Calls the given function for each data element of the datalist.
The function is called with each data element's Quark id and data, together with the given
user_data parameter. Note that this function is NOT thread-safe. So unless datalist can be protected from any
modifications during invocation of this function, it should not be called.
func can make changes to datalist, but the iteration will not reflect changes made during the
@foreach call, other than skipping over elements that are removed.
| func |
the function to call for each data element. |
| datalist |
a datalist. |
| user_data |
user data to pass to the function. |