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Got it.
What about this "it is easier to set attribute value to None rather then
deleting attribute from object if default value is desired, especially for
generic clients"?
Do I understand correctly that setting some property to None should leads
to automatic assignment of the default value?

As for me, this looks not obvious, and I would prefer not to have such
implicit assignments

--
Regards,
Alexander Tivelkov


On Tue, May 27, 2014 at 4:05 PM, Stan Lagun <slagun at mirantis.com> wrote:

> Hi Alex,
>
> 1. This is directly related to the bug mentioned above.
>
> Taking an example from launchpad
>
> networks:
>     Contract:
>       environment: $.bool().notNull()
>       flat: $.bool().notNull()
>       custom: [$.class(Network).notNull()]
>
>     Default:
>       environment: true
>       flat: false
>
>
> currently if no value was passed for 'networks' property (and no value
> means no such key, null value doesn't count) then default value would be
> used ({"environment": true, "flat": false}) and contract would be applied
> to that value. If you pass "networks": {"flat": "true"} you will get
> exception as this value doesn't matched contract (for 'environment' key).
> With new approach exactly the same Default is interpreted as a set of
> independent default - default for 'environment' attribute and default for
> 'flat' attribute. So {"flat": "true"} turns into {"environment": "true",
> "flat": true}. With proposed changes the example above can be rewritten as
>
> networks:
>     Contract:
>       environment: $.bool()
>       flat: $.bool()
>       custom: [$.class(Network)]
>
>     Default:
>       environment: true
>
> 2. Yes, I do agree it breaks compatibility. I believe we will brake it
> anyway because MuranoPL is still far from mature and it is very likely that
> some new future would require incompatible change. Thats was one of the
> purpose of app-incubator repository - if we make breaking changes we can
> walk through all applications and fix them. I hope that all such changes
> will be made in Juno time frame.
> As for this particular change it is 1% chance that $.int() contract was
> intentionally written to accept null and 99% that it was mistake. So making
> $.int() not-nullable we do much more good than cause harm
>
>
> Sincerely yours,
> Stan Lagun
> Principal Software Engineer @ Mirantis
>
>  <slagun at mirantis.com>
>
>
> On Tue, May 27, 2014 at 1:37 PM, Alexander Tivelkov <
> ativelkov at mirantis.com> wrote:
>
>> Hi Stan,
>>
>> I don't understand well your third problem/solution statement. Could you
>> please give an example of the proposed behaviour vs the current one?
>>
>> The last solution - to have not-nullable primitive types by default -
>> looks good to me, but it breaks the compatibility with the existing
>> contracts. If some package developers have already got "$.int()" in their
>> classes, we cannot be sure if this is a mistake or an intention to make the
>> property nullable. In the latter case their code will be broke, as the null
>> values will not be accepted anymore.
>>
>> I understand that currently the adoption of MuranoPL is quite low and
>> most of its users are murano-contributors and so are probably reading this
>> mailing list, so this breaking change should not surprise them, but I'd
>> rather be careful with such things anyway. I suggest to discuss this today
>> on a weekly meeting in IRC. If everybody are fine with such changes, then
>> let's do them, as they are reasonable.
>>
>>
>>
>> --
>> Regards,
>> Alexander Tivelkov
>>
>>
>> On Mon, May 26, 2014 at 6:40 PM, Stan Lagun <slagun at mirantis.com> wrote:
>>
>>> Hello everyone!
>>>
>>> Recently I've been looking for a best way to fix incorrect handling of
>>> defaults in MuranoPL's property contracts [1]. I analyzed how contracts
>>> used in applications that are currently in app-incubator, typical mistakes
>>> and usage patterns. I've found number of places where contract system can
>>> be significantly improved. Here is my list of proposed improvements I'd
>>> like to deliberate with you:
>>>
>>> Problem: when contract violation happens it is very hard to tell without
>>> debugger what went wrong. Especially this is a problem for complex nested
>>> structures. Currently it even impossible to tell which property caused
>>> exception during class load
>>>
>>> Solution: during contract traversal keep track of path to a part of
>>> contract being processed (for example 'foo/0/bar'). Include that path in
>>> every thrown exception alongside with human-readable description describing
>>> what value was processed and what contract was violated. Prepend property
>>> name to exception text so that is would be clear what property cannot be
>>> initialized. The same for method arguments.
>>>
>>> ---
>>>
>>> Problem: Single default value is not enough for properties that are not
>>> of scalar type.
>>> For example if we have contract like
>>> - name: $.string()
>>>   disabled: $.bool()
>>>
>>> (array of structures) it is reasonable to have default value for
>>> "disabled" attribute of each list entry whereas there is no meaningful
>>> default for "name" attribute. Current approach allows to provide initial
>>> filling of entire array which is obviously not what developer expects.
>>>
>>> Solution: to have Default reflect contract structure so that different
>>> parts of Default can serve as an independent defaults for corresponding
>>> parts of the contract. Default need to be traversed in parallel with
>>> contract spec and provided value. Default value need not provide value for
>>> every single attribute mentioned in contract spec and thus can be simpler
>>> than contract itself.
>>>
>>> ---
>>>
>>> Problem: Default value is only used when no property value provided at
>>> all. Null (None) value is a valid value even if it not valid for particular
>>> contract. Thus is null is passed Default is not used.
>>>
>>> Solution: to treat every missing value as null. Missing Default is also
>>> considered to be null. This greatly simplifies understanding of contracts
>>> and client development (it is easier to set attribute value to None rather
>>> then deleting attribute from object if default value is desired, especially
>>> for generic clients)
>>>
>>> ---
>>>
>>> Problem: most of the time developer writes $.int() contract he doesn't
>>> realize that null is also valid value for such contract and the correct
>>> form is $.int().notNull(). This is even more obvious in case of bool().
>>> Developers are lazy and usually forget to wright long verbose contracts.
>>>
>>> Solution: to make all primitive types be not-nullable with obvious
>>> defaults (0 for int, false for bool).
>>> This is enough for 95% of use cases. For the rest 5% where null value
>>> does valid to have separate contract methods optionalInt(), optionalBool()
>>> etc.
>>>
>>> As for strings I think that the same approach should be used -
>>> $.string() is a non-nullable sequence of chars with empty string as a
>>> default. And there should also be optionalString() that es equal to current
>>> $.string(). But in most cases when developer writes $.string() what he
>>> really wants is $.string().notNull().trim().notEmpty(). So while it is
>>> reasonable to have all this helper functions (notEmpty(), notBlank(),
>>> trim() etc.) is would be great to have one contract function that does
>>> exactly that.
>>>
>>> ---
>>>
>>> While some of proposed changes can possible break existing contracts in
>>> practice they just legalize current state of affairs so nothing would break.
>>>
>>> What do you think?
>>>
>>> [1]: https://bugs.launchpad.net/murano/+bug/1313694
>>>
>>> Sincerely yours,
>>> Stan Lagun
>>> Principal Software Engineer @ Mirantis
>>>
>>>  <slagun at mirantis.com>
>>>
>>> _______________________________________________
>>> OpenStack-dev mailing list
>>> OpenStack-dev at lists.openstack.org
>>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>>>
>>>
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