0 release.

[felixmulder]

Cool, thanks for having a look, will update this tomorrow 👍

[codecov-io]

Codecov Report

Merging #2020 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2020      +/-   ##
==========================================
+ Coverage   95.08%   95.08%   +<.01%     
==========================================
  Files         301      301              
  Lines        4943     4946       +3     
  Branches      124      125       +1     
==========================================
+ Hits         4700     4703       +3     
  Misses        243      243

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/foldable.scala	100% <100%> (ø)	⬆️

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[felixmulder]

@kailuowang - since the syntax suite doesn't actually run, I guess codecov doesn't pick it up. Is there somewhere I should add a runtime test for this so as not to affect codecov?

[kailuowang]

@felixmulder a good way to add test coverage for such aliases is doctest. How about we mention these aliases and provide some doctests in scala docs for foldRight and foldLeft in Foldable. Examples of doctests can be found also in the file here

[felixmulder]

@kailuowang - added the doctest 👍

[kailuowang]

Awesome! Thanks very much!

[felixmulder]

NP, thanks for the pointers :)