Scala state monad in for comprehension

Question

I'm studying Michael Pilquist's excellent state monad lecture here. I get stuck at 54 min with two questions.

If ofs is an Option[FollowerStats], what is the ? operator? I can't find a ternary operator on Option in Scala 2.10.2
How does the last generator put an updated cache (with hit or miss incremented) back into the State result of the checkCache method? The returned State seems to be be discarded and the for comprehension seems to only yielding the Option[FollowerStats]

.

def checkCache(u: String): State[Cache, Option[FollowerState]] = for {
    c <- State.get[Cache]
    ofs <- State.state {
        c.get(u).collect {
            case Timestamped(fs, ts) if !state(ts) => fs
        }
    }
    _ <- State.put(ofs ? c.recordHit | c.recordMiss)
} yield ofs

To try and understand I attempted to re-write the for comprehension, but it's not helped.

State.get[Cache].flatMap{ c => 
    State.state{c.get(u).collect(...)}.flatMap{ ofs =>
        State.put(ofs ? c.recordHit | c.recordMiss).map{ _ =>
            ofs
        }
    }
}

Update: I think I've grasped the key to point 2 thanks to the answers. I didn't realise that that the yield is essentially saying: take the last state s => (s,()) from the put and replace the Unit value type with ofs, to get s =>(s,ofs). I guess the key is realising that the yield isn't literally returning ofs, but it's translated as State.map.

Update Understand the option bit now. I guess the presentation still uses the implicits from Scalaz although it's deriving the state monad.

The Option ? ... | ... bit comes from an implicit conversion (specifically an implicit class OptionOps). You have probably already dealt with implicit conversions such as being able to call toInt on a String due to an implicit conversion to StringOps - this is much the same. — Shadowlands
At 1:06:12 he actually mentions the option thing. Should have continued watching, but OCD prevented it. — Pengin
Well hopefully the next OCD sufferer watching that vid will find this question, read all the comments, and know to jump straight back to watch the rest of the vid! :) — Shadowlands

Travis Brown Travis Brown · Accepted Answer · 2013-10-08T10:12:57

First for the ternary operator.

ofs ? c.recordHit | c.recordMiss

Is to equivalent to:

if (ofs.isDefined) c.recordHit else c.recordMiss

Or:

ofs.fold(c.recordMiss)(_ => c.recordHit)

It's provided by Scalaz and implemented as an implicit class (OptionOps) that provides a ? method for Option that returns a Conditional class that has a | method. This is a case where I'm not sure the extra concision is worth the confusion, but people use it, so it's worth knowing about.

For the state update, consider the following simplified example:

val inc: State[Int, Unit] = for { i <- get[Int]; _ <- put(i + 1) } yield ()

It might look at first like we're "discarding" the state and only yielding unit, but the value of inc is actually the entire state-manipulating computation (in this case one that doesn't return any relevant or interesting value), including the update.

Scala state monad in for comprehension

2 Answers