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Add more doctest to Monad and FlatMap #4427

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Add more doctest to Monad and FlatMap #4427

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7f45e3b
Wip: doctests for FlatMap and Monad
diogocanut Apr 12, 2023
f8a5b37
Merge branches 'flatMap-monad-doctest' and 'main' of https://github.c…
diogocanut Apr 17, 2023
7fd2fa2
Adding more doctests to Monad
diogocanut Apr 17, 2023
2510abe
fix: FlatMap counter example
diogocanut Apr 17, 2023
b61ca6f
Merge branch 'main' of github.com:diogocanut/cats into flatMap-monad-…
diogocanut May 20, 2023
ecac102
improve ifM doctests, add string instead of boolean
diogocanut May 20, 2023
4d72a8b
Using State instead of StateT
diogocanut Aug 22, 2023
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26 changes: 26 additions & 0 deletions core/src/main/scala/cats/FlatMap.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -137,6 +137,22 @@ trait FlatMap[F[_]] extends Apply[F] with FlatMapArityFunctions[F] {

/**
* `if` lifted into monad.
*
* Example:
* {{{
* scala> import cats.Eval
* scala> import cats.syntax.all._
*
* scala> val b1 = Eval.now(true)
* scala> val asString1 = b1.ifM(Eval.now("it's true!"), Eval.now("it's false!"))
* scala> asString1.value
* res0: String = it's true!

* scala> val b2 = Eval.now(false)
* scala> val asString2 = b2.ifM(Eval.now("it's true!"), Eval.now("it's false!"))
* scala> asString2.value
* res1: String = it's false!
* }}}
*/

def ifM[B](fa: F[Boolean])(ifTrue: => F[B], ifFalse: => F[B]): F[B] =
Expand Down Expand Up @@ -207,6 +223,16 @@ trait FlatMap[F[_]] extends Apply[F] with FlatMapArityFunctions[F] {
* This repeats an F until we get defined values. This can be useful
* for polling type operations on State (or RNG) Monads, or in effect
* monads.
*
* Example:
* {{{
* scala> import cats.data.State
* scala> import cats.syntax.all._
* scala> val counter = State { i: Int => (i+1, if(i>100) Some(i) else None)}
* scala> val eval = counter.untilDefinedM.run(0)
* scala> eval.value
* res0: (Int, Int) = (102,101)
* }}}
*/

def untilDefinedM[A](foa: F[Option[A]]): F[A] = {
Expand Down
90 changes: 90 additions & 0 deletions core/src/main/scala/cats/Monad.scala
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Expand Up @@ -40,6 +40,18 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
* Collects the results into an arbitrary `Alternative` value, such as a `Vector`.
* This implementation uses append on each evaluation result,
* so avoid data structures with non-constant append performance, e.g. `List`.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val appendStr: StateT[Id, String, Unit] = StateT.modify(_ + "a")
* scala> val appendStrAndGet: StateT[Id, String, String] = appendStr *> StateT.get
* scala> val (result, agg) = appendStrAndGet.whileM[Vector](StateT.inspect(i => !(i.length >= 5))).run("")
* result: String = aaaaa
* agg: Vector[String] = Vector(a, aa, aaa, aaaa, aaaaa)
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Instead of StateT[Id, ...] can't we just use State? The issue with StateT[Id, ...] is that actually it is not stacksafe.

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Sure, should I use State also in all other examples that uses StateT or just this one?

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Yes. You should never use StateT[Id.

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Done. I'm using State now, but there's some results that's previous result + 1, like in iterateUntilM. Is that okay?

* }}}
*/

def whileM[G[_], A](p: F[Boolean])(body: => F[A])(implicit G: Alternative[G]): F[G[A]] = {
Expand All @@ -60,6 +72,16 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
* Execute an action repeatedly as long as the given `Boolean` expression
* returns `true`. The condition is evaluated before the loop body.
* Discards results.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val appendStr: StateT[Id, String, Unit] = StateT.modify(_ + "a")
* scala> val (result, _) = appendStr.whileM_(StateT.inspect(i => !(i.length >= 3))).run("")
* result: String = aaa
* }}}
*/

def whileM_[A](p: F[Boolean])(body: => F[A]): F[Unit] = {
Expand All @@ -80,6 +102,18 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
* arbitrary `Alternative` value, such as a `Vector`.
* This implementation uses append on each evaluation result,
* so avoid data structures with non-constant append performance, e.g. `List`.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
* scala> val incrementAndGet: StateT[Id, Int, Int] = increment *> StateT.get
* scala> val (result, aggregation) = incrementAndGet.untilM[Vector](StateT.inspect(_ >= 5)).run(-1)
* result: Int = 5
* aggregation: Vector[Int] = Vector(0, 1, 2, 3, 4, 5)
* }}}
*/
def untilM[G[_], A](f: F[A])(cond: => F[Boolean])(implicit G: Alternative[G]): F[G[A]] = {
val p = Eval.later(cond)
Expand All @@ -89,6 +123,16 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
/**
* Execute an action repeatedly until the `Boolean` condition returns `true`.
* The condition is evaluated after the loop body. Discards results.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
* scala> val (result, _) = increment.untilM_(StateT.inspect(_ >= 5)).run(-1)
* result: Int = 5
* }}}
*/
def untilM_[A](f: F[A])(cond: => F[Boolean]): F[Unit] = {
val p = Eval.later(cond)
Expand All @@ -98,6 +142,17 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
/**
* Execute an action repeatedly until its result fails to satisfy the given predicate
* and return that result, discarding all others.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
* scala> val incrementAndGet: StateT[Id, Int, Int] = increment *> StateT.get
* scala> val (result, _) = incrementAndGet.iterateWhile(_ < 5).run(-1)
* result: Int = 5
* }}}
*/
def iterateWhile[A](f: F[A])(p: A => Boolean): F[A] =
flatMap(f) { i =>
Expand All @@ -107,6 +162,17 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
/**
* Execute an action repeatedly until its result satisfies the given predicate
* and return that result, discarding all others.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
* scala> val incrementAndGet: StateT[Id, Int, Int] = increment *> StateT.get
* scala> val (result, _) = incrementAndGet.iterateUntil(_ == 5).run(-1)
* result: Int = 5
* }}}
*/
def iterateUntil[A](f: F[A])(p: A => Boolean): F[A] =
flatMap(f) { i =>
Expand All @@ -116,6 +182,18 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
/**
* Apply a monadic function iteratively until its result fails
* to satisfy the given predicate and return that result.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
* scala> val incrementAndGet: StateT[Id, Int, Int] = increment *> StateT.get
* scala> val (n, sum) = 0.iterateWhileM(s => incrementAndGet.map(_ + s))(_ < 5).run(0)
* n: Int = 3
* sum: Int = 6
* }}}
*/
def iterateWhileM[A](init: A)(f: A => F[A])(p: A => Boolean): F[A] =
tailRecM(init) { a =>
Expand All @@ -128,6 +206,18 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
/**
* Apply a monadic function iteratively until its result satisfies
* the given predicate and return that result.
*
* Example:
* {{{
* scala> import cats.{Id, Monad}
* scala> import cats.data.StateT
* scala> import cats.syntax.all._
* scala> val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
* scala> val incrementAndGet: StateT[Id, Int, Int] = increment *> StateT.get
* scala> val (n, sum) = 0.iterateUntilM(s => incrementAndGet.map(_ + s))(_ > 5).run(0)
* n: Int = 3
* sum: Int = 6
* }}}
*/
def iterateUntilM[A](init: A)(f: A => F[A])(p: A => Boolean): F[A] =
iterateWhileM(init)(f)(!p(_))
Expand Down