RFR: 8177466: Add compiler support for local variable type-inference
Maurizio Cimadamore
maurizio.cimadamore at oracle.com
Tue Sep 19 11:40:30 UTC 2017
On 19/09/17 01:21, Sergey Bylokhov wrote:
> It is also interesting how the new 'var' should work in case of
> conditional expression:
>
> String s = true ? new String() : new int[1]; // incompatible types:
> bad type in conditional expression
> int[] s = true ? new String() : new int[1]; // incompatible types:
> bad type in conditional expression
In this case, both arms of the conditional return reference types, so
the conditional is a poly expression (see JLS 15.25.3). That means that
in the first case, you need to check that:
String is assignable to String, ok
int[] is assignable to String, not ok
In the second case you have the dual case:
String is assignable to int[], not ok
int[] is assignable to int[], ok
So, in neither declarations, both branches of the check are satisfied -
which is why you get an error.
>
> var s = true ? new String() : new int[1]; // works fine
> Is it expected that this code compiles?
Now, in this case the conditional is treated as a standalone conditional
- because there's no target type. The type of the initializer is then
computed using the classic rules, which in this case give lub(String,
int[]) = Serializable. So the type of 'var' is Serializable (all arrays
have Serializable as superinterface, see JLS 4.10.3).
Maurizio
>
>
> One related example:
> var s = true ? new String() : new int[1];
> s[0]=123;
>
> testVar.java:11: error: array required, but Serializable found
> s[0]=123;
> ^
> 1 error
>
> Looks like a surprise - 's' is of type "Serializable"?
>
> On 9/18/17 15:44, Sergey Bylokhov wrote:
>> Hi, Maurizio.
>> I am not sure is it expected or not, but in some cases the new 'var'
>> produce some non-easy to read error messages:
>>
>> var s = true ? new ArrayList<String>() : new ArrayList<Integer>();
>> s.add(new String());
>>
>> testVar.java:9: error: no suitable method found for add(String)
>> s.add(new String());
>> ^
>> method Collection.add(CAP#1) is not applicable
>> (argument mismatch; String cannot be converted to CAP#1)
>> method List.add(CAP#1) is not applicable
>> (argument mismatch; String cannot be converted to CAP#1)
>> method AbstractCollection.add(CAP#1) is not applicable
>> (argument mismatch; String cannot be converted to CAP#1)
>> method AbstractList.add(CAP#1) is not applicable
>> (argument mismatch; String cannot be converted to CAP#1)
>> method ArrayList.add(CAP#1) is not applicable
>> (argument mismatch; String cannot be converted to CAP#1)
>> where CAP#1 is a fresh type-variable:
>> CAP#1 extends INT#1 from capture of ? extends INT#1
>> where INT#1,INT#2 are intersection types:
>> INT#1 extends Object,Serializable,Comparable<? extends INT#2>
>> INT#2 extends Object,Serializable,Comparable<?>
>> Note: Some messages have been simplified; recompile with
>> -Xdiags:verbose to get full output
>>
>>
>> On 9/18/17 09:14, 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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