Typeclasses implicit objects

[raulraja]

This allows Typeclasses instances of arbitrary arity to be discovered implicitly by providing a default factory based on naming conventions that can be used to lookup the instance.
It also introduces granularity in the typeclass definitions for better composition.

Fixes #260

• Function0
• Eval
• ListKW
• Option
• Try
• Const
• Coproduct
• Either
• EitherT
• FreeApplicative
• Free
• Function1
• Ior
• Iterable
• Kleisli
• NonEmptyList
• Number
• SequenceKW
• StateT
• String
• Validated
• WriterT
• Yoneda

Example on EitherTFunctorInstance

interface EitherTFunctorInstance<F, L> : Functor<EitherTKindPartial<F, L>> {
    fun FF(): Functor<F>
    override fun <A, B> map(fa: EitherTKind<F, L, A>, f: (A) -> B): EitherT<F, L, B> = fa.ev().map({ f(it) }, FF())
}

object EitherTFunctorInstanceImplicits {
    @JvmStatic
    fun <F, L> instance(FF: Functor<F>): EitherTFunctorInstance<F, L> = object : EitherTFunctorInstance<F, L> {
        override fun FF(): Functor<F> = FF
    }
}

You can now safely invoke without prior registration:

functor<EitherTKindPartial<OptionHK, Throwable>>()

Note the EitherT Functor instance depends on the Functor<F>. Kategory can now automatically resolve constrains recursively as declared in the EitherTFunctorInstanceImplicits#instance arguments.

Once all instances are migrated I will introduce a new processor @typeclass which generates the implicit object automatically and also the companion helpers for the datatype that the instance is targeting in the same way now @deriving does.

As a final pass we will remove all calls that manually provide instances that can be resolved implicitly in reified contexts.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9b47f64). Click here to learn what that means.
The diff coverage is 61.15%.

@@            Coverage Diff            @@
##             master     #264   +/-   ##
=========================================
  Coverage          ?   45.57%           
  Complexity        ?      315           
=========================================
  Files             ?      143           
  Lines             ?     3721           
  Branches          ?      423           
=========================================
  Hits              ?     1696           
  Misses            ?     1891           
  Partials          ?      134

Impacted Files	Coverage Δ	Complexity Δ
...-test/src/main/kotlin/kategory/laws/FunctorLaws.kt	5.26% <ø> (ø)	1 <0> (?)
...e/src/main/kotlin/kategory/typeclasses/Composed.kt	67.64% <ø> (ø)	0 <0> (?)
...src/main/kotlin/kategory/laws/FunctorFilterLaws.kt	5.55% <ø> (ø)	1 <0> (?)
...y-core/src/main/kotlin/kategory/arrow/FunctionK.kt	60% <ø> (ø)	0 <0> (?)
...core/src/main/kotlin/kategory/typeclasses/Monad.kt	65.67% <0%> (ø)	0 <0> (?)
...n/kotlin/kategory/instances/SequenceKWInstances.kt	0% <0%> (ø)	0 <0> (?)
...ry-test/src/main/kotlin/kategory/laws/MonadLaws.kt	1.4% <0%> (ø)	2 <0> (?)
...ore/src/main/kotlin/kategory/typeclasses/Monoid.kt	0% <0%> (ø)	0 <0> (?)
...y-core/src/main/kotlin/kategory/data/SequenceKW.kt	76.31% <0%> (ø)	8 <0> (?)
...ore/src/main/kotlin/kategory/typeclasses/Inject.kt	0% <0%> (ø)	0 <0> (?)
... and 71 more

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[pakoito]

Why have they lost their variance?

[raulraja]

Inferred types with the type-literal in java reflection not liking contravariant positions. Also these are usually mapped to root ADTs and typeclasses which are just invariant positions AFAIK.

[pakoito]

Magic!