NullPointerException when a Null CalValue is returned from a foreign function.

[rdwebster]

In certain situations, a NullPointerException will be generated in the CAL runtime code when a foreign function returns a Null value for a CalValue result.
My code was attempting to test for the null value, but the exception seems to happen before it gets to this.

This seems to happen in more cases with the 'debug_capable' option turned on.

The following code demonstrates the problem:

nullTest :: Maybe String;
public nullTest = 
    let
        val :: CalValue;
        val = eager nullCalValue;
    in
        if isNullCalValue val then Nothing
        else Just $ unsafeFromCalValue val;

Running this code in ICE with the org.openquark.cal.machine.debug_capable option on will result in a NullPointerException.
If the 'eager' function is removed, then the problem happens when the debug_capable option is on or off.

The call stack for the error:
java.lang.NullPointerException
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.setResult(RTResultFunction.java:63)
at org.openquark.cal.internal.runtime.lecc.RTFullApp$General$_0.reduce(RTFullApp.java:225)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.unsynchronizedEvaluate(RTResultFunction.java:136)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.evaluate(RTResultFunction.java:103)
at org.openquark.cal_RichSandbox_NullLookupBugTest.Null_Test.val$1$def_Strict(Null_Test.java)
at org.openquark.cal_RichSandbox_NullLookupBugTest.Null_Test.f(Null_Test.java)
at org.openquark.cal.internal.runtime.lecc.RTFullApp$General$_0.reduce(RTFullApp.java:225)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.unsynchronizedEvaluate(RTResultFunction.java:136)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.evaluate(RTResultFunction.java:103)
at org.openquark.cal_RichSandbox_NullLookupBugTest.Io__iceruntarget__0__0.f(Io__iceruntarget__0__0.java)
at org.openquark.cal.internal.runtime.lecc.RTFullApp$General$_0.reduce(RTFullApp.java:225)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.unsynchronizedEvaluate(RTResultFunction.java:136)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.evaluate(RTResultFunction.java:103)
at org.openquark.cal_Cal_Core_Prelude.Id.f(Id.java)
at org.openquark.cal.internal.runtime.lecc.RTApplication.reduce(RTApplication.java:156)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.unsynchronizedEvaluate(RTResultFunction.java:136)
at org.openquark.cal.internal.runtime.lecc.RTResultFunction.evaluate(RTResultFunction.java:103)
at org.openquark.cal.internal.machine.lecc.Executor.exec(Executor.java:173)
at org.openquark.cal.ICE$FunctionRunThread.run(ICE.java:7226)
at java.lang.Thread.run(Thread.java:637)

[indicee-michald]

Missing functions in example above:

foreign unsafe import jvm "isNull" isNullCalValue :: CalValue -> Boolean;
foreign unsafe import jvm "null" nullCalValue :: CalValue;

Note that nullCalValue is implemented as a RTSuperCombinator with an f() that returns null and a make() that wraps itself in a RTFullApp.General._0. This causes RTResultFunction (ie. RTFullApp.General._0) to blow up due to being unable to handle null valued results. It is possible to 'fix' this error by preventing the setResult(), unsynchronizedEvaluate() and synchronizedEvaluate() functions of RTResultFunction from failing due to result being null.

A better solution may be to implement nullCalValue as an RTData that would just return the null value and wouldn't have to go through RTResultFunction.*Evaluate().

Input appreciated.