trying to tie the knot

[Hal Daume III]

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Hi All,

I thought I was starting to get my head around the whole tying the knot
phenomenon, but when my program produced "*** Exception: <<loop>>" I
figured I was probably missing something :).

Here's what I'm trying to do (I know I could do it without tying the knot,
but that seemed like a reasonable approach and I wanted to get my hands
dirty or feat wet):

I'm reading in a file; that is I have a function String ->
MyDataType.  More specifically I'm reading in a Tree, and the tree is
specified in the file with these "Subtree" fields.  So you can be reading
along in the tree and then instead of actually seeing a branch or leaf,
you'll see somethign like "[S1]" which means "stick subtree S1 in
here".  later in the file (arbitrarily later), you will see something like
"Subtree [S1]" and then following that is that tree.

So what I did was I changed my function to a String -> [(String,Tree)] ->
(Tree, [(String,Tree)] function where the list is a lookup for the
subtrees.  I then said:

readTrees s = let (t, st) = readTrees' s st in t

and then the readTrees' basically read the tree in as normal; except when
it gets to something like "[S1]" it does lookup "[S1]" subTrees to get the
subtree and then when it gets to "Subtree [S1]" it reads in the subtree
and then adds it to the subtree list.

Is this not an appropriate use of tying the knot or amd I just doing
something wrong?  I've attached my code if you want to read it; It's
marginally more complex that I've written here jsut because of some of the
idiosyncracies in the file format...

 - Hal

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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