RFR: 8177466: Add compiler support for local variable type-inference
Vicente Romero
vicente.romero at oracle.com
Mon Sep 25 14:20:31 UTC 2017
Hi Maurizio,
Thanks for the updated version. I don't have any major comments. Looks
good to me,
Vicente
On 09/22/2017 06:52 AM, Maurizio Cimadamore wrote:
> Here's another revision:
>
> http://cr.openjdk.java.net/~mcimadamore/8177466_v3/
>
> diagnostics:
>
> http://cr.openjdk.java.net/~mcimadamore/8177466_v3/diags.html
>
> Changes:
>
> * Rewrote logic of type projection in Types.java to use an enum
> * Added several javadoc on Types.java
> * added call to baseType() in Attr::attribLazyConstantValue
> * fixed an issue in TypeHarness where type var was created with null
> lower bound (as of this patch, javac is stricter and will assert on
> null bounds, as they are problematic to deal with when doing projections)
>
> If there are no major comments, my plan is to push this on
> jdk10/master early next week.
>
> Thanks!
> Maurizio
>
>
> On 19/09/17 16:04, Maurizio Cimadamore wrote:
>> Hi,
>> I have put together a slightly updated webrev:
>>
>> * as pointed out, one diagnostic used to say '9' instead of '10'
>> (this will likely need to change again because of new release
>> cadence, but good enough for now)
>> * the previous webrev contained a spurious added file (TypeHarness)
>> which was caused by a glitch in the lvti branch in the amber repo.
>>
>> New webrev:
>>
>> http://cr.openjdk.java.net/~mcimadamore/8177466_v2/
>>
>> New diags:
>>
>> http://cr.openjdk.java.net/~mcimadamore/8177466_v2/diags.html
>>
>> Maurizio
>>
>>
>> On 18/09/17 17:14, Maurizio Cimadamore wrote:
>>> Hi,
>>> this change adds support for local variable type inference (JEP 286
>>> [1]). A webrev of the change is available here:
>>>
>>> http://cr.openjdk.java.net/~mcimadamore/8177466
>>>
>>> The patch is relatively straightforward: implicitly typed locals are
>>> modeled in a similar fashion to implicit lambda parameters: their
>>> AST node is a JCVariableDecl whose 'vartype' field is not set (e.g.
>>> null).
>>>
>>> There are few tricky parts to this changeset:
>>>
>>> 1) tweak the parser to give 'var' special meaning depending on the
>>> version number and context
>>>
>>> 2) Add logic in name resolution to catch bad reference to types
>>> named 'var'
>>>
>>> 3) add logic to map initializer type back to a suitable variable
>>> declared type
>>>
>>> As for (1), the parser has been extended so as to special case local
>>> variables with special name 'var', so that the type will be left out
>>> of the corresponding AST representing the variable declaration. This
>>> behavior will only affect latest source version.
>>>
>>> The parser has a number of extra checks to prevent 'var to be used
>>> in places where it does not belong (according to the spec draft
>>> [2]); for instance, declaring a class whose name is 'var' is
>>> rejected in the parser. As a general rule, I tried to implement all
>>> such checks in the parser, as that gives very early and precise
>>> feedback about what's wrong with the code. The changes are
>>> implemented in Parser.java.
>>>
>>> There are however errors which cannot be caught in the parser, and
>>> that's why (2) is needed. Basically, whenever 'var' is used in a
>>> position where it could be either a type or a package name, the
>>> parser can't simply rule that out, so we have to accept the code,
>>> and give an error if, later on, we discover that 'var' was really
>>> used in a type position (see changes in Resolve.java).
>>>
>>> As far as (3) is concerned, we need to 'uncapture' captured types
>>> from initializers. That means that if we have a 'var' like this:
>>>
>>> class Foo {
>>> void test() {
>>> var x = getClass().getSuperClass();
>>> }
>>> }
>>>
>>> The initializer type will be something like Class<? super #CAP>,
>>> where #CAP <: Foo
>>>
>>> In this case, the compiler will project this type back to the less
>>> specific type Class<?>, and use that as the declared type for 'x'.
>>> This logic is defined in Types.java. As this logic is the same logic
>>> needed by jshell to render type of standalone expressions, jshell
>>> class VarTypePrinter has been removed and jshell has been rewired to
>>> point at the (now) official routine in Types. Jshell also needed
>>> several other tweaks to (i) accept 'var' and (ii) to deal with
>>> non-denotable types (intersection types and anonymous class types)
>>> that can be produced by the LVTI machinery (many thanks to Jan for
>>> doing those changes!)
>>>
>>>
>>> As far as testing is concerned, I wrote several tests to check that
>>> the parser was behaving as expected; to check the behavior of the
>>> LVTI inference machinery, I wrote a test harness which leverages
>>> annotation on 'var' so that we can write down assertions such as:
>>>
>>> @InferredType("java.util.List<? extends java.lang.String>")
>>> var s = extString();
>>>
>>>
>>> Regarding compiler diagnostics, for those interested, a
>>> comprehensive list of examples of new diagnostics triggered by the
>>> LVTI compiler can be found here:
>>>
>>> http://cr.openjdk.java.net/~mcimadamore/8177466/lvti-diags.html
>>>
>>> Finally, a finder has been added to detect local variable decls
>>> whose declared type can be replaced by 'var' - to enable it, the
>>> hidden option -XDfind=local should be used.
>>>
>>>
>>> Thanks
>>> Maurizio
>>>
>>> [1] - http://openjdk.java.net/jeps/286
>>> [2] - http://cr.openjdk.java.net/~dlsmith/local-var-inference.html
>>>
>>>
>>
>
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