Hi everyone<br>
<br>
I&#39;m trying to write a piece of Haskell code, but for some reason I<br>
can&#39;t make it working. I&#39;m asking for your help in finding a bug in<br>
this code:<br>
<br>
&gt; import Data.Tree<br>
&gt; import Data.List<br>
<br>
&gt; expandTree :: Int -&gt; [(Int,Int)] -&gt; ((Tree Int),[(Int,Int)])<br>
&gt; expandTree rootLabel [] = ((Node rootLabel []),[]) -- probably obsolete line<br>
&gt; expandTree rootLabel nodePairs = ((Node rootLabel forest),unused)<br>
&gt; &nbsp; &nbsp;where<br>
&gt; &nbsp; &nbsp; &nbsp;(marked,unmarked) = partition (\ (x,y) -&gt; (x==rootLabel) || (y==rootLabel)) nodePairs<br>
&gt; &nbsp; &nbsp; &nbsp;(forest,unused) = foldl<br>
&gt; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;(\ (forestAcc, unusedPairs) node -&gt; let (a,b) = expandTree node unused in ((a:forestAcc),b) )<br>
&gt; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;([],unmarked)<br>
&gt; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;(map (\(o,p)-&gt; if o == rootLabel then p else o ) marked)<br>
<br>
<br>
It is supposed to create tree with root labeled with rootLabel, while<br>
nodePairs is a description of a tree. If a list is not exhausted, the<br>
remaining should be returned. It should work so that:<br>
<br>
expandTree 1 [(1,2)] == ((Node 1 [(Node 2 [])]),[])<br>
expandTree 1 [(2,3)] == ((Node 1 []),[(2,3)])<br>
expandTree 1 [(1,2),(3,4)] == ((Node 1 [(Node 2 [])]),[(3,4)])<br>
<br>
However, the code loops. Any clues?<br>
<br>
Thanks in advance.<br>
<br>
Regards<br>
Christopher Skrzêtnicki