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From Paul King <>
Subject Re: Making @Immutable a meta-annotation
Date Wed, 17 Jan 2018 00:48:49 GMT
Response inline.

On Wed, Jan 17, 2018 at 9:54 AM, MG <> wrote:

> Hmmm.... If the argument for naming the marker annotation @KnownImmutable
> was that the existing parameters have similar names (and cannot be changed)
> then it seems to me the "KnownImmutable" name choice was pretty immutable
> to begin with...
> Apart from that, there is still the inconsistency what @KnownImmutable
> really expresses:
>    - Class that carries @KnownImmutable is "fully immutable": When a
>    developer puts the annotation on a class
>    - Class that carries @KnownImmutable is "bse immutable" (i.e. no
>    defense-copying ctors etc): When being put by the Groovy compiler on a
>    class after having applied @ImmutableBase transformations to it.
> This second bullet is the wrong way to look at what is going on. Just
using @ImmutableBase (or whatever name) on a class doesn't add the
@KnownImmutable annotation. The @Immutable annotation collector adds
@KnownImmutable knowing that @ImmutableBase and the various constructor
annotations are going to be processed. So in fact it's just the first case
but with the compiler indicating that it is "fully immutable". So I don't
see a conflict as far as the @Immutable annotation goes. What you could
argue is that users of the constructor transformations might want the
defensive copying etc. but might not want to make the class fully immutable
in which case having an annotation attribute like
@MapConstructor(makeImmutable=true) would make more sense. This would
provide maximum flexibility and remove the slight dual usage of the
annotation. But the other way to look at this is that having a
"makeImmutable" annotation attribute smells of implementation detail and
just using @KnownImmutable is the more declarative way to express what we
want to achieve with less noise.

> The way it looks to me you are trying to express two different things
> through the same annotation - but to have a clean design you would need two
> seperate annotations. Maybe that is also why you do not like any of my
> alternatives, because you are looking for one name that expresses both use
> cases - and that does not exist, because the use cases differ (?)
> I am still convinced that while knownUmmutable semi-works as a parameter
> name inside of @Immutable (I would have picked guaranteed here also), that
> does not mean it is a good choice for the annotation name. But as I said,
> if you are convinced that one requires the other, this discussion is mute
> anyway...
> On 16.01.2018 01:56, Paul King wrote:
> Explanations below.
> On Tue, Jan 16, 2018 at 12:56 AM, MG <> wrote:
>> Hi Paul,
>> 1) for me, if you have to explain a name better, then it is already a bad
>> name. Intuitively suggesting the correct interpretation to another
>> developer, without requiring him to thoroughly read through the
>> documentation, is the art of picking good names (which imho Groovy overall
>> does a good job at).
>> With regards to @KnownImmutable, "someone (the compiler or the developer)
>> knows" is even more confusing. If it is in fact irrelevant who knows it,
>> why is there a "Known" in the name in the first place ? And why is
>> therefore e.g. @IsImmutable not a better name (it could also carry a
>> parameter which can be true or false, with false allowing a developer to
>> express that a class is definitely not immutable (even if it might look
>> that way on first glance; e.g. effectively blocking or issuing a warning in
>> certain parallel execution scenarios)).
> We have since the introduction of @Immutable used the knownImmutable and
> knownImmutableClasses annotation attributes and people seem to grok what
> they mean. This is a very similar use case. I think it would be hard to
> justify renaming @KnownImmutable without renaming the annotation attributes
> as well.
>> 2) There seems to be a contradiction in your statements: You say that
>> "Once @ImmutableBase (or whatever name) processing has finished its checks,
>> it can then vouch for the class and puts the marker interface
>> [@KnownImmutable] "rubber stamp" on it", but further down you say that
>> "These changes [that @ImmutableBase applies] alone don't guarantee
>> immutability.". Is it a "known immutable" after @ImmutableBase has done its
>> thing, or not ?
> Only after all transformations have completed it is guaranteed (see below).
>> 3) If I did not miss something the new @Immutable meta annotation is made
>> up of the following annotations:
>> @ImmutableBase
>> @KnownImmutable
>> @ToString
>> @EqualsAndHashCode
>> @MapConstructor
>> @TupleConstructor
>> How is any of the last four necessary for a class to be immutable ?
>> Immutability to me means, that the state of the class cannot be changed
>> after it has been created. How are @ToString, @EqualsAndHashCode,
>> @MapConstructor, and @TupleConstructor helping with this ?
>> At least one ctor to initialize the class fields is basically necessary
>> to make this a practically usable immutable class, yes, but @IsImmutable it
>> must be after @ImmutableBase does its job, or it will not be immutable in
>> the end. All the other annotations are just icing on the cake (see
>> "@Immutable should be named @ImmutableCanonical").
> @MapConstructor and @TupleConstructor do different things if they find the
> @KnownImmutable marker interface on the class they are processing
> (defensive copy in/clone/wrap etc.) which is needed for immutable classes.
> We could have used an additional annotation attribute (makeImmutable =
> true) or something but the marker interface is useful in its own right and
> it seems sensible to not duplicate the information it conveys. Besides we'd
> have to choose a name for "makeImmutable" and again since it's only part of
> the immutable story good naming would be hard.
>> If you keep @ImmutableBase, at least consider replacing @KnownImmutable
>> with @GuaranteedImmutableTag or @GuaranteedImmutableMarker ? The "Tag" or
>> "Marker" postfix at least expresses that this annotation just tags the
>> class as having certain properties, and that this is a general fact, and
>> not only known to developers or compilers in the know...
> Marker interfaces are commonplace within the Java world and we don't name
> them as such. It's not CloneableTag or SerializableMarker. I think adding
> such a suffix would be confusing.
>> I hope I do not completely miss your point, but this is how it looks to
>> me from what I read :-),
>> Cheers,
>> mg
>> On 15.01.2018 14:08, Paul King wrote:
>> Response below.
>> On Sun, Jan 14, 2018 at 6:11 AM, MG <> wrote:
>>> Hi Paul,
>>> now I get where you are coming from with @KnownImmutable. I agree with
>>> splitting the two concepts: Flexible & elegant :-)
>>> Transferring the parameter name knownImmutables (which exists inside the
>>> @Immutable context) to the annotation name KnownImmutable (which has no
>>> such context) still does not work for me, though.
>>> In addition having @Immutable = @KnownImmutable + @ImmutableBase
>>> violates the definition you give for @KnownImmutable, because either the
>>> class is "known to be immutable" = "immutable by implementation by the
>>> developer", or it becomes immutable through @ImmutableBase & Groovy...
>> Well that is perhaps an indication that it needs to be explained better
>> rather than necessarily a bad name. I'll try again. It just means that
>> someone (the compiler or the developer) knows that it is immutable. If that
>> marker interface is on the class there is no need to look further inside
>> the class, you can assume it is vouched for as immutable. Once
>> @ImmutableBase (or whatever name) processing has finished its checks, it
>> can then vouch for the class and puts the marker interface "rubber stamp"
>> on it.
>>> What do you think about:
>>> @IsImmutable
>>> @ImmutableContract
>>> @GuaranteedImmutable
>>> instead
>>> ?
>>> Thinking about this some more, still don't like @ImmutableBase. Sounds
>>> too much like a base class to me - and what would be the "base"
>>> functionality of being immutable ? Something either is immutable, or not
>>> (@ImmutableCore also fails in this regard ;-) ).
>>> So still would prefer @ImmutableOnly o.s. ..
>> @ImmutableOnly indicates that it is somehow immutable at that point - it
>> isn't really a finished immutable class until all the other related
>> transforms have done their thing. Perhaps it is useful to reiterate what it
>> does. It does a whole pile of validation (you can't have public fields, you
>> can't have certain annotation attributes on some of the other annotations
>> that wouldn't make sense for an immutable object, you can't have your own
>> constructors, it can't be applied on interfaces, it checks spelling of
>> property names referenced in annotation attributes) plus some preliminary
>> changes (makes class final, ensures properties have a final private backing
>> field and a getter but no setter, makes a copyWith constructor if needed).
>> These changes alone don't guarantee immutability. Would you prefer
>> @ImmutablePrelim?
>>> Cheers,
>>> mg
>>> -------- Urspr√ľngliche Nachricht --------
>>> Von: Paul King <> <>
>>> Datum: 13.01.18 13:17 (GMT+01:00)
>>> An: MG <> <>
>>> Betreff: Re: Making @Immutable a meta-annotation
>>> I should have explained the @KnownImmutable idea a bit more. I guess I
>>> was thinking about several possibilities for that in parallel. What I
>>> really think is the way to go though is to split out the two different
>>> aspects that I was trying to capture. One is triggering the AST
>>> transformation, the other is a runtime marker of immutability. With that in
>>> mind I'd suggest the following:
>>> @KnownImmutable will be a marker interface and nothing more. Any class
>>> having that annotation will be deemed immutable.
>>> E.g. if I write my own Address class and I know it's immutable I can
>>> mark it as such:
>>> @KnownImmutable
>>> class Address {
>>>   Address(String value) { this.value = value }
>>>   final String value
>>> }
>>> Now if I have:
>>> @Immutable
>>> class Person {
>>>   String name
>>>   Address address
>>> }
>>> Then the processing associated with @Immutable won't complain about a
>>> potentially mutable "Address" field.
>>> Then we can just leave @ImmutableBase (or similar) as the AST transform
>>> to kick off the initial processing needed for immutable classes.
>>> The @Immutable annotation collector would be replaced by the constructor
>>> annotations, ToString, EqualsAndHashcode and both ImmutableBase and
>>> KnownImmutable.
>>> The name KnownImmutable matches existing functionality. Two alternatives
>>> to annotating Address with KnownImmutable that already exist would be using
>>> the following annotation attributes on @Immutable:
>>> @Immutable(knownImmutableClasses=[Address]) or
>>> @Immutable(knownImmutables=[address]).
>>> Cheers, Paul.
>>> On Sat, Jan 13, 2018 at 1:43 PM, MG <> wrote:
>>>> Hi Paul,
>>>> I think the core of the problem is, that @Immutable as a
>>>> meta-annotation woud be better off being called something along the line
>>>> @ImmutableCanonical (see: If you do no need the immutability, use
>>>> @Canonical), since it does not solely supply immutability support - then
>>>> would be natural to call the actual core immutability annotation just
>>>> "Immutable".
>>>> That is probably off the table, since it would be a breaking change -
>>>> so we are stuck with the problem of naming the immutability annotation part
>>>> something else.
>>>> @ImmutableClass would imply to me that the "Class" part carries some
>>>> meaning, which I feel it does not, since
>>>> a) "Class" could be postfixed to any annotation name that applies to
>>>> classes
>>>> b) The meta-annotation should accordingly also be called
>>>> "ImmutableClass"
>>>> Because of that I find postfixing "Immutable" with "Class" just
>>>> confusing. It also is not intuitive to me, which annotation does only
>>>> supply the core, and which supplies the extended (canonical)
>>>> functionality...
>>>> I do not understand where you are going with @KnownImmutable (known to
>>>> whom ?-) To me this seems less intuitive/more confusing than
>>>> @ImmutableClass...).
>>>> @ImmutableCore is similar to @ImmutableBase (because I intentionally
>>>> based it on it :-) ), but different in the sense that it imho expresses the
>>>> semantics of the annotation: Making the object purely immutable-only,
>>>> without any constructors, toString functionality, etc.
>>>> How about:
>>>> @ImmutableOnly
>>>> @PureImmutable
>>>> @ModificationProtected
>>>> @Locked
>>>> @Frozen
>>>> @Unchangeable
>>>> @Changeless
>>>> @InitOnly
>>>> @InitializeOnly
>>>> @Constant
>>>> @Const
>>>> @NonModifieable
>>>> @NonChangeable
>>>> ?
>>>> mg
>>>> On 12.01.2018 08:01, Paul King wrote:
>>>> @ImmutableCore is similar to @ImmutableBase - probably okay but I don't
>>>> think ideal. Another alternative would be @ImmutableInfo or have an
>>>> explicit marker interface with a different package, e.g.
>>>> groovy.transform.marker.Immutable but that might cause IDE completion
>>>> headaches. Perhaps @KnownImmutable as a straight marker interface might be
>>>> the way to go - then it could be used explicitly on manually created
>>>> immutable classes and avoid the need to use the
>>>> knownImmutableClasses/knownImmutables annotation attributes for that
>>>> case.
>>>> Cheers, Paul.
>>>> On Thu, Jan 11, 2018 at 9:34 PM, mg <> wrote:
>>>>> Hi Paul,
>>>>> great to make @Immutable more fine granular / flexible :-)
>>>>> what about
>>>>> @ImmutabilityChecked
>>>>> or
>>>>> @ImmutableCore
>>>>> instead of @ImmutableClass ?
>>>>> Cheers
>>>>> mg
>>>>> -------- Urspr√ľngliche Nachricht --------
>>>>> Von: Paul King <>
>>>>> Datum: 11.01.18 08:07 (GMT+01:00)
>>>>> An:
>>>>> Betreff: Making @Immutable a meta-annotation
>>>>> There has been discussion on and off about making @Immutable a
>>>>> meta-annotation (annotation collector) in much the same way as @Canonical
>>>>> was re-vamped. (This is for 2.5+).
>>>>> I have a preliminary PR which does this:
>>>>> Preliminary because it still needs a bit of refactoring to reduce some
>>>>> duplication of code that exists between the normal and immutable map
>>>>> tuple constructors. I still need to do this but that can happen
>>>>> transparently behind the scenes as an implementation detail if we don't
>>>>> finish it straight away. As well as reducing duplication, the pending
>>>>> refactoring will enable things like the pre and post options for
>>>>> MapConstructor and TupleConstructor which aren't currently working.
>>>>> I am keen on any feedback at this point. In particular, while most of
>>>>> the functionality is pushed off into the collected annotations/transforms,
>>>>> I ended up with some left over checks which I kept in an annotation
>>>>> currently called @ImmutableClass. I tried various names for this class,
>>>>> e.g. @ImmutableBase and @ImmutableCheck but finally settled on
>>>>> @ImmutableClass since the annotation causes the preliminary checks to
>>>>> performed but also acts as a marker interface for the MapConstructor
>>>>> TupleConstructor transforms to do the alternate code needed for
>>>>> immutability and to indicate that a class is immutable when it might
>>>>> be a property of another immutable class. Let me know if you can think
of a
>>>>> better name or have any other feedback.
>>>>> Cheers, Paul.

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