Add Applicative IorT instance

[LukaJCB]

This was a missing instances cc @arosien

[codecov-io]

Codecov Report

Merging #2181 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2181      +/-   ##
==========================================
+ Coverage   94.71%   94.75%   +0.04%     
==========================================
  Files         329      330       +1     
  Lines        5558     5572      +14     
  Branches      215      218       +3     
==========================================
+ Hits         5264     5280      +16     
+ Misses        294      292       -2

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/IorT.scala	97.82% <100%> (+0.06%)	⬆️
core/src/main/scala/cats/syntax/option.scala	100% <0%> (ø)	⬆️
.../src/main/scala/cats/syntax/applicativeError.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/syntax/TrySyntax.scala	100% <0%> (ø)
core/src/main/scala/cats/syntax/either.scala	99.16% <0%> (ø)	⬆️
core/src/main/scala/cats/ApplicativeError.scala	100% <0%> (+11.76%)	⬆️

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[peterneyens]

EitherT and OptionT explicitly don't have this instance because it is inconsistent with the Monad instance (which is one of the reason for the toNested methods on those data types). (See #1467 for some discussion and links).

I haven't checked it for IorT, but I suspect it will have the same issue.

[martin-g]

The description says List but the code uses ListWrapper. Is this intended ?

[LukaJCB]

Thanks for pointing that out @peterneyens! I wasn't aware this would lead to inconsistencies, I understand how OptionT and EitherT would differ, but I'm not 100% certain I'd be the same for IorT I'll have to investigate some more.

[arosien]

@peterneyens @LukaJCB perhaps we could add some kind of scaladoc that would let the user know to call toNested to use this in an Applicative manner.

[kailuowang]

I created #2272, shall we close this?

[LukaJCB]

Yup :)