Cats: Lack of whileM, whileM_, untilM and untilM_ methods for Monad

[Atry]

These methods present in scalaz.Monad. Should we add them in cats as well?

[kailuowang]

Forgive my ignorance ( I never used these before), I am just curious is there any usecases for this other than the MonadState, in particular, MonadState[StateT[Trampoline, S, ?], S (with Stack safety) or MonadState[State[S, ?], S] (without) .

[peterneyens]

There is this closed PR #1216 by @tpolecat (from before the move of tailRecM to FlatMap).

[tpolecat]

@kailuowang these can be useful for IO-like types, or things that will be interpreted into IO … in doobie for instance get[Foo].whileM[Vector](next) is a program of type Free[ResultSetOp, Vector[Foo]] that expresses the intent to read a resultset into a vector.

I closed that PR because I lost interest in the tailRecM discussion, but it should be straightforward to implement now.

[Atry]

@kailuowang I am porting ThoughtWorks Each to cats.

At the moment, ThoughtWorks Each requires whileM_ on scalaz.Monad in order to transform do and while loops.

I will be able to transform do and while loops on cats, if cats support whileM_, too.

[johnynek]

Now that we have tailRecM on FlatMap these methods are easy (and stack
safe).
On Wed, Mar 22, 2017 at 15:33 杨博 (Yang Bo) [e-mail] wrote:

@kailuowang https://github.com/kailuowang I am porting ThoughtWorks Each
https://github.com/ThoughtWorksInc/each to cats.

At the moment, ThoughtWorks Each https://github.com/ThoughtWorksInc/each
requires whileM_ on scalaz.Monad in order to transform do and while loops.

I will be able to transform do and while loops on cats, if cats support
whileM_, too.

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