Add iterateWhileM and iterateUntilM #1809

drbild · 2017-08-09T18:12:26Z

Adds iterateWhileM and iterateUntilM to complement the existing iterateWhile and iterateUntil methods.
Reimplements iterateWhile and iterateUntil in terms of the new iterateWhileM and iterateUntilM.

kailuowang · 2017-08-09T18:21:51Z

looks great! Thanks! 👍 pending build pass.

codecov-io · 2017-08-09T20:21:25Z

Codecov Report

Merging #1809 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1809      +/-   ##
==========================================
+ Coverage   94.96%   94.97%   +<.01%     
==========================================
  Files         241      241              
  Lines        4193     4195       +2     
  Branches      117      109       -8     
==========================================
+ Hits         3982     3984       +2     
  Misses        211      211

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/monad.scala	`100% <100%> (ø)`	⬆️
core/src/main/scala/cats/Monad.scala	`96% <100%> (-0.16%)`	⬇️

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peterneyens · 2017-08-09T21:34:52Z

Just wondering, these iterateUntil / iterateWhile methods only make sense for something like Task or IO, right ?
Are there cases where the f: A => F[A] would actually return something different every time, so you couldn't use f(a).iterateUntil(pred) instead of a.iterateUntilM(f)(pred) ?

Edit: I missed that in the M variants, the f is reused. Makes more sense now 😄. Thanks for this PR.
I would like to see some less trivial scaladoc examples as the ones using Id.

drbild · 2017-08-09T22:45:10Z

@peterneyens It's a common idiom in Haskell where A represents some state and A => F[A] is a side-effecting update function and you want to iterate until some terminal state is reached.

Simple games are a frequent example. A is the current state of the world. A => IO[A] is the step function that reads user input, updates the state accordingly, and refreshes the UI. A => Boolean checks for a termination condition like the player dying.

So the main loop becomes something like

def initialWorld: World = ??? // initial world state

def stepWorld(old): IO[World] = getUserInput >>= updateWorld(old) >>! refreshUI

def keepRunning(current): Boolean = isPlayerAlive(current)

def loop: IO[World] = initialWorld.iterateWhileM(stepWorld)(keepRunning)

This example could all be rewritten using StateT[IO, World, ?] so that

def loop = stepWorld.iterateWhile(keepRunning).run(initialWorld)

would work. But pulling in StateT often seems like overkill.

drbild · 2017-08-09T22:49:36Z

I think of iterateWhile as a specialized version of iterateWhileM. As an analogy:

iterateWhile : iterateWhileM :: followedBy : flatMap

kailuowang · 2017-08-10T00:03:45Z

I agree that some less trivial tests/examples would be nice.

tpolecat

This looks fine to me, but I agree it would be nice to see a test with something less trivial, like Eval maybe.

johnynek

this is great, in my opinion, but I'd also like to see a more meaty monad in the tests. I'd like to see State or Reader for instance.

johnynek · 2017-08-11T17:37:32Z

tests/src/test/scala/cats/tests/MonadTest.scala

@@ -74,4 +74,28 @@ class MonadTest extends CatsSuite {
    result should ===(50000)
  }

+  test("iterateWhileM") {
+    forAll(Gen.posNum[Int]) { (max: Int) =>


might this not run a super long time? any reason why max shouldn't be something from like: Gen.choose(0, 500000)?

johnynek · 2017-08-11T17:38:33Z

core/src/main/scala/cats/Monad.scala

+    * Apply a monadic function iteratively until its result fails
+    * to satisfy the given predicate and return that result.
+    */
+  def iterateWhileM[A](init: A)(f: A => F[A])(p: A => Boolean): F[A] = {


no need for the outer { } here.

johnynek · 2017-08-11T17:38:47Z

core/src/main/scala/cats/Monad.scala

+    * Apply a monadic function iteratively until its result satisfies
+    * the given predicate and return that result.
+    */
+  def iterateUntilM[A](init: A)(f: A => F[A])(p: A => Boolean): F[A] = {


no need for the outer { }

peterneyens · 2017-08-15T08:33:30Z

core/src/main/scala/cats/Monad.scala

+      if (p(a))
+        map(f(a))(_.asLeft[A])
+      else
+        pure(a.asRight[A])


Can we replace the asLeft and asRight with just Left(..) and Right(..) (in whileM as well) ?

kailuowang · 2017-08-28T15:31:24Z

LGTM, @peterneyens @johnynek looks like that the comments were addressed. any other feedback?

drbild · 2017-08-28T15:52:39Z

I've left the Id-based example in the scaladoc, because nothing with a non-trivial monad but simple enough for a scaladoc has come to mind. I could drop the scaladoc entirely for now. Some simple iterative algorithm - like Euclid's GCD - but with a possible failure condition (so in the Option or Either monad) would be good. I just haven't been able come up with one.

I'm also working on some additions to the Monad doc, using some real examples - finding all solutions of a subset-sum variant and all solutions of the n-queens problem, both in the List monad. Those can go in a different PR though.

kailuowang · 2017-08-28T16:31:22Z

I love the doc ideas, they sound awesome. I'd say we can have them in a separate PR so that we can have the binary breaking change merged in asap.

non · 2017-08-30T15:17:35Z

Looks good to me. 👍

kailuowang · 2017-08-30T15:20:14Z

since all feedback seems addressed and I am going to merge with 3 sign-offs.

* Limit runtime of monad loop tests * Do not use Either syntax in monad loops * Add iterateWhileM and iterateUntilM * Implement iterateWhile in terms of iterateWhileM

kailuowang added this to the 1.0.0-RC1 milestone Aug 9, 2017

kailuowang requested a review from tpolecat August 9, 2017 20:54

tpolecat approved these changes Aug 10, 2017

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johnynek reviewed Aug 11, 2017

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drbild force-pushed the add-iterateuntilm-etc branch 2 times, most recently from 5742379 to 3ae823b Compare August 14, 2017 17:00

peterneyens reviewed Aug 15, 2017

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drbild force-pushed the add-iterateuntilm-etc branch 5 times, most recently from bcfd5e6 to 075ae1f Compare August 20, 2017 22:01

kailuowang approved these changes Aug 28, 2017

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drbild added 4 commits August 29, 2017 19:56

Limit runtime of monad loop tests

14e1695

Do not use Either syntax in monad loops

9a31560

Add iterateWhileM and iterateUntilM

3f7dd4d

Implement iterateWhile in terms of iterateWhileM

60d8e5a

drbild force-pushed the add-iterateuntilm-etc branch from 075ae1f to 60d8e5a Compare August 30, 2017 00:58

kailuowang merged commit 406d984 into typelevel:master Aug 30, 2017

drbild deleted the add-iterateuntilm-etc branch August 30, 2017 16:53

kailuowang added the enhancement label Oct 15, 2017

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Add iterateWhileM and iterateUntilM #1809

Add iterateWhileM and iterateUntilM #1809

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Add iterateWhileM and iterateUntilM #1809

Add iterateWhileM and iterateUntilM #1809

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