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Add invariant docs #3492

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merged 3 commits into from
Jul 3, 2020
Merged

Add invariant docs #3492

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068909d
improve Invariant docs
kazchimo Jun 20, 2020
44ad3bf
improve type annotations
kazchimo Jun 20, 2020
e8c0604
fix docs but
kazchimo Jun 20, 2020
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73 changes: 62 additions & 11 deletions core/src/main/scala/cats/Invariant.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,7 @@ import scala.annotation.implicitNotFound
* {{{
* scala> import cats.implicits._
* scala> import scala.concurrent.duration._
*
* scala> val durSemigroup: Semigroup[FiniteDuration] =
* | Invariant[Semigroup].imap(Semigroup[Long])(Duration.fromNanos)(_.toNanos)
* scala> durSemigroup.combine(2.seconds, 3.seconds)
Expand All @@ -31,22 +32,72 @@ import scala.annotation.implicitNotFound
*/
def imap[A, B](fa: F[A])(f: A => B)(g: B => A): F[B]

/**
* Compose Invariant `F[_]` and `G[_]` then produce `Invariant[F[G[_]]]` using their `imap`.
*
* Example:
* {{{
* scala> import cats.implicits._
* scala> import scala.concurrent.duration._
*
* scala> val durSemigroupList: Semigroup[List[FiniteDuration]] =
* | Invariant[Semigroup].compose[List].imap(Semigroup[List[Long]])(Duration.fromNanos)(_.toNanos)
* scala> durSemigroupList.combine(List(2.seconds, 3.seconds), List(4.seconds))
* res1: List[FiniteDuration] = List(2 seconds, 3 seconds, 4 seconds)
* }}}
*/
def compose[G[_]: Invariant]: Invariant[λ[α => F[G[α]]]] =
new ComposedInvariant[F, G] {
val F = self
val G = Invariant[G]
val F: Invariant[F] = self
val G: Invariant[G] = Invariant[G]
}

/**
* Compose Invariant `F[_]` and Functor `G[_]` then produce `Invariant[F[G[_]]]`
* using F's `imap` and G's `map`.
*
* Example:
* {{{
* scala> import cats.implicits._
* scala> import scala.concurrent.duration._
*
* scala> val durSemigroupList: Semigroup[List[FiniteDuration]] =
* | Invariant[Semigroup]
* | .composeFunctor[List]
* | .imap(Semigroup[List[Long]])(Duration.fromNanos)(_.toNanos)
* scala> durSemigroupList.combine(List(2.seconds, 3.seconds), List(4.seconds))
* res1: List[FiniteDuration] = List(2 seconds, 3 seconds, 4 seconds)
* }}}
*/
def composeFunctor[G[_]: Functor]: Invariant[λ[α => F[G[α]]]] =
new ComposedInvariantCovariant[F, G] {
val F = self
val G = Functor[G]
val F: Invariant[F] = self
val G: Functor[G] = Functor[G]
}

/**
* Compose Invariant `F[_]` and Contravariant `G[_]` then produce `Invariant[F[G[_]]]`
* using F's `imap` and G's `contramap`.
*
* Example:
* {{{
* scala> import cats.implicits._
* scala> import scala.concurrent.duration._
*
* scala> type ToInt[T] = T => Int
* scala> val durSemigroupToInt: Semigroup[ToInt[FiniteDuration]] =
* | Invariant[Semigroup]
* | .composeContravariant[ToInt]
* | .imap(Semigroup[ToInt[Long]])(Duration.fromNanos)(_.toNanos)
* // semantically equal to (2.seconds.toSeconds.toInt + 1) + (2.seconds.toSeconds.toInt * 2) = 7
* scala> durSemigroupToInt.combine(_.toSeconds.toInt + 1, _.toSeconds.toInt * 2)(2.seconds)
* res1: Int = 7
* }}}
*/
def composeContravariant[G[_]: Contravariant]: Invariant[λ[α => F[G[α]]]] =
new ComposedInvariantContravariant[F, G] {
val F = self
val G = Contravariant[G]
val F: Invariant[F] = self
val G: Contravariant[G] = Contravariant[G]
}
}

Expand Down Expand Up @@ -104,7 +155,7 @@ object Invariant extends ScalaVersionSpecificInvariantInstances with InvariantIn

def imap[A, B](fa: Monoid[A])(f: A => B)(g: B => A): Monoid[B] =
new Monoid[B] {
val empty = f(fa.empty)
val empty: B = f(fa.empty)
def combine(x: B, y: B): B = f(fa.combine(g(x), g(y)))
override def combineAllOption(bs: IterableOnce[B]): Option[B] =
fa.combineAllOption(bs.iterator.map(g)).map(f)
Expand Down Expand Up @@ -137,7 +188,7 @@ object Invariant extends ScalaVersionSpecificInvariantInstances with InvariantIn

def imap[A, B](fa: CommutativeMonoid[A])(f: A => B)(g: B => A): CommutativeMonoid[B] =
new CommutativeMonoid[B] {
val empty = f(fa.empty)
val empty: B = f(fa.empty)
def combine(x: B, y: B): B = f(fa.combine(g(x), g(y)))
override def combineAllOption(bs: IterableOnce[B]): Option[B] =
fa.combineAllOption(bs.iterator.map(g)).map(f)
Expand All @@ -149,7 +200,7 @@ object Invariant extends ScalaVersionSpecificInvariantInstances with InvariantIn

def imap[A, B](fa: BoundedSemilattice[A])(f: A => B)(g: B => A): BoundedSemilattice[B] =
new BoundedSemilattice[B] {
val empty = f(fa.empty)
val empty: B = f(fa.empty)
def combine(x: B, y: B): B = f(fa.combine(g(x), g(y)))
override def combineAllOption(bs: IterableOnce[B]): Option[B] =
fa.combineAllOption(bs.iterator.map(g)).map(f)
Expand All @@ -161,7 +212,7 @@ object Invariant extends ScalaVersionSpecificInvariantInstances with InvariantIn

def imap[A, B](fa: Group[A])(f: A => B)(g: B => A): Group[B] =
new Group[B] {
val empty = f(fa.empty)
val empty: B = f(fa.empty)
def combine(x: B, y: B): B = f(fa.combine(g(x), g(y)))
def inverse(b: B): B = f(fa.inverse(g(b)))
override def combineAllOption(bs: IterableOnce[B]): Option[B] =
Expand All @@ -174,7 +225,7 @@ object Invariant extends ScalaVersionSpecificInvariantInstances with InvariantIn

def imap[A, B](fa: CommutativeGroup[A])(f: A => B)(g: B => A): CommutativeGroup[B] =
new CommutativeGroup[B] {
val empty = f(fa.empty)
val empty: B = f(fa.empty)
def combine(x: B, y: B): B = f(fa.combine(g(x), g(y)))
def inverse(b: B): B = f(fa.inverse(g(b)))
override def combineAllOption(bs: IterableOnce[B]): Option[B] =
Expand Down