binary search in erlang

t.erl

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

chop(Key, List) ->
    chop(Key, List, 0).
  
chop(Key, List, Acc) ->
    Mid = length(List) div 2,
    {List1, List2} = lists:split(Mid, List),
    %io:format("List1> ~p~nList2> ~p~n Mid> ~p~n Acc> ~p~n", [List1, List2, Mid, Acc]),
    case hd(List2) of
        Item when Key /= Item andalso List1 =:= [] ->
            erlang:error(notfound);
        Item when Key < Item ->
            chop(Key, List1, Acc);
        Item when Key > Item ->
            chop(Key, List2, Acc + Mid);
        Item when Key == Item ->
            Acc+Mid+1
    end.