prefixtree.hs

data Ptree a = Pnode a [Ptree a] deriving (Eq, Show)

ptLeaf :: Char -> Ptree Char
ptLeaf c = Pnode c []

ptInsert :: String -> Ptree Char -> Ptree Char
pInsert [] tree = tree
ptInsert (c:cs) (Pnode v children) = case children of
    []                         -> undefined -- the node has no children
    (current:rest) | v == c    -> undefined -- the node has children, and the first one is what we're looking for
                   | otherwise -> undefined -- the node has children, but the first one is not what we're looking for