[Haskell-cafe] Painting logs to get a coloured tree

minh thu noteed at gmail.com
Tue Feb 10 08:38:02 EST 2009

2009/2/10 Joachim Breitner <mail at joachim-breitner.de>:
> Hi,
> Am Dienstag, den 10.02.2009, 10:05 +0100 schrieb minh thu:
>> I forgot to mention you can try to tie the knot too, using the result
>> of the processing in the first mapping (and then you don't need the
>> second one)...
> could you elaborate who to tie that particular knot? I unfortunately, I
> don't see it.
> Thanks,
> Joachim

I can post some code later but here is the idea.

You conceptually label the tree with Int's. If you go through the tree
visiting the node in a specific order, you don't have to actually
label it since the label of a node is just its position in the

The goal is to map the tree with some data drawn from an
association-list. Again, a straight-forward association is just a
plain list indexed by Int's.

Thus, when visiting the nodes of the tree, if you have the
above-mentionned list, you can use that information when doing the
mapping, replacing the data in the node by the data in the list (where
the index used for the list is the 'label' of the node).

The list is the result of going to the tree too, thus tying the knot.
To construct it, you simply make some kind of mapAccum, using [] as
the starting value and : (cons) to accumulate the data.

To understand this intuitiveley, just note that a three can be
flattened into a list. Thus if you want to process the
'association-list' which is represented by a plain list, just zipWith
it [0..].


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