Implement basic UI

src/Profold/LineNode.hs

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+module Profold.LineNode
+  ( LineNode(..)
+  , newLineNode
+  , Path
+  , flatten
+  , modify
+  ) where
+
+import qualified Data.Text    as T
+import qualified Data.Vector  as V
+
+import           Profold.Util
+
+data LineNode = LineNode
+  { lineText     :: T.Text
+  , lineChildren :: V.Vector LineNode
+  , lineFolded   :: Bool
+  } deriving (Show)
+
+newLineNode :: T.Text -> [LineNode] -> LineNode
+newLineNode text children = LineNode text (V.fromList children) True
+
+type Path = [Int]
+
+flatten :: LineNode -> V.Vector (Path, LineNode)
+flatten ln
+  | lineFolded ln = V.singleton ([], ln)
+  | otherwise =
+    V.cons ([], ln) $
+    V.imap (\i (is, n) -> (i : is, n)) $ V.concatMap flatten $ lineChildren ln
+
+modify :: (LineNode -> LineNode) -> Path -> LineNode -> LineNode
+modify f [] ln = f ln
+modify f (i:is) ln =
+  ln {lineChildren = modifyAtIndex i (modify f is) $ lineChildren ln}