リストをStream化する

有限長のリストを，あたかも無限長のStreamであるかのように扱いたい時があったので，実装してみたところ，うまくいきましたので，ご報告します．

実装例

fn list, default_value ->
  Stream.unfold(list, fn
    [] -> {default_value, []}
    [h | t] -> {h, t}
  end)
end

使用例

iex> f = 
...>   fn list, default_value ->
...>     Stream.unfold(list, fn
...>       [] -> {default_value, []}
...>       [h | t] -> {h, t}
...>     end)
...>   end
#Function<41.105768164/2 in :erl_eval.expr/6>
iex> s = f.([1, 2], 0)
#Function<63.53678557/2 in Stream.unfold/2>
iex> Enum.take(s, 10)
[1, 2, 0, 0, 0, 0, 0, 0, 0, 0]
iex> s = f.([9, 8, 7, 6, 5, 4, 3, 2, 1], 0)
#Function<63.53678557/2 in Stream.unfold/2>
iex> Enum.take(s, 10)
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
iex> s = f.(9..1, 0)
#Function<63.53678557/2 in Stream.unfold/2>
iex> Enum.take(s, 10)
** (FunctionClauseError) no function clause matching in :erl_eval."-inside-an-interpreted-fun-"/1    
    
    The following arguments were given to :erl_eval."-inside-an-interpreted-fun-"/1:
    
        # 1
        9..1//-1
    
    (stdlib 5.2) :erl_eval."-inside-an-interpreted-fun-"/1
    (stdlib 5.2) erl_eval.erl:963: :erl_eval.eval_fun/8
    iex:3: (file)
    iex:3: (file)
iex> s = f.([:a, :b], :c)
#Function<63.53678557/2 in Stream.unfold/2>
iex> Enum.take(s, 10)
[:a, :b, :c, :c, :c, :c, :c, :c, :c, :c]
iex>