Ad hoc type restriction

Tue Oct 14 00:30:09 UTC 2025

Let me frame this a different way.

The compiler already provides an extension mechanism in the form of
annotation processors. We already have examples out there in the world of
annotation processors that augment the compiler with extra checks,
including those that treat an annotated type as a distinct type and perform
flow-analysis (https://checkerframework.org/manual/#writing-annotations).
The primary restriction on this mechanism is that a processor cannot alter
if a given compilation unit is valid Java or how that compilation unit will
translate to bytecode.

The bar for a new compiler extension mechanism would reasonably begin at
"the existing mechanism is insufficient." As I see it, your proposal #1 is
already viable. Proposal #2 is not without lifting (in part or in whole)
the primary restriction on annotation processors, but if those checks are
not synthetic - i.e. a warning is provided until an explicit check is
inserted into the source code - that sidesteps the need for it.

You could also perform bytecode rewriting after program compilation to
insert checks. That is allowed.

> because Java doesn't make it very easy to do, it's often not being done
at all

Maybe let's interrogate this from the other side: Why is this not easy to
do? What friction points are essential (you do need to get a processor and
put it on paths - what else?) and which are artificial? Clearly it's harder
than a "normal library," why is that?

On Mon, Oct 13, 2025 at 4:28 PM Archie Cobbs <archie.cobbs at gmail.com> wrote:

> Ethan McCue <ethan at mccue.dev> wrote:
>
>> However there is nothing conceptually preventing the tools validating
>> @NonNull usage from also emitting an error until you have inserted a known
>> precheck.
>> ...
>> But for other single-value invariants, like your @PhoneNumber example, it
>> seems fairly practical. Especially since, as a general rule, arbitrary cost
>> computations really shouldn't be invisible. How would one know if (@B A) is
>> going to thread invocations of some validation method everywhere?
>
>
> This is why 3rd party tools aren't as good as having the compiler handle
> it, because the compiler is in a position to provide both stronger and more
> efficient guarantees - think generic types and runtime erasure.
> Compiler-supported typing allows the developer to move the burden of proof
> from the method receiving a parameter to the code invoking that method, and
> onward back up the call chain, so that validations tend to occur "early",
> when they are first known to be true, instead of "late" at the (many more)
> points in the code where someone actually cares that they are true.
>
> So if phone numbers are central to your application, and they are passed
> around and used all over the place as type @PhoneNumber String, then they
> will only need to actually be validated at a few application entry points,
> not at the start of every method that has a phone number as a parameter. In
> other words, the annotation is ideally not a "to-do" list but rather an
> "it's already done" list.
>
> The guarantee that the compiler would then provide is ideally on the same
> level as with generics: while it's being provided by the compiler, not the
> JVM, so you can always get around it if you try hard enough (native code,
> reflection, class file switcheroo, etc.), as long as you "follow the rules"
> you get the guarantee - or if not, an error or at least a warning.
>
> Brian Goetz <brian.goetz at oracle.com> wrote:
>
>> I think the best bet for making this usable would be some mechanism like
>> a "view", likely only on value types, that would erase down to the
>> underlying wrapped type, but interpose yourself on construction, and
>> provided a conversion from T to RefinedT that verified the requirement.
>> But this is both nontrivial and presumes a lot of stuff we don't even have
>> yet...
>>
>
> I think that is close to what I was imagining. It seems like it could be
> done with fairly minimal impact/disruption...? No need for wrappers or
> views.
>
> But first just to be clear, what I'm getting at here is a fairly narrow
> idea, i.e., what relatively simple thing might the compiler do, with a
> worthwhile cost/benefit ratio, to make it easier for developers to reason
> about the correctness of their code when "type restriction" is being used,
> either formally or informally (meaning, if you're using an int to pass
> around the size of collection, you're doing informal type restriction).
>
> What's the benefit? Type restriction is fairly pervasive, and yet because
> Java doesn't make it very easy to do, it's often not being done at all, and
> this ends up adding to the amount of manual work developers must do to
> prove to themselves their code is correct. The more of this burden the
> compiler could take on, the bigger the benefit would be.
>
> What's the cost? That depends on the solution of course.
>
> To me the giant poster-child for this kind of pragmatic language addition
> is generics. It had all kinds of minor flaws from the point of view of
> language design, but the problem it addressed was so pervasive, and the new
> tool it provided to developers for verifying the correctness of their code
> was so powerful, that nobody thinks it wasn't worth the trade-off.
>
> OK let me throw out two straw-man proposals. I'll just assume these are
> stupid/naive ideas with major flaws. Hopefully they can at least help map
> out the usable territory - if any exists.
>
> *Proposal #1*
>
> This one is very simple, but provides a weaker guarantee.
>
>    1. The compiler recognizes and tracks "type restriction annotations",
>    which are type annotations having the meta-annotation @TypeRestriction
>    2. For all operations assigning some value v of type S to type T:
>       1. If a type restriction annotation A is present on T but not S,
>       the compiler generates a warning in the new lint category
>       "type-restriction"
>
> That's it. A cast like var pn = (@PhoneNumber String)input functions
> simply as a developer assertion that the type restriction has been
> verified, but the compiler does not actually check this. There is no change
> to the generated bytecode. If the developer chooses to write a validation
> method that takes a string, validates it (or throws an exception), and then
> returns the validated string, that method will need to be annotated with
> @SuppressWarnings("type-restriction") because of the cast in front of the
> return statement.
>
> Guarantee provided: Proper type restriction as long as "type-restriction"
> warnings are enabled and not emitted. However, this is a "fail slow"
> guarantee: it's easy to defeat (just cast!). So if you write a method that
> takes a @PhoneNumber String parameter that is passed an invalid value,
> you won't find out until something goes wrong later down the line (or
> never). In other words, *your* code will be correct, but you have to be
> trusting of any code that *invokes* your code, which in practice is not
> always a sound strategy.
>
> *Proposal #2*
>
> This is proposal is more complex but provides a stronger guarantee:
>
>    1. The compiler recognizes and tracks "type restriction annotations",
>    which have the meta-annotation @TypeRestriction
>       1. The annotation specifies a user-supplied "constructor" class
>       providing a user-defined construction/validation method validate(v)
>       2. We add class TypeRestrictionException extends RuntimeException and
>       encourage validate() methods to throw (some subclass of) it
>    2. For all operations assigning some value v of type S to type T:
>       1. If a type restriction annotation A is present on T but not S,
>       the compiler generates a "type-restriction" warning AND adds an
>       implicit cast added (see next step)
>    3. For every cast like  var pn = (@PhoneNumber String)"+15105551212"  the
>    compiler inserts bytecode to invoke the appropriate enforcer
>    validate(v) method
>    4. The JLS rules for method resolution, type inference, etc., do not
>    change (that would be way over-complicating things)
>       1. Two methods void dial(String pn) and void dial(@PhoneNumber
>       String pn) will still collide
>
> Guarantee provided: Proper type restriction unless you are going to
> extremes (native code, reflection, runtime classfile switcheroo, etc.).
> This is a "fail fast" guarantee: errors are caught at the moment an invalid
> value is assigned to a type-restricted variable. If your method parameters
> have the annotation, you don't have to trust 3rd party code that calls
> those methods (as long as it was compiled properly). I.e., same level of
> guarantee as generics.
>
> These are by no means complete or particularly elegant solutions from a
> language design point of view. They are pragmatic and relatively
> unobtrusive add-ons, using existing language concepts, to get us most of
> what we want, which is:
>
>    - User-defined "custom" type restrictions with compile-time
>    checking/enforcement
>       - As with generics, the goal is not language perfection, but rather
>       making it easier for developers to reason about correctness
>    - Compile-time guarantees that type restricted values in source files
>    will be actually type restricted at runtime
>    - Efficient implementation
>       - Validation only happens "when necessary"
>       - No JVM changes needed (erasure)
>    - No changes to language syntax; existing source files are 100%
>    backward compatible
>
> The developer side of me says that the cost/benefit ratio of something
> like this would be worthwhile, in spite of its pragmatic nature, simply
> because the problem being addressed seems so pervasive. I felt the same way
> about generics (which was a much bigger change addressing a
> much bigger pervasive problem).
>
> But I'm sure there are things I'm missing... ?
>
> -Archie
>
> --
> Archie L. Cobbs
>
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