[erlang-questions] orddict - is it an expected behavior?

[Kostis Sagonas]

On 01/12/2016 01:50 PM, Minin Maxim wrote:
> Hi All,
>
> is it an expected behavior?
>
> 1> orddict:merge(fun(_, _X, Y) -> Y end,  orddict:from_list([{1,a},{2,b}]), [{1,c},{1,f}]).
> [{1,c},{1,f},{2,b}]
>
> Here returns merge function an invalid orddict.
>
> I know, if the second argument is a valid orddict, all is fine:
>
> 2> orddict:merge(fun(_, _X, Y) -> Y end,  orddict:from_list([{1,a},{2,b}]), orddict:from_list([{1,c},{1,f}])).
> [{1,f},{2,b}]
>
> I would expect, that merge function either generates an exception or returns a proper orddict, but here is that not the case. Am I wrong?

Yes, you are.

If you violate the preconditions of a function (as expressed by the 
properties that are expected by its arguments), you cannot expect that 
its postcondition (its return) is what the documentation promises.

Kostis