On Mon, Apr 5, 2010 at 4:54 PM, Douglas Philips <[email protected]> wrote:
> Immutability is orthogonal to reference-ness.
> There is nothing "wrong" with having immutable cyclic graphs of values.
>
There is something wrong with immutable cyclic data structures: an undefined
(or unexpected) behaviour on update because of identity and state being
conflated again.
Let say one can write:
(def john-doe {:name "John Doe" :email "[email protected]" :account {:owner
#<cyclic reference to the root map> :balance 1000}})
Then you want to add 10$ to John's account:
(update-in john-doe [:account :balance] + 10)
The resulting data structure is:
{:name "John Doe" :email "[email protected]" :account {:owner #<cyclic reference
to the root map> :balance 1010}}
but what would #<cyclic reference to the root map> point to? to the previous
value of john-doe! So if we expands the reference:
{:name "John Doe" :email "[email protected]" :account {:owner {:name "John Doe"
:email "[email protected]" :account {:owner #<cyclic reference to the root map>
:balance 1000}} :balance 1010}}
This is not exactly the expected result. The result is confusing because the
cyclic reference must be to the identity of the user account not to its
actual value.
The same problem arises even when you update a non-nested field:
(assoc john-doe :email "[email protected]")
The resulting value would be:
{:name "John Doe" :email "[email protected]" :account {:owner {:name "John
Doe" :email "[email protected]" :account {:owner #<cyclic reference to the root
map> :balance 1000}} :balance 1000}}
You really need an indirection level to properly handle cyclic data
structures.
hth,
Christophe
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