From: Luana Fagarasan <luana_0105@yahoo.com>

Hello,

I am quite new to Haskell and I am seeking the best way to display trees graphically when using Haskell. I have looked on the web, but am still not clear!

IMHO Haskell is a good language for this sort of task; there are several graphics libraries that I would consider fairly mature and reliable. I have a project that uses "diagrams" (http://hackage.haskell.org/package/diagrams) to draw a tree; the code is:

import Diagrams.Prelude

drawTree (Node dt children) = fmap (queryFunc $ nodePath dt) (circle 1 # scaleY 0.5 # fc darkblue # lw 0.1 # lc deepskyblue # pad 1.1 # named (show $ nodePath dt) ) === (hcat' with {sep = 0.2} (map drawTree children) # centerX)

drawTree could be understood as "draw the current node on top of the diagram produced by applying the node's children to drawTree". This function just draws each node as an oval. "#" is a diagrams-provided operator to apply parameters to a diagram. The "queryFunc" and "named" stuff is to enable queries into the diagram, so that mouse click coordinates can be converted into a label for the node (as retrieved by "nodePath"). Depending on what you want to display for each node, the image could be as simple or complex as you would like. Probably the most full-featured diagrams backend uses Cairo, so your drawings can be rendered in any format Cairo supports (including GTK widgets). Other backends are also available although I don't know as much about them. HTH, John Lato