homework.hs

showSTENs :: Int -> [StackTrace] -> String
showSTENs tabLen = unlines . showSTENs' 0 . build
    where
        showTabs :: Int -> String
        showTabs numSpaces = replicate numSpaces ' '
        showSTENs' :: Int -> [STENode] -> [String]
        showSTENs' tabs [] = [""]
        showSTENs' tabs stens = concat $ map showSTEN (groupAndJoin . sort $ stens)
            where
                showSTEN (STENode ste []) = [showTabs (tabs * tabLen) ++ show ste]
                showSTEN (STENode ste stens') = (showTabs (tabs * tabLen) ++ show ste) : (showSTENs' (tabs+1) stens')
        groupAndJoin :: [STENode] -> [STENode]
        groupAndJoin stens = map merge (group stens)
            where
                merge :: [STENode] -> STENode
                merge stens@((STENode ste _):rest) = STENode ste (concat . map (\(STENode _ ss) -> ss) $ stens)
        build :: [StackTrace] -> [STENode]
        build traces = map buildList traces
            where 
                buildList :: StackTrace -> STENode
                buildList trace = foldr makeNewSTENode (STENode (last trace) []) (init trace)
                makeNewSTENode :: StackTraceElement -> STENode -> STENode
                makeNewSTENode parent child = STENode parent [child]