Got a new gotcha today
Elixir’s ++/2 operator (list concatenation) has a special clause: If the right-hand side is not a list, the operation returns that right-hand side value instead of raising an error.
Never knew about it. Did bite me. Type checker did not warn that the enumeration would possibly
raise an exception. 
iex(1)> [] ++ nil
nil
iex(2)> [] ++ :foo
:foo
iex(3)> [] ++ "bla"
"bla"
The source in erlang is clear. This is the comment on the nil as right-hand side:
if (is_nil(lhs)) {
/* This is buggy but expected, `[] ++ 'not_a_list'` has always resulted
* in 'not_a_list'. */
return rhs;
Doubt I like the behavior, but alas…there it is.
