Erlang Language Loop and Recursion Iolist and Bitstring

Example

Like list, simplest function over iolist and bitstring is:

-spec loop(iolist()) -> ok | {ok, iolist} .
loop(<<>>) ->
  ok;
loop(<<Head, Tail/bitstring>>) ->
  loop(Tail);
loop(<<Rest/bitstring>>) ->
  {ok, Rest}

You can call it like this:

loop(<<"abc">>).

Here the recursive function expansion:

loop(<<"a"/bitstring, "bc"/bitstring>>) ->
  loop(<<"b"/bitstring, "c"/bitstring>>) ->
    loop(<<"c"/bitstring>>) ->
      loop(<<>>) ->
        ok.

Recursive function over variable binary size

This code take bitstring and dynamically define binary size of it. So if, if we set a size of 4, every 4 bits, a data will be matched. This loop do nothing interesting, its just our pillar.

loop(Bitstring, Size) 
  when is_bitstring(Bitstring), is_integer(Size) ->
    case Bitstring of
      <<>> -> 
        ok;
      <<Head:Size/bitstring,Tail/bitstring>> ->
        loop(Tail, Size);
      <<Rest/bitstring>> ->
        {ok, Rest}
    end.

You can call it like this:

loop(<<"abc">>, 4).

Here the recursive function expansion:

loop(<<6:4/bitstring, 22, 38, 3:4>>, 4) ->
  loop(<<1:4/bitstring, "bc">>, 4) ->
    loop(<<6:4/bitstring, 38,3:4>>, 4) ->
      loop(<<2:4/bitstring, "c">>, 4) ->
        loop(<<6:4/bitstring, 3:4>>, 4) ->
          loop(<<3:4/bitstring>>, 4) ->
            loop(<<>>, 4) ->
              ok.

Our bitstring is splitted over 7 patterns. Why? Because by default, Erlang use binary size of 8 bits, if we split it in two, we have 4 bits. Our string is 8*3=24 bits. 24/4=6 patterns. Last pattern is <<>>. loop/2 function is called 7 times.

recursive function over variable binary size with actions

Now, we can do more interesting thing. This function take one more argument, an anonymous function. Everytime we match a pattern, this one will be passed to it.

-spec loop(iolist(), integer(), function()) -> ok.
loop(Bitstring, Size, Fun) ->
  when is_bitstring(Bitstring), is_integer(Size), is_function(Fun) ->
        case Bitstring of
      <<>> -> 
        ok;
      <<Head:Size/bitstring,Tail/bitstring>> ->
        Fun(Head),
        loop(Tail, Size, Fun);
      <<Rest/bitstring>> ->
        Fun(Rest),
        {ok, Rest}
    end.

You can call it like this:

Fun = fun(X) -> io:format("~p~n", [X]) end.
loop(<<"abc">>, 4, Fun).

Here the recursive function expansion:

loop(<<6:4/bitstring, 22, 38, 3:4>>, 4, Fun(<<6:4>>) ->
  loop(<<1:4/bitstring, "bc">>, 4, Fun(<<1:4>>)) ->
    loop(<<6:4/bitstring, 38,3:4>>, 4, Fun(<<6:4>>)) ->
      loop(<<2:4/bitstring, "c">>, 4, Fun(<<2:4>>)) ->
        loop(<<6:4/bitstring, 3:4>>, 4, Fun(<<6:4>>) ->
          loop(<<3:4/bitstring>>, 4, Fun(<<3:4>>) ->
            loop(<<>>, 4) ->
              ok.

Recursive function over bitstring returning modified bitstring

This one is similar to lists:map/2 but for bitstring and iolist.

% public function (interface).
-spec loop(iolist(), fun()) -> iolist() | {iolist(), iolist()}.
loop(Bitstring, Fun) ->
  loop(Bitstring, 8, Fun).

% public function (interface).
-spec loop(iolist(), integer(), fun()) -> iolist() | {iolist(), iolist()}.
loop(Bitstring, Size, Fun) ->
  loop(Bitstring, Size, Fun, <<>>)

% private function.
-spec loop(iolist(), integer(), fun(), iolist()) -> iolist() | {iolist(), iolist()}.
loop(<<>>, _, _, Buffer) ->
  Buffer;
loop(Bitstring, Size, Fun, Buffer) ->
  when is_bitstring(Bitstring), is_integer(Size), is_function(Fun) ->
    case Bitstring of
      <<>> -> 
        Buffer;
      <<Head:Size/bitstring,Tail/bitstring>> ->
        Data = Fun(Head),
        BufferReturn = <<Buffer/bitstring, Data/bitstring>>,
        loop(Tail, Size, Fun, BufferReturn);
      <<Rest/bitstring>> ->
        {Buffer, Rest}
    end.

This code seems more complexe. Two functions were added: loop/2 and loop/3. These two functions are simple interface to loop/4.

You can execute it like this:

Fun = fun(<<X>>) -> << (X+1) >> end.
loop(<<"abc">>, Fun).
% will return <<"bcd">>

Fun = fun(<<X:4>>) -> << (X+1) >> end.
loop(<<"abc">>, 4, Fun).
% will return <<7,2,7,3,7,4>>

loop(<<"abc">>, 4, Fun, <<>>).
% will return <<7,2,7,3,7,4>>