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POHaskell version of yin-yang puzzle : Kind incompatibility error
 

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  1. POHaskell version of yin-yang puzzle : Kind incompatibility error
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    <p>I want to implement the <a href="https://stackoverflow.com/questions/9250474/does-the-yin-yang-continuations-puzzle-make-sense-in-a-typed-language">yin-yang puzzle</a> in Haskell. Here is my attempt (unsucceful):</p> <pre><code>-- The data type in use is recursive, so we must have a newtype defined newtype Cl m = Cl { goOn :: MonadCont m =&gt; Cl m -&gt; m (Cl m) } yinyang :: (MonadIO m, MonadCont m) =&gt; m (Cl m) yinyang = do yin &lt;- (callCC $ \k -&gt; return (Cl k)) &gt;&gt;= (\c -&gt; liftIO (putStr "@") &gt;&gt; goOn c) yang &lt;- (callCC $ \k -&gt; return (Cl k)) &gt;&gt;= (\c -&gt; liftIO (putStr "*") &gt;&gt; goOn c) goOn yin yang </code></pre> <p>When look at the types, obviously <code>callCC $ \k -&gt; return (Cl k)</code> gives a <code>m (Cl m)</code>, so <code>yin</code> is of type <code>Cl m</code>. <code>yang</code> is the same thing. So I expect <code>goOn yin yang</code> gives the final type <code>m (Cl m)</code>.</p> <p>This implementation looks good, but the problem is it does not compile! here is the error I got:</p> <pre><code>Couldn't match kind `*' against `* -&gt; *' Kind incompatibility when matching types: m0 :: * -&gt; * Cl :: (* -&gt; *) -&gt; * In the first argument of `goOn', namely `yin' In a stmt of a 'do' block: goOn yin yang </code></pre> <p>Any idea to fix this?</p> <p><strong>UPDATE</strong></p> <p>Although I found the answer by myself, I still don't understand what that error message means. Can any one explain to me? What I know already is, in the problematic version, <code>goOn c</code> returns something like <code>Cl m -&gt; m (Cl m)</code>, not the expected <code>m (Cl m)</code>. But that is not what you can get from the error message.</p>
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