On 15/06/07, Bert Freudenberg <bert at freudenbergs.de> wrote:
>
> On Jun 15, 2007, at 14:45 , sig wrote:
>
> > actually i missing these operations for dictionaries.
> > in current implementation if you diff/union on two dictionaries,
> > you'll find out that associations play role as set elements, not keys.
> >
> > for dictionaries 'a difference: b' i get not exactly what i would
> > expect.
>
> Dictionaries are like other collections - a collection of
> "elements" (the values). The keys are only interesting for accessing
> - like indices on Arrays.  #do: operates on the values. So does
> #select:, and #difference:. You wouldn't expect #difference: on an
> Array to work on its indices, would you?
>
Consider following example (union of two dictionaries):

| dict1 dict2 result1 result2 |
dict1 := Dictionary new.
dict2 := Dictionary new.
dict1 at: 1 put: 5.
dict2 at: 1 put: 6.

result1 := dict1 union: dict2.
result2 := dict2 union: dict1.

union defined as commutative operation fails in this example, because
result1<>result2.
it can be considered as commutative only in case if we taking
resulting keys into account, not values.
from this point same logics must be followed when doing
intersection/difference for dictionaries - take only keys into
account, not their associative values.

So, i'd expect 'dict1 difference: dict2'  must return empty
dictionary, as well as
dict1 intersection: dict2  return non-empty dictionary.

Btw, all this is based on wrong semantics of Association '=' mehod,
which compares not only keys, but values also. This was discussed in
other topic before.