Erlang: choosing unique items from a list, using recursion

Question

Given any list in Erlang, e.g.:

L = [foo, bar, foo, buzz, foo].

How can I only show the unique items of that list, using a recursive function? I do not want to use an built-in function, like one of the lists functions (if it exists).

In my example, where I want to get to would be a new list, such as

SL = [bar, buzz].

My guess is that I would first sort the list, using a quick sort function, before applying a filter?

Any suggestions would be helpful. The example is a variation of an exercise in chapter 3 of Cesarini's & Thompson's excellent "Erlang Programming" book.

Answer 1

I propose this one:

unique(L) ->
    unique([],L).
unique(R,[]) -> R; 
unique(R,[H|T]) ->
    case member_remove(H,T,[],true) of
        {false,Nt} -> unique(R,Nt);
        {true,Nt} -> unique([H|R],Nt)
    end.

member_remove(_,[],Res,Bool) -> {Bool,Res};
member_remove(H,[H|T],Res,_) -> member_remove(H,T,Res,false);
member_remove(H,[V|T],Res,Bool) -> member_remove(H,T,[V|Res],Bool).

The member_remove function returns in one pass the remaining tail without all occurrences of the element being checked for duplicate and the test result.