How can a cfparam'd variable be undefined? - coldfusion

Question

A lot of sources call a single resource, typically through <cfthread ..>, but some use <cfinclude ..>.
Ideally, the code looks for the variable previous_state. If some variants are passed, then the resource will attempt to use them.
I received this error:
Variable PREVIOUS_STATE is undefined.
The line record points to the <cfif ..> in this chunk of code.
<cfparam name= "previous_state"
default= "" />
<cfif isSimpleValue( previous_state )
and len( previous_state ) eq 0>
<cfset previous_state= previousState />
</cfif>
My question is how can previous_state be undefined?
I can duplicate it in the application, but it's a fairly complex chain of code using threads. Perhaps the reference was eaten by the garbage collector?
I'm having trouble duplicating it in a simple code segment. I've tried setting the variable to the return of a function with returnType= "void", but <cfparam ..> seems to reset it to an empty string.
Here's the full code context. I removed the unrelated vars and such.
// Page
oComponent.foo();
// Component.foo()
<cfset var local= {
previous_state= QueryNew( "foo" , "varchar" )
} />
<cfthread name= "foo_#createUUID()#"
previousState= "#local.previous_state#">
<!--- Module does unrelated things... --->
<cfmodule template= "some_module.cfm">
<cfoutput>
// unrelated things
<cfparam name= "previous_state"
default= "" />
<!--- Next line is throwing error. --->
<cfif isSimpleValue( previous_state )
and len( previous_state ) eq 0>
<cfset previous_state= previousState />
</cfif>
</cfoutput>
</cfmodule>
</cfthread>
I'm now thinking cfparam is trying to use a scope that no longer exists by the time this code executes.

Answer 1

As the code is within a CFTHREAD tag I thing you should be passing previous_state as a CFTHREAD attribute, such as:
<cfparam name="previous_state" default="" />
<CFTHREAD previous_state = previous_state
previousState= "#local.previous_state#">
<cfif isSimpleValue( ATTRIBUTES.previous_state ) ........
</CFTHREAD>
To quote the CF Docs:
"The Attributes scope contains attributes that are passed to the scope, and is available only within the thread and only for the life of the thread."

Answer 2

For previous_state to be undefined it would need to be null. You would need to do something like
<cfif isNull(previous_state)>true</cfif>
To prove this try the following
<cfset previous_state = "" />
<!--- Change to set previous_state --->
<cfset previous_state = javacast( "null", previous_state ) />
<cfparam name="previous_state" default="" />
<cfif isSimpleValue( previous_state ) and len( previous_state ) eq 0 >
<cfset previous_state = previousState />
</cfif>

Answer 3

Make sure you scope your variables properly and your example above has a typo in it. I'm not having an issue running this code on both ACF9.0.1 and Railo 3.3.1.000
<cfparam name= "previous_state"
default= "" />
<cfdump var="#previous_state#">