[erlang-bugs] lists:usort bug 1

[Hans Bolinder]

[Lev Walkin:]
>         usort(Fun, List1) -> List2
> 
>                Types  Fun = fun(Elem1, Elem2) -> bool()
>                        Elem1 = Elem2 = term()
>                       List1 = List2 = [term()]
> 
>        Returns a list which contains the sorted elements of List1 where
>        all but the  first  element  of  the  elements  comparing  equal
>        according to the ordering function Fun have been deleted. Fun(A,
>        B) should return true if A comes before B in the ordering, false
>        otherwise.
> 
> Actual behavior:
> 
> 18> lists:usort(fun(A, B) -> A < B end, [a, a]).
> [a,a]
> 19> lists:usort(fun(A, B) -> A =< B end, [a, a]).
> [a]
> 20> lists:usort(fun(A, B) -> A >= B end, [a, a]).
> [a]
> 21> lists:usort(fun(A, B) -> A > B end, [a, a]).
> [a,a]
> 22>
> 
> Contradicts description "should return true if A comes before B in the 
> ordering, false otherwise".

Note that it is assumed by sort/2, usort/2, merge/3, and umerge/3 that
the fun is an "ordering function". Neither </2 nor >/2 is an ordering
function.

Best regards,

Hans Bolinder, Erlang/OTP team, Ericsson