structs_dialyzer.ex

defmodule BST do
  defstruct data: nil, left: nil, right: nil

  @spec insert(BST.t, any) :: BST.t
  def insert(nil, value) do
    %BST{data: value}
  end
  def insert(%BST{data: d, left: l, right: _r} = bst, value) when value > d do
    %BST{data: d, left: l, right: insert(bst.right, value)}
  end
  def insert(%BST{data: d, left: _l, right: r} = bst, value) when value <= d do
    %BST{data: d, left: insert(bst.left, value), right: r}
  end
end

# Dialyzer output:
# BST.ex:9: The call _@1:'right'() requires that _@1 is of type atom() | tuple() not #{}
# BST.ex:12: The call _@1:'left'() requires that _@1 is of type atom() | tuple() not #{}