Hi all,<div><br></div><div><div>Suppose I have a compound data type -</div><div><br></div><div>data M o = M (String,o)</div><div><br></div><div>Now, I can define a function that works for ALL M irrespective of o. For example -</div>

<div><br></div><div>f :: M o -&gt; M o</div><div>f (M (s,o)) = M (s++&quot;!&quot;, o)</div><div><br></div><div>I can also use this function in an expression, applying it to different types without problem -</div><div><br>

</div><div><div>p = (m1&#39;,m2&#39;) where</div><div>  m1 = M (&quot;1&quot;, ())</div><div>  m2 = M (&quot;2&quot;, True)</div><div>  m1&#39; = f m1</div><div>  m2&#39; = f m2</div></div><div><br></div><div>Main*&gt; p</div>

<div>(M (&quot;1!&quot;,()),M (&quot;2!&quot;,True))</div><div><br></div><div>However, if I try to parameterise over the function &#39;f&#39; it does not work!  -</div><div><br></div><div>p f = (m1&#39;,m2&#39;) where</div>

<div>  m1 = M (&quot;1&quot;, ())</div><div>  m2 = M (&quot;2&quot;, True)</div><div>  m1&#39; = f m1</div><div>  m2&#39; = f m2</div><div><br></div><div>It doesn&#39;t even typecheck, producing the error - &quot;Couldn&#39;t match expected type &#39;Bool&#39; with actual type &#39;()&#39;&quot;</div>

<div><br></div><div>Is there a particular reason for this? How can I define a function like &#39;p&#39; within Haskell?</div></div><div><br></div><div>Thanks,</div><div>Anupam Jain</div>