Binary chop in Erlang

chop.erl

-module(chop).
-export([chop/2]).

chop(_Value, []) ->	
  -1;

chop(Value, List) ->
  case (length(List) + 1) div 2 of 
    0 -> 
      -1;
    Middle ->
      Current = lists:nth(Middle, List),
      chop(Value, List, Current, 0, length(List), Middle - 1)
  end.

chop(_Value, [], _Current, _Low, _Hi, _Middle) -> 
  -1;
	
chop(_Value, _List, _Current, Low, Hi, _Middle) when Hi < Low -> 
  -1;
	
chop(Value, _List, Current, Low, Hi, _Middle) when Value /= Current, Hi == Low ->	
  -1;

chop(Value, List, Current, Low, Hi, _Middle) when Value < Current, Hi > Low ->
  MidIndex = (Low + Hi) div 2,
  chop(Value, List, lists:nth(MidIndex + 1, List), Low, MidIndex, MidIndex);

chop(Value, List, Current, Low, Hi, _Middle) when Value > Current, Hi > Low ->
  MidIndex = (Low + Hi) div 2,
  chop(Value, List, lists:nth(MidIndex + 1, List), MidIndex + 1, Hi, MidIndex);

chop(Value, _List, Current, _Low, _Hi, Middle) when Value == Current ->
  Middle.

chop_test.erl

-module(chop_test).

-include_lib("eunit/include/eunit.hrl").

chop_test_() ->
  [?_assertEqual(-1, chop:chop(3, [])),
  ?_assertEqual(-1, chop:chop(3, [ 1 ])),
  ?_assertEqual(0, chop:chop(1, [ 1 ])),
  ?_assertEqual(0, chop:chop(1, [ 1, 3, 5 ])),
  ?_assertEqual(1, chop:chop(3, [ 1, 3, 5 ])),
  ?_assertEqual(2, chop:chop(5, [ 1, 3, 5 ])),
  ?_assertEqual(-1, chop:chop(0, [1, 3, 5])),
  ?_assertEqual(-1, chop:chop(2, [1, 3, 5])),
  ?_assertEqual(-1, chop:chop(4, [1, 3, 5])),
  ?_assertEqual(-1, chop:chop(6, [1, 3, 5])),
  ?_assertEqual(0,  chop:chop(1, [1, 3, 5, 7])),
  ?_assertEqual(1,  chop:chop(3, [1, 3, 5, 7])),
  ?_assertEqual(2,  chop:chop(5, [1, 3, 5, 7])),
  ?_assertEqual(3,  chop:chop(7, [1, 3, 5, 7])),
  ?_assertEqual(-1, chop:chop(0, [1, 3, 5, 7])),
  ?_assertEqual(-1, chop:chop(2, [1, 3, 5, 7])),
  ?_assertEqual(-1, chop:chop(4, [1, 3, 5, 7])),
  ?_assertEqual(-1, chop:chop(6, [1, 3, 5, 7])),
  ?_assertEqual(-1, chop:chop(8, [1, 3, 5, 7]))
  ].