Add IO#redeem and IO#redeemWith

[alexandru]

This is related to typelevel/cats#2237

This uses IO's internals to expose an optimized redeemWith operation, that's equivalent with doing attempt.flatMap, but avoiding a second flatMap boundary, or extra boxing in Either.

The performance improvements on doing this are pretty good, see: typelevel/cats#2161 — and the capability of doing this has been in IO ever since I optimized its internal encoding, we just need to expose it.

[codecov-io]

Codecov Report

Merging #191 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
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+ Coverage   89.63%   89.69%   +0.05%     
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  Files          50       50              
  Lines        1081     1087       +6     
  Branches       82       77       -5     
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+ Hits          969      975       +6     
  Misses        112      112

[LukaJCB]

Looks good to me! Sounds pretty useful in general :)

[alexandru]

Actually I want to think about this some more, so I'm closing it to prevent accidental merging. Will reopen if the case.

[alexandru]

OK, thought about it 🙂

As mentioned in typelevel/cats#2237:

I've carefully considered possibilities, doing benchmarks with alternatives, like a variant that takes a function with an Either[Throwable, A] parameter, plus I modified some Monix code to see how it feels like and I think this approach is the best.

The signature is nice too, it's basically a fold (catamorphism?) that happens in the F[_] context.

Unfortunately for abstracting over it we'll have to wait for Cats 2.0 I think.
Or maybe introduce a MonadRedeemBinCompat in Cats 1.x.

[LukaJCB]

👍

[alexandru]

Got verbal confirmation that @LukaJCB and @kailuowang are interested in approving the related PR in Cats.

I think it's safe to move forward with it and merge this.

[LukaJCB]

Sounds good to me! Though Travis is failing with a syntax error :)

[alexandru]

Broken merge with master, I fixed it.

[LukaJCB]

👍