Binary search in Erlang

binary_search.erl

%% I wondered if I'm in the 10% of devs that can write binary search correctly:
%% http://reprog.wordpress.com/2010/04/19/are-you-one-of-the-10-percent/

%% The automated test cases at http://www.mac-guyver.com/switham/2010/04/Binary_Search/
%% (edited for Erlang below) say I am. Well, nice to know :)

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

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

binary_search(List, N) ->

  Length = length(List),
  Middle = (Length + 1) div 2, %% saves us hassle with odd/even indexes

  case Middle of
    0 -> -1; %% empty list -> item not found
    _ -> 
   
      Item = lists:nth(Middle, List),
      
      case Item of
        N -> Middle; %% yay, found it!
        _ -> case Item > N of
               true  -> binary_search(lists:sublist(List, Length - Middle), N); %% LT, search on left side
               false -> binary_search(lists:nthtail(Middle, List), N)           %% GT, search on right side
             end
      end
  end.

Makefile

run:
	erlc binary_search.erl
	erl -noshell -s binary_search test -s init stop

tests:
	python write_tests.py >binary_search_tests.erl
	erlc binary_search_tests.erl

all: tests run

sample_run.txt

$ make all
python write_tests.py >binary_search_tests.erl
erlc binary_search_tests.erl
erlc binary_search.erl
erl -noshell -s binary_search test -s init stop
  All 4096 tests passed.

write_tests.py

#!/usr/bin/env python

# based on http://www.mac-guyver.com/switham/2010/04/Binary_Search/
# edited for Erlang :)
# now essentially prints out a file with all the EUnit tests

from random import *
# 0.0 <= random() < 1.0;  a <= randrange( a, b ) < b  for int(a) < int(b).


if __name__ == "__main__":

    print "-module(binary_search_tests)."
    print "-include_lib(\"eunit/include/eunit.hrl\")."
    print

    for n in range( 1, 4097 ):
        size = int( 2.0 ** (random() * 8) - 1 )        # size can be zero.
        if random() < .5:
            A = range( size )                          # every number in range
        else:          
            A = [ randrange(size) for i in range(size) ] # random numbers
            A.sort()
        want_there = size > 0  and  random() < .5
        if want_there:
            T = A[ randrange(size) ]
        else:
            theres = dict( [ ( x, True ) for x in A ] )
            not_theres = [ x for x in range( -1, size+1 ) if not x in theres ]
            T = not_theres[ randrange( len(not_theres) ) ]
        is_there = T in A
        assert( is_there == want_there )

        print "a_%d_test() ->" % (n)
        print "  ?assert%s(binary_search:binary_search(%s, %d) =:= -1)." % (["","Not"][is_there], A, T)
        print

It seems like this solution does not return the index of the element. So I created another one based on your solution which returns the index.

https://gist.github.com/ColdFreak/888e9a0a5cbed5ddc1a3