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  1. PO
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    <p>First off, <code>Step</code> looks perfect to me. As for <code>Stream</code>, I think you're on the right track with the abstract type. Here's what I came up with (including implementations of the remaining methods in section 2.1 of the Coutts paper):</p> <pre><code>abstract class Stream[A] { protected type S def next: S =&gt; Step[A, S] def state: S def map[B](f: A =&gt; B): Stream[B] = { val next: S =&gt; Step[B, S] = this.next(_) match { case Done =&gt; Done case Skip(s) =&gt; Skip(s) case Yield(a, s) =&gt; Yield(f(a), s) } Stream(next, state) } def unstream: List[A] = { def unfold(s: S): List[A] = next(s) match { case Done =&gt; List.empty case Skip(s) =&gt; unfold(s) case Yield(a, s) =&gt; a :: unfold(s) } unfold(state) } } object Stream { def apply[A, S0](n: S0 =&gt; Step[A, S0], s: S0) = new Stream[A] { type S = S0 val next = n val state = s } def apply[A](as: List[A]): Stream[A] = { val next: List[A] =&gt; Step[A, List[A]] = { case a :: as =&gt; Yield(a, as) case Nil =&gt; Done } Stream(next, as) } def unapply[A](s: Stream[A]): Option[(s.S =&gt; Step[A, s.S], s.S)] = Some((s.next, s.state)) } </code></pre> <p>A couple things to note:</p> <ul> <li>My <code>unapply</code> has a dependent method type: it depends on the <code>s.S</code>. I think that might have been your stumbling block.</li> <li>The <code>unfold</code> method in <code>unstream</code> is not tail-recursive.</li> </ul> <p>The thing I'm still not really clear on myself is why it's important for <code>S</code> to be existential / hidden / whatever. If it's not, you could just write:</p> <pre><code>case class Stream[A, S](next: S =&gt; Step[A, S], state: S) </code></pre> <p>... but I assume there's a reason for it. That being said, I'm also not sure this approach actually hides <code>S</code> the way you want. But this is my story and I'm sticking to it.</p>
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    1. POCorrect encoding of this existential type in Scala?
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    1. USmergeconflict
    1. USmergeconflict
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    1. POCorrect encoding of this existential type in Scala?
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    1. COI agree that the abstract type is most promising approach to take.
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      1. USMiles Sabin
    2. COI will follow the abstract type member approach then, I'm still feel consistently torn on what the best approach is when encoding existentials. @mergeconflict thanks for pointing out my stumbling on the dependent type, I see the much clearer solution.
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      1. USjroesch
    3. COAlso the hidden type is to enable stream's of the same type to be equivalent regardless of their internal state/producer, in that regard I think it works well enough.
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