Mutable records

Dmitry Petrashko dmitry.petrashko at
Mon Mar 26 18:31:58 UTC 2018

Hi Brian,

> What did I miss?

There is one more usecase for mutable record types in C++ structs in which
I find some value.
Consider a method that requires a dozen of arguments. In case this method
serves a single purpose,
those argument are very likely to be somehow related. Instead of passing 10
arguments separately they can
be grouped into structures that both make it easier to document the code
and to write it.
Tuples don't serve this purpose well, a they don't give names or
explanations, while classes are too verbose.

In order to keep those structured arguments on par with normal arguments in
Java, they have to be mutable,
as normal arguments are re-assignable in Java, unlike in Scala.

>  - People will just work around it anyway, as they do with lambdas.  If a
class has N-1 final fields, and one mutable one, what do we think they're
going to do?
>  - Mutability introduces pain, but so does repetition and boilerplate --
it gives bugs a place to hide.  Making the feature less applicable consigns
more users to using a more error-prone mechanism.

In case records are just syntactic sugar that does not have magic that
cannot be reproduced by expanding the definition side,
it might be worth considering making records immutable only in case you
want to steer culture towards immutability.
As long as they can expand the definition site manually without changing
public API, they can do it.
This would also sidestep adding a new keyword to the language.


On Fri, Mar 23, 2018 at 11:03 AM, Brian Goetz <brian.goetz at>

> A few people have asked, "wouldn't it just be easier to prohibit
> mutability in records"?  And while it surely would be easier (most of the
> issues I raised in my writeup go away without mutability), I think it would
> also greatly restrict the utility of the feature.  Let me talk about why,
> and give some examples -- and then I'd like to talk about what we can do,
> if anything, to make the mutable use cases easier.
> ## General argument: Mutability is pervasive in Java; you can only push it
> away a bit.
> We saw this with lambdas; developers are all too eager to "work around"
> the limitation on mutable local capture by wrapping their mutables in a
> one-element array.  In fact, IDEs even "helpfully" offers to do this for
> you, thus ensuring that everyone thinks this is OK.
> We will see this again with value types; even though value types are
> immutable, value types can contain references to mutable objects, and
> trying to enforce "values all the way down" would result in fairly useless
> value types.
> (That doesn't mean we can't nudge towards immutability where we think it
> makes sense, if we think the value of the nudge exceeds the irregularity or
> complexity it entails.)
> ## Records and value types: goals and similarities
> While records and value types have some features in common (getting
> equals() for free), they have different motivations.
> Value types are about treating aggregates as, well, values, with all the
> things that entails; they can be freely shared, the runtime can routinely
> optimize them by putting them on the stack or in registers and flatten them
> into enclosing values, classes, or arrays (yielding better density and
> flatness.)  What they ask you to give up in exchange is identity, which
> means giving up mutability and layout polymorphism.
> Records are about treating data as data; when modeling aggregates with
> records, the result is transparent classes whose API and representation are
> the same thing.  This means that records can be freely interconverted
> between their exploded and aggregate forms with no loss of information.
> What they ask you to give up is the freedom to define the mapping between
> representation and API (constructors, accessors, equals, hashCode,
> deconstruction) in a nontransparent way.  (Essentially, you give up all
> encapsulation except for the ability to control writes to their state.)
> My claim is that the goals are mostly orthogonal, and the benefits and
> tradeoffs of each are as well.  All four quadrants make sense to me.  Some
> aggregates are values but not transparent (think cursors that hold
> references into the internals of a data structure, or hold a native
> resource); some are "just their data" but not values (graph nodes, as well
> as the mutable examples below), and others are both (value records).
> The superficial commonalities between records and values (both are
> restricted forms of aggregate, and these restrictions make it possible to
> provide sensible defaults for things like equals) tease us into thinking
> they are the same thing, but I don't think they are.
> Assuming this to be true, how can we justify having two new constructs?
> Value types, by nature of what they require the developer to give up,
> enable the runtime to make significant optimizations it could not otherwise
> make.  So if we want flat and dense data, this is basically our only option
> -- make the programmer consent to the handcuffs.  The argument for records
> is more of a contingent one; records allow you to express more with less.
> The "more with less" has at least two aspects; in addition to the obvious
> reduction in boilerplate, libraries and frameworks can make more reasonable
> assumptions about what construction or deconstruction means, and therefore
> can build useful functionality safely (such as marshaling to/from XML.)
> But records don't let you do anything you can't already do with classes.
> So if I had a quota, I'd have to pick values over records.
> In a language with values on the roadmap, immutable-only records seem to
> offer a pretty lame return-on-complexity.  Nothing about values requires
> you to use encapsulation, so you could model most immutable records with a
> value type, with less boilerplate than a class (but more than none), and
> the remainder with classes. (Immutable records buy you one thing that
> values do not -- pointer polymorphism.  That lets you make graphs or trees
> of them.)  But I think it is clear that this model of records is a kind of
> weird half-one, half-the-other thing, and its not entirely clear it would
> carry its weight.
> And, when users ask "why can't record components be mutable, after all,
> records are about data, and some data is mutable", I don't think we have a
> very good answer other than "immutability is good for you."  I much prefer
> the argument of "there are two orthogonal sets of tradeoffs; pick one, the
> other, or both."
> ## Use cases for mutable records
> Here are two use cases that immediately come to mind; please share others.
> Groups of related mutable state.  An example here is a set of counters.
> If I define:
>     record CacheCounters(public int hitCount, public int accessCount) {
>         float hitRate() { ... }
>     }
> then I can treat them as a single entity; store a counter-pair in a
> variable, have arrays of them, use them as values of Maps, pass them
> around, etc.  (The fact that they're mutable introduces constraints, but
> not new constraints; we deal with this problem every day.)  I can even lock
> on it, if that's how I want to do it.
> Domain objects.  Another common use is domain agregates:
>     record Person(String first, String last);
> If I want to marshal one of these to or from XML using a framework like
> JAXB, I can provide mapping metadata between XML schema and classes, and
> the framework will gladly populate the object for me.  The way these
> frameworks want to work is to instantiate a mutable object with a no-arg
> constructor, and then set the fields (or call setters) as components become
> available on the stream. Yes, you can write a binding framework that waits
> until it has all the stuff and then calls a N-arg constructor, but that's a
> lot harder, and uses a lot more memory.  Mutable records will play nicely
> with these frameworks.
> ## Embracing mutability
> I cheated a bit in the two examples I gave; neither had a no-arg
> constructor.  We could do a few things about this:
>  - Make the user write a no-arg constructor (and hopefully make this easy
> enough)
>  - Provide a no-arg constructor for all records that just pass the default
> values for that type to the default constructor (which might reject them,
> if it doesn't like nulls)
>  - Try to provide a "minimal" constructor that only takes the final
> fields.  (I don't like this because changing a field between final and not
> changes the signature of an implicit constructor, which won't be binary
> compatible.)
> Similarly, you could object that deriving equals/hashCode from mutable
> state is dangerous.  (But List does do this.)  Again, there are a few ways
> to deal.  We could adjust the standard equals/hashCode to only take into
> account final fields.  But, I'm skeptical of this, because I could easily
> imagine people constructing records via mutation but then using them in an
> effectively immutable way thereafter, and they might want the stronger
> equals contract.  Or, we could tell people, as we do with List, not to use
> them as keys in hash-based collections.  (We could even have compiler
> warnings about this.)
> ## Additional considerations
> Here are a few less fundamental points about accepting mutable records,
> none of which are slam-dunks, but might still be useful to consider:
>  - People will just work around it anyway, as they do with lambdas.  If a
> class has N-1 final fields, and one mutable one, what do we think they're
> going to do?
>  - C# embraced mutable records.  This isn't surprising, but what is
> surprising is that Scala's case classes did also.  While I don't have data
> from either Neal or Martin, I suspect that they went through a similar
> analysis -- that it would leave out too many desirable use cases for the
> feature, and still not protect us from deeper mutability anyway.
>  - Mutability introduces pain, but so does repetition and boilerplate --
> it gives bugs a place to hide.  Making the feature less applicable consigns
> more users to using a more error-prone mechanism.
> ## Fields: final by default?
> One of the nudges we've considered is making fields final by default, but
> letting them be declared non-final.  This is a nudge, in that it sends a
> message that records are best served immutable, but if you want your
> revenge warm, you can have it.  I think there are reasonable arguments on
> both sides of this story, but one argument I am not particularly motivated
> by is "but then we'd have to introduce non-final as a keyword."  If we
> think final-by-default is a good idea, I don't think the lack of a
> denotation should be the impediment.
> ## Clone
> Clone is a mess, and I'm not sure there's a good answer here, but there's
> surely a good discussion.
> As a user, I find the ability to clone arrays (despite being shallow) is
> super useful, and it makes it far easier to be good about doing defensive
> copies all the time.  If cloning were harder (new array/arraycopy), I'd
> probably cut more corners.  If we can deliver the same benefit for records,
> that seems enticing.
> There's a fair argument over whether the standard clone should be shallow
> (easy to specify and implement) or should try to deeply clone Cloneable
> components.  Or maybe both options suck.  Or maybe it should be opt in; if
> the record extends Clonable, you get a clone() method.
> What did I miss?

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