script that parses indented text into a tree structure

parse_indents.hs

import Data.Char
import qualified Data.String as S
import System.IO
import System.Environment

-- Parses indented text into a tree structure, where each node is represented as a (content, children) tuple
-- Example invocation:
--     echo -e "a\n b\n c\n  d\n  d2\n e\nf\n g\n  h\n   i\n j\n k\nl\n" | ./parse_indents
-- a {b, c {d, d2}, e}, f {g {h {i}}, j, k}, l

-- Parsing functions

getIndent line =
    let (spaces, content) = span (\c -> c == ' ') line 
    in (length(spaces), content)

data Node = Node { content :: String, children :: [Node] }

parse list = parseNodes $ map getIndent list

parseNodes :: [(Int, String)] -> [Node]
parseNodes [] = []
parseNodes ((_indent, ""):rest) = parseNodes rest
parseNodes ((indent, content):rest) =
    -- the children of this node are indented further
    let (childLines, siblingLines) = span (\(i,c) -> i > indent) rest
        children = parseNodes childLines
        siblings = parseNodes siblingLines
    in (Node content children:siblings)


-- Display functions

join :: String -> [String] -> String
join d [s] = s
join d (h:t) = h ++ d ++ join d t

format :: String -> [Node] -> [String]
format indent [] = []
format indent (node:nodes) =
    let nodeLine = indent ++ (content node)
        childLines = format ("  " ++ indent) (children node)
        sibLines = format indent nodes
    in concat [[nodeLine], childLines, sibLines]

terseFmt :: [Node] -> String
terseFmt [] = ""
terseFmt (node:nodes) =
    let nodeLine = content node
        childLines = case terseFmt (children node) of
            "" -> ""
            c -> " {" ++ c ++ "}"
        sibLines = case terseFmt nodes of
            "" -> ""
            c -> ", " ++ c
    in nodeLine ++ childLines ++ sibLines

-- Entry point
main = do
    content <- getContents
    let tree = parse $ S.lines content
    putStrLn ("Top nodes: " ++ (show (length tree)))
    putStrLn (unlines $ format ""  tree)
    putStrLn $ terseFmt tree