Rename NaturalTransformation to FunctionK.

[peterneyens]

This would rename NaturalTransformation to FunctionK #1065, but we should probably wait until some more people have expressed their opinion about this change ?

I'm also not sure how to change the documentation, should we go from "a natural transformation" to "a FunctionK".

[peterneyens]

This could then be something like :

To do this, we will use a FunctionK (also known as a natural transformation). A FunctionK is like a function but between type containers instead of values. A FunctionK between types F[_] and G[_] could be written as FunctionK[F, G] (or as F ~> G using the symbolic alternative).

[non]

I would say that we can describe the type FunctionK as a natural transformation. So, when we would put the term in backticks (i.e. code) we'd say FunctionK but otherwise we'd say transformation or natural transformation. Does this make sense?

[peterneyens]

That does make sense, yes. It felt weird to use FunctionK in the documentation, that's why I left the documentation itself untouched.

[non]

This is great, thanks!

I think for now we should remove type NaturalTransformation. We can add it back in if a bunch of people complain but having too many names for the same thing will probably confuse the issue.

Relatedly, I think we should remove the <~ alias. I don't think there's ever a situation where code can't just use ~>.

[codecov-io]

Current coverage is 88.37%

Merging #1072 into master will increase coverage by <.01%

1. 2 files (not in diff) in ...main/scala/cats/laws were modified. more
2. 4 files (not in diff) in .../main/scala/cats/std were modified. more
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3. 2 files (not in diff) in ...n/scala/cats/functor were modified. more
4. 5 files (not in diff) in ...main/scala/cats/data were modified. more
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5. 4 files (not in diff) in .../src/main/scala/cats were modified. more
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6. 2 files (not in diff) in ...cats/laws/discipline were deleted. more
7. 2 files (not in diff) in ...main/scala/cats/laws were deleted. more
8. 2 files (not in diff) in .../src/main/scala/cats were deleted. more
9. File ...cats/free/Free.scala was modified. more
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10. File ...a/cats/package.scala was modified. more
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@@             master      #1072   diff @@
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  Files           221        215     -6   
  Lines          2798       2743    -55   
  Methods        2743       2689    -54   
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  Branches         50         49     -1   
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- Hits           2472       2424    -48   
+ Misses          326        319     -7   
  Partials          0          0          

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[ceedubs]

@peterneyens thanks, this looks great! Sorry, but it has a merge conflict. Would you mind resolving it?

[non]

Perfect. 👍 Thanks!

[ceedubs]

👍