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POScala state monad in for comprehension
 

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    <p>I'm studying Michael Pilquist's excellent state monad lecture <a href="http://www.youtube.com/watch?v=Jg3Uv_YWJqI" rel="nofollow">here</a>. I get stuck at 54 min with two questions.</p> <ol> <li><p>If <code>ofs</code> is an <code>Option[FollowerStats]</code>, what is the <code>?</code> operator? I can't find a ternary operator on Option in Scala 2.10.2</p></li> <li><p>How does the last generator put an updated cache (with hit or miss incremented) back into the <code>State</code> result of the checkCache method? The returned <code>State</code> seems to be be discarded and the for comprehension seems to only yielding the <code>Option[FollowerStats]</code></p></li> </ol> <p>.</p> <pre><code>def checkCache(u: String): State[Cache, Option[FollowerState]] = for { c &lt;- State.get[Cache] ofs &lt;- State.state { c.get(u).collect { case Timestamped(fs, ts) if !state(ts) =&gt; fs } } _ &lt;- State.put(ofs ? c.recordHit | c.recordMiss) } yield ofs </code></pre> <p>To try and understand I attempted to re-write the for comprehension, but it's not helped.</p> <pre><code>State.get[Cache].flatMap{ c =&gt; State.state{c.get(u).collect(...)}.flatMap{ ofs =&gt; State.put(ofs ? c.recordHit | c.recordMiss).map{ _ =&gt; ofs } } } </code></pre> <p><strong>Update:</strong> 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 <code>s =&gt; (s,())</code> from the <code>put</code> and replace the <code>Unit</code> value type with ofs, to get <code>s =&gt;(s,ofs)</code>. I guess the key is realising that the yield isn't literally returning ofs, but it's translated as State.map.</p> <p><strong>Update</strong> Understand the option bit now. I guess the presentation still uses the implicits from Scalaz although it's deriving the state monad.</p>
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    1. COThe `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.
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    2. COAt 1:06:12 he actually mentions the option thing. Should have continued watching, but OCD prevented it.
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    3. COWell 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! :)
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