Make Free.foldMap stack-safe.

[TomasMikula]

Fixes #721.

The fix is now possible by strengthening constraints on the target monad from Monad to MonadRec.

This is not binary compatible, and will not be source compatible for users that used this with a non-MonadRec monad.

[TomasMikula]

To fix the tutorial, it will require at least MonadRec instance for State.

Quite ironically, the tutorial emphasizes stack-safety,

An important aspect of foldMap is its stack-safety. It evaluates
each step of computation on the stack then unstack and restart. This
process is known as trampolining.

As long as your natural transformation is stack-safe, foldMap will
never overflow your stack. Trampolining is heap-intensive but
stack-safety provides the reliability required to use Free[_] for
data-intensive tasks, as well as infinite processes such as streams.

but this has been wrong until this PR.

[non]

This looks great! Do you mind adding the necessary MonadRec instance? If not, I can take a crack at it in a different PR.

[TomasMikula]

I can give it a go.

[TomasMikula]

Turns out also MonadRec[Eval] is required, due to the way State is defined. See #1076.

[non]

I'm re-running Travis -- I think after your last PR this should be green now.

[non]

Looks like maybe there are conflicts -- but once this is green and ready-to-merge 👍 from me.

[codecov-io]

Current coverage is 88.34%

Merging #1075 into master will decrease coverage by <.01%

1. 3 files (not in diff) in .../src/main/scala/cats were deleted. more
2. 3 files (not in diff) in ...main/scala/cats/data were modified. more
   • Misses -1
3. 3 files (not in diff) in .../src/main/scala/cats were modified. more
   • Misses +1
   • Hits -1
4. File ...Transformation.scala (not in diff) was created. more

@@             master      #1075   diff @@
==========================================
  Files           223        221     -2   
  Lines          2808       2804     -4   
  Methods        2751       2749     -2   
  Messages          0          0          
  Branches         52         50     -2   
==========================================
- Hits           2481       2477     -4   
  Misses          327        327          
  Partials          0          0          

Powered by Codecov. Last updated by 04b2501...5dc6518

[milessabin]

👍

[milessabin]

Needs rebasing ...

[TomasMikula]

rebased

[ceedubs]

Thanks @TomasMikula!

[mandubian]

cool I had totallly missed this one :D