-- i have those data types.
-- TimeStamp is an Int
-- MessageType is not important for this question :-).

data LogMessage = LogMessage MessageType TimeStamp String
                | Unknown String
  deriving (Show, Eq)

data MessageTree = Leaf
                 | Node MessageTree LogMessage MessageTree
  deriving (Show, Eq)

{-
the exercise is this:
A MessageTree should be sorted by timestamp: that is, the timestamp
of a LogMessage in any Node should be greater than all timestamps
of any LogMessage in the left subtree, and less than all timestamps
of any LogMessage in the right child.

Unknown messages should not be stored in a MessageTree since
they lack a timestamp.

i need to create this function:    -}
insert :: LogMessage -> MessageTree -> MessageTree

{-
which inserts a new LogMessage into an existing MessageTree, producing
a new MessageTree. insert may assume that it is given a
sorted MessageTree, and must produce a new sorted MessageTree
containing the new LogMessage in addition to the contents of the
original MessageTree.
However, note that if insert is given a LogMessage which is
Unknown, it should return the MessageTree unchanged.

can you please give me a hint?
what i have done so far is this:
-}

insert :: LogMessage -> MessageTree -> MessageTree
insert (Unknown _) m    = m
insert l           Leaf = Node Leaf l Leaf

{-
but i can get a `insert` with a non empty MessageTree and i don't understand how to insert a new LogMessage to a non empty MessageTree.
please do not post answers.
just a hint how to proceed.
thanks!
-}