Hi Cafe,<div><br></div><div><div>I try to implement some sort of monadic fold, where traversing is polymorphic over monad type.</div><div>The problem is that the code below does not compile. It works with any monad except for ST. </div>

<div>I suspect that monomorphism is at work here, but it is unclear for me how to change the code to make it work with ST.</div><div><br></div><div>fold :: Monad m =&gt; (Int -&gt; m ()) -&gt; m ()</div><div>fold f = mapM_ f [0..20]</div>

<div><br></div><div>selectFold :: Monad m =&gt; String -&gt; IO ((Int -&gt; m ()) -&gt; m ())</div><div>selectFold method = do</div><div>    -- in real program I&#39;d like to choose between </div><div>    -- different fold methods, based on some IO context</div>

<div>    return fold</div><div><br></div><div>useFold :: Monad m =&gt; ((Int -&gt; m ()) -&gt; m ()) -&gt; m ()</div><div>useFold fold&#39; = fold&#39; f</div><div>    where f _ = return () -- some trivial iterator</div>
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<br></div><div>main = do</div><div>    fold&#39;&#39; &lt;- selectFold &quot;some-method-id&quot;</div><div>    print $ runST $ useFold fold&#39;&#39;</div></div><div><br></div><div><br></div><div>Thank you!</div><div>Dmitry</div>