[patterns] Unchecked cast warning is absent?

[forax at univ-mlv.fr]

----- Mail original -----
> De: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "Tagir Valeev" <amaembo at gmail.com>, "amber-dev" <amber-dev at openjdk.java.net>
> Envoyé: Mardi 3 Octobre 2017 10:57:07
> Objet: Re: [patterns] Unchecked cast warning is absent?

> On 03/10/17 09:16, Remi Forax wrote:
>> Maurizio,
>> __matches <=> instanceof + cast,
>> the instanceof part is currently illegal, so the implication is that __matches
>> should be illegal too.
> From a code gen perspective I agree with you. From a type checking
> perspective I don't otherwise you would make pattern matching
> incompatible with generics:
> 
> List<String> ls = ...
> if (x matches ArrayList<String> a) {
>   ...
> } else if (x matches LinkedList<String> ll) {
>   ...
> }
> 
> I think accepting programs such as these is useful (and actually the
> above is not even an unsafe program!). But, using your strict
> interpretation, I can't do an instanceof whose target type is
> ArrayList<String> - so this should be rejected?

written with instanceof + cast, it's

if (x instanceof ArrayList) {
  ArrayList<String> a = (ArrayList<String>)ls; 
  ...
} else {
  if (x instanceof LinkedList) {
    LinkedList<String> a = (LinkedList<String>)ls; 
    ...
  } 
}

which is as you said is ok.

A quick idea is to try to separate how we handle type variables from how we handle parameterized types.
But i think there is a better idea.

One interesting things with __matches is that this is not the only mechanism to match a type, one can use instanceof + cast,
so you can only allow __match and pattern matching if it's typesafe (no warning) and let people write instanceof + cast if it's unsafe.

It's like the enhanced for, you can not write a loop that need an index but that fine because you can write the classical for loop in that case.

So in my opinion, __match and the pattern matching should only work if it's typesafe, so people reading the code will just think it's just a dumb type cases,
if it's not safe, lets people think twice about that and write the code with instanceof and cast to see on which branches they want to hang themselves. 

> 
> I think instanceof makes sense to describe what the generated code
> should do - but I believe applying the same type checking rules you have
> for instanceof would result in overly restrictive semantics.
> 
> Maurizio

Rémi

>>
>> Rémi
>>
>> ----- Mail original -----
>>> De: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
>>> À: "Tagir Valeev" <amaembo at gmail.com>, "amber-dev" <amber-dev at openjdk.java.net>
>>> Envoyé: Mardi 3 Octobre 2017 10:12:11
>>> Objet: Re: [patterns] Unchecked cast warning is absent?
>>> I don't think you want to disallow. It's same as cast - you can do
>>>
>>> (T)obj
>>>
>>> Which is a no-op, and you get an unchecked warning for it. If T has some
>>> more interesting bound, the cast is not a no-op.
>>>
>>> Maurizio
>>>
>>>
>>> On 03/10/17 05:28, Tagir Valeev wrote:
>>>> It's even more interesting that method type parameter is allowed in __matches:
>>>>
>>>>     static <T> void matches(Object obj) {
>>>>       if(obj __matches T t) {
>>>>         System.out.println("Matches!");
>>>>       }
>>>>     }
>>>>
>>>> Bytecode shows that "instanceof Object" is generated which is pretty
>>>> useless. I think, such construct should be disabled and result in
>>>> compilation error (similarly to instanceof).
>>>>
>>>> With best regards,
>>>> Tagir Valeev.
>>>>
>>>> On Mon, Oct 2, 2017 at 4:45 PM, Tagir Valeev <amaembo at gmail.com> wrote:
>>>>> Hello!
>>>>>
>>>>> Just tried to play with pattern matching implementation. Seems that
>>>>> the matching against a non-reifiable type is unsafe, which is
>>>>> expected. Should the warning be displayed in this case?
>>>>>
>>>>> E.g.:
>>>>>
>>>>> import java.util.*;
>>>>>
>>>>> public class ListTest {
>>>>>
>>>>>     static void matches(Object obj) {
>>>>>       if(obj __matches List<String> list) {
>>>>>         System.out.println(list.get(0));
>>>>>       }
>>>>>     }
>>>>>
>>>>>     public static void main(String[] args) {
>>>>>       matches(Collections.singletonList(1));
>>>>>     }
>>>>> }
>>>>>
>>>>> $ ~/j/bin/javac -Xlint:all ListTest.java
>>>>> (compilation successful, no warning)
>>>>> $ ~/j/bin/java ListTest
>>>>> Exception in thread "main" java.lang.ClassCastException:
>>>>> java.base/java.lang.Integer cannot be cast to
>>>>> java.base/java.lang.String
>>>>> at ListTest.matches(ListTest.java:7)
>>>>> at ListTest.main(ListTest.java:12)
>>>>>
>>>>> With best regards,
> >>>> Tagir Valeev.