[Haskell-cafe] Pretty printing a tree

[wren ng thornton]

José Romildo Malaquias wrote:
> Hello.
> 
> I would like to pretty print a tree in a way that its structure is
> easily perceived.
> 
> For instance, consider the declarations:
> 
>    data Node a = Node a [Node a]
> 
>    type Tree a = [ Node a ]
> 
>    t = [ Node "a" [ Node "b" []
>                   , Node "c" [ Node "c1" []
>                              , Node "c2" [] ]
>                   , Node "d" [ Node "d1" [ Node "d1a" [] ]
>                              , Node "d2" [] ] ] ]
> 
> Then the resulting of pretty printing the given tree would be something
> like the following:
> 
>        a
>        |
> +-------------+
> |    |        |
> b    c        d
>      |        |
>    +---+    +---+
>    |   |    |   |
>    c1  c2   d1  d2
>             |
>            d1a

If you're just curious about how one would write such a thing, you can 
look at Data.Trie.Internal.showTrie[1]--- it's horizontal rather than 
vertical, and it doesn't center labels above their children, but it 
should give you a starting idea. Data.Map and Data.IntMap also have 
examples (showTree, showTreeWith) which are a bit simpler.

This is a common homework assignment (because it's a great exercise!) 
though I haven't seen any prepackaged generic solutions. Perhaps we need 
more enterprising students :)


[1] 
http://community.haskell.org/~wren/bytestring-trie/src/Data/Trie/Internal.hs

-- 
Live well,
~wren