tuxji commented on a change in pull request #674:
URL: https://github.com/apache/daffodil/pull/674#discussion_r742845282
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File path:
daffodil-runtime1/src/main/scala/org/apache/daffodil/processors/VariableMap1.scala
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@@ -155,6 +155,7 @@ class VariableInstance private (val rd: VariableRuntimeData)
this.setState(this.priorState)
this.setValue(this.priorValue)
}
+ case (VariableDefined, _, _) => // This should only occur for
pre-defined variables
Review comment:
This change definitely looks too simple to me. At the very least it
should verify some invariant or assumption so that we will be alerted if a
deeper problem exists. I'm not sure exactly what the original problem is or
what invariant needs to be checked, but surely there must be something you can
put into an `Assert.invariant(...)` call at this line.
Perhaps the invariant should be whether the variable has ever been read? Or
whether it ever has been shadowed by a new variable instance? Again, I'm not
sure what problem is occurring or which part of the DFDL specification applies
to this situation (the verb "reset" does not occur anywhere in the
specification!), although I looked at [7.7 DFDL Variable
Annotations](https://daffodil.apache.org/docs/dfdl/#_Toc62570090) and noted the
following statements:
> Variables are defined at the top-level of a schema and have a specific
simple type. A distinction is made between the variable as defined, and an
instance of the variable where a value can be stored. The dfdl:defineVariable
also introduces a single unique global instance of the variable. Additional
instances may be allocated in a stack-like fashion using
dfdl:newVariableInstance which causes new instances to come into existence upon
entry to the scope of a model group, and these instances go away on exit from
the same. **[Is this where the function "reset" comes in?]**
> DFDL variables only vary in the sense that different instances of the same
variable can have different values. A single instance of a variable only ever
takes on a single value. Each variable instance is a single-assignment location
for a value[9]. Once a variable instance's value has been read, it can never be
assigned again. If it has not yet been assigned, and its default value has not
been read, then a variable instance can be assigned once using
dfdl:setVariable. **[Can you find some invariant to check from this part of the
specification?]**
> The following variables are predefined, and their names are in the DFDL
namespace (http://www.ogf.org/dfdl/dfdl-1.0/). These variables are expected to
be commonly set externally so are predefined for convenience. **[Can a
predefined variable ever take on a different value (I know it has a default
value) if it was set externally but did not have its value read before a
dfdl:setVariable?]**
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