Dear Haskell Community,<br>
<br>
I have been bedeviled by a persistent confusion about one aspect of 
where/let clauses, which I think I can illustrate more easily with code 
than with words.  Given code like this:<br>
<br>
averageR pxls = map (pcast . setR ravg) pxls&#39;<br>
  where (ravg,_,_) = averages pxls&#39;<br>
        pxls&#39;              = map pcast pxls<br>
<br>
What happens if I substitute pxls&#39; for its right-hand side,<br>
<br>
averageR pxls = map (pcast . setR ravg) (map pcast pxls)<br>

  where (ravg,_,_) = averages (map pcast pxls)<br>
<br>
My &quot;superstition&quot; here has been that the former only evaluates pxls&#39; 
once, whereas the latter computes it twice.  This seems like a basic 
issue which must have been confirmed or debunked somewhere in my 
readings, but it hasn&#39;t sunk in with me.