Units Checker and null literals

[John Hovell]

I have some projects that I am upgrading from checker 1.x to 2.x and getting some new failures that I am not sure how to resolve:

For example:

error: [return.type.incompatible] incompatible types in return.

            return null;

                   ^

 found   : @UnknownUnits  null

 required: @frac(Prefix.deci) Float

The error makes sense, but I am wondering if there is some way I can annotate my return of null to include the units. 

@frac is a custom units annotation I wrote for fractions but figure it would be the same for @m (meters) or a different unit

@frac @Nullable public Float myMethod(final String param) {

        if (param.equals("foo") {

            return null;

        }

        return MyCustomUnitsTools.FRAC * 3f;

    }

I have searched documentation for how to annotate null literals but haven't found how to handle this case, or if my code needs refactoring if there is a better pattern to follow. Is there a way to resolve these sorts of errors or a better pattern to follow for nullable units?

Thanks!

[Werner Dietl]

Hi John,

thanks for the report! `null` actually is the bottom in the type
hierarchy and for built-in units annotations this works as you expect.
I've opened issue
https://github.com/typetools/checker-framework/issues/2087 to look
into why this doesn't work for custom units.

Best,
cu, WMD.

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[Jeff Luo]

Hi John,

The type of the null literal is programatically set by Units Checker to be @UnitsBottom, which is the bottom type of all units qualifiers. It shouldn't be necessary to annotate or cast, the null literal to any other unit as it is always a subtype of every unit.

I tried to reproduce your snippet in our live demo tool, but I'm not seeing any errors when using one of the built-in units as the return type. The issue here is that somehow null takes on @UnknownUnits as its unit when it shouldn't at all.

If absolutely necessary, you can cast the null literal, ie:  return (@m Float) null;

Jeff

On Wed, Aug 1, 2018 at 1:25 PM John Hovell <jho...@sunrun.com> wrote:

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[John Hovell]

Thanks all. I will try to put together a test case if this is not expected behavior. It could be a bug/issue on my end as the project is rather large/complex. I thought it might be because I had some checker 1.x classes sitting around on the class path but I cleaned that up and am still seeing the same behavior but need to do some more work to create an example.

I'll see if I can re-create in a simple project (using that demo tool assuming I can reproduce it there) and get back to you all. Thank you for the replies!

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[John Hovell]

Here is another example in returning a literal null where the units checker does not complain but the nullness checker does, even though I have the return type annotated as @Nullable.

Any advice how to fix this? 

Error: [return.type.incompatible] incompatible types in return.

   found   : null

   required: @Initialized @Nullable Float @Initialized @NonNull []

Thanks all!

http://eisop.uwaterloo.ca/live/#mode=display&code=import+org.checkerframework.checker.units.qual.*%3B%0Aimport+org.checkerframework.checker.units.UnitsTools%3B%0A%0Aimport+org.checkerframework.checker.nullness.qual.Nullable%3B%0A%0Apublic+class+Test+%7B%0A%0A++public+%40Nullable+%40m+Float%5B%5D+test(final+String+param)+%7B%0A++++++if+(param.equals(%22foo%22))+%7B%0A++++++++return+null%3B%0A++++++%7D%0A++++++%0A++++++final+%40m+Float%5B%5D+retVal+%3D+new+Float%5B1%5D%3B%0A++++++retVal%5B0%5D+%3D+UnitsTools.m+*+3f%3B%0A++++++return+retVal%3B+%0A++%7D%0A%7D&typeSystem=nullness

[John Hovell]

I created a simple test but unfortunately it causes the checker framework to crash. Maybe this would be a good place to start? Not sure if that's because I am defining annotations as inner static classes but wasn't sure how else to do it in the demo tool.

Error: Could not load class 'Test.frac' for field 'quantity' in annotation @org.checkerframework.checker.units.qual.UnitsMultiple(quantity=Test.frac.class, prefix=org.checkerframework.checker.units.qual.Prefix.centi); The Checker Framework crashed.  Please report the crash.  To see the full stack trace invoke the compiler with -AprintErrorStack

http://eisop.uwaterloo.ca/live/#mode=display&code=import+org.checkerframework.checker.units.qual.*%3B%0A%0Aimport+org.checkerframework.framework.qual.SubtypeOf%3B%0A%0Aimport+java.lang.annotation.Documented%3B%0Aimport+java.lang.annotation.ElementType%3B%0Aimport+java.lang.annotation.Retention%3B%0Aimport+java.lang.annotation.RetentionPolicy%3B%0Aimport+java.lang.annotation.Target%3B%0A%0Apublic+class+Test+%7B%0A%0A++++/**%0A+++++*+Fraction%0A+++++*/%0A++++%40Documented%0A++++%40Retention(RetentionPolicy.RUNTIME)%0A++++%40Target(%7BElementType.TYPE_USE,+ElementType.TYPE_PARAMETER%7D)%0A++++%40SubtypeOf(UnknownUnits.class)%0A++++public+static+%40interface+frac+%7B%0A++++++++Prefix+value()+default+Prefix.one%3B%0A++++%7D%0A++++%0A++++/**%0A+++++*+Percent%0A+++++*/%0A++++%40Documented%0A++++%40Retention(RetentionPolicy.RUNTIME)%0A++++%40Target(%7BElementType.TYPE_USE,+ElementType.TYPE_PARAMETER%7D)%0A++++%40SubtypeOf(UnknownUnits.class)%0A++++%40UnitsMultiple(quantity%3Dfrac.class,+prefix%3DPrefix.centi)%0A++++public+static+%40interface+pct+%7B%0A++++%7D%0A%0A++++public+%40pct+Float+test(final+String+param)+%7B%0A++++++++if+(param.equals(%22foo%22))+%7B%0A++++++++++++return+null%3B%0A++++++++%7D%0A++++++%0A++++++++return+toFrac(3f)%3B%0A++++%7D%0A++++%0A++++%40SuppressWarnings(%22units%22)%0A++++%40pct+Float+toFrac(float+number)+%7B%0A++++++++return+number%3B%0A++++%7D%0A++++++++%0A%7D&typeSystem=units

Thanks all for the help!

On Wednesday, August 1, 2018 at 7:15:36 PM UTC-7, John Hovell wrote:

[Werner Dietl]

Hi John,

re 1) In this example you have:

public @Nullable @m Float[] test(final String param) {
if (param.equals("foo")) {
return null;
}

The Units Checker is not complaining about this, but the Nullness
Checker gives you the correct error:

Error: [return.type.incompatible] incompatible types in return.

found : null
required: @Initialized @Nullable Float @Initialized @NonNull []

You probably meant `@m Float @Nullable []` as the return type. You
want to express that the method returns a possibly null array of @m
Float objects.
So you need to put the @Nullable in front of the array and the @m in
front of the component type.
The error message tries to highlight this by showing the `@NonNull []`
array type.
Does this make sense now?

re 2) Thanks for letting us know about the restriction with inner
static classes. I've filed
https://github.com/typetools/checker-framework/issues/2090 to look
into fixing this.
However, when I change your example to use top-level classes, I do not
get an error from the Units Checker about the return null.
Can you reproduce the original problem
https://github.com/typetools/checker-framework/issues/2087 with
top-level classes?

Best,
cu, WMD.

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>>> Jeff Luo
>>> PhD Candidate, ECE, UWaterloo
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