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Reduce stack depth in StateT #1466

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merged 4 commits into from
Dec 31, 2016
Merged

Reduce stack depth in StateT #1466

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df3137f
Reduce stack depth in StateT
johnynek Nov 15, 2016
9bbbf5d
only use FlatMap for product
johnynek Nov 16, 2016
595be31
Lower required typeclasses
johnynek Nov 16, 2016
d5e371f
Add more methods
johnynek Nov 22, 2016
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42 changes: 29 additions & 13 deletions core/src/main/scala/cats/data/StateT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -12,25 +12,32 @@ import cats.syntax.either._
final class StateT[F[_], S, A](val runF: F[S => F[(S, A)]]) extends Serializable {

def flatMap[B](fas: A => StateT[F, S, B])(implicit F: Monad[F]): StateT[F, S, B] =
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this only needs FlatMap[F] in this formulation, but I didn't want to change binary compatibility for a negligible gain.

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I think that makes sense for now. Maybe add a comment to that effect? If we ever want to tighten things up later that will be handy.

StateT(s =>
F.flatMap(runF) { fsf =>
F.flatMap(fsf(s)) { case (s, a) =>
StateT.applyF(F.map(runF) { sfsa =>
sfsa.andThen { fsa =>
F.flatMap(fsa) { case (s, a) =>
fas(a).run(s)
}
})
}
})

def flatMapF[B](faf: A => F[B])(implicit F: Monad[F]): StateT[F, S, B] =
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only needs FlatMap[F] in the current formulation.

StateT(s =>
F.flatMap(runF) { fsf =>
F.flatMap(fsf(s)) { case (s, a) =>
F.map(faf(a))((s, _))
}
StateT.applyF(F.map(runF) { sfsa =>
sfsa.andThen { fsa =>
F.flatMap(fsa) { case (s, a) => F.map(faf(a))((s, _)) }
}
)
})

def map[B](f: A => B)(implicit F: Monad[F]): StateT[F, S, B] =
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if we fix transform to only use Functor[F] then this only needs Functor[F], which is fitting for map.

transform { case (s, a) => (s, f(a)) }

def product[B](sb: StateT[F, S, B])(implicit F: Monad[F]): StateT[F, S, (A, B)] =
StateT.applyF(F.map2(runF, sb.runF) { (ssa, ssb) =>
ssa.andThen { fsa =>
F.flatMap(fsa) { case (s, a) =>
F.map(ssb(s)) { case (s, b) => (s, (a, b)) }
}
}
})
/**
* Run with the provided initial state value
*/
Expand Down Expand Up @@ -70,9 +77,12 @@ final class StateT[F[_], S, A](val runF: F[S => F[(S, A)]]) extends Serializable
* Like [[map]], but also allows the state (`S`) value to be modified.
*/
def transform[B](f: (S, A) => (S, B))(implicit F: Monad[F]): StateT[F, S, B] =
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this only needs Functor[F] in this formulation.

transformF { fsa =>
F.map(fsa){ case (s, a) => f(s, a) }
}
StateT.applyF(
F.map(runF) { sfsa =>
sfsa.andThen { fsa =>
F.map(fsa) { case (s, a) => f(s, a) }
}
})

/**
* Like [[transform]], but allows the context to change from `F` to `G`.
Expand Down Expand Up @@ -231,6 +241,12 @@ private[data] sealed trait StateTMonad[F[_], S] extends Monad[StateT[F, S, ?]] {

override def map[A, B](fa: StateT[F, S, A])(f: A => B): StateT[F, S, B] = fa.map(f)

override def ap[A, B](ff: StateT[F, S, A => B])(fa: StateT[F, S, A]): StateT[F, S, B] =
map2(ff, fa) { case (f, a) => f(a) }

override def product[A, B](fa: StateT[F, S, A], fb: StateT[F, S, B]): StateT[F, S, (A, B)] =
fa.product(fb)

def tailRecM[A, B](a: A)(f: A => StateT[F, S, Either[A, B]]): StateT[F, S, B] =
StateT[F, S, B](s => F.tailRecM[(S, A), (S, B)]((s, a)) {
case (s, a) => F.map(f(a).run(s)) { case (s, ab) => ab.bimap((s, _), (s, _)) }
Expand Down