Changed Behavior in Variable Definition

[the Otter]

As most people here probably know, the Let ( ) function can be used to define a Local variable. As such, it is possible to build a custom function that defines such a variable, and it is further possible to set said variable to a value including itself. An example would be the following custom function, ErrorList, consisting of the following calculation:

Let ( $ErrorList = List ( $ErrorList ; Get ( LastError ) ) ; "" )

If a Set Variable script step sets the same variable as a custom function like the one above, e.g.

Set Variable [ $ErrorList ; Value: ErrorList ]

…the script step will run appropriately, so long as the contradictory variable—in this case, $ErrorList—is not yet defined. However, once this variable has been defined, executing the preceding script step will cause FileMaker 14 (and perhaps other versions) to suffer an Error #1213 and crash the application. The workaround for this behavior is to have the Set Variable script step set a dummy variable, e.g.

Set Variable [ $x ; Value: ErrorList ]

Even if $x is not referenced anywhere, having a script call the ErrorList function passes the variable $ErrorList to the script’s own context, thus allowing its value to be accessed by later steps in the same script (including subsequent calls to the ErrorList function itself).

In FileMaker 15, this behavior has been changed: local variables defined within a custom function are now valid only within the scope of the function itself, including any recursions. While this alleviates the problem of application crashes, it also results in unexpected behavior when scripts written in earlier versions of FileMaker rely on custom functions to set local variables. When migrating to FileMaker 15, each affected script step must be updated to set the target variable explicitly instead of relying on the custom function to do the work. In other words, the code:

Set Variable [ $ErrorList ; Value: ErrorList ]

…which proved fatal in FileMaker 14, is now required grammar for FileMaker 15: FileMaker 15 believes that what happens in the function stays in the function, instead returning the result of the calculation to the variable defined in the Set Variable script step. The FileMaker 14 grammar,

Set Variable [ $x ; Value: ErrorList ]

…thus sets $x to the intended value of $ErrorList while leaving the value of $ErrorList as null.

Unfortunately, this cannot work effectively in a mixed-installation environment: the FileMaker 14 grammar leaves FileMaker 15 clients with unintended null values; the FileMaker 15 grammar causes FileMaker 14 to crash. When upgrading all users to FileMaker 15 is not feasible, the best workaround is to use the FileMaker 14 grammar, then once all relevant script steps are complete, check the value of the intended variable (e.g. $ErrorList) and, if empty, set it to the value of the dummy variable (e.g. $x).

 

ErrorTest.fmp12

Edited January 11, 2017 by the Otter
Updated Demo File

[jbante]

4 hours ago, the Otter said:

In FileMaker 15, this behavior has been changed: local variables defined within a custom function are now valid only within the scope of the function itself, including any recursions. While this alleviates the problem of application crashes, it also results in unexpected behavior when scripts written in earlier versions of FileMaker rely on custom functions to set local variables. When migrating to FileMaker 15, each affected script step must be updated to set the target variable explicitly instead of relying on the custom function to do the work.

I just tested this, and FileMaker 15 custom functions were happy to set local variables that persist outside the function for me. It looks like the ErrorList function in your demo file isn't setting the $Error variable because the custom function chooses not to save error code 0, and the script doesn't trigger any other errors for it to save.

[the Otter]

Thanks, jbante. Actually, the custom function only ignores error #0 if any error (including 0) was previously recorded. However, you are correct that I have apparently misunderstood the issue: I tried a simpler custom function that sets a variable and it was able to pass that variable to the script.

Despite my misdiagnosis of the problem, there is still a change in behavior: the first version of the script still crashes FileMaker 14, but not 15; and the second version still fails to set $Error to 0 in FileMaker 15, but not 14. I will update the original post once I have more accurately diagnosed the scenario in which this occurs.

Edited January 11, 2017 by the Otter

[jbante]

Let ( [
         #error = Get ( LastError ) ;
         #newError = If ( #error ≠ 0 ; Trim ( #error & " " & scriptStep ) ) ;

It sure looks to me like the function is trying to ignore error code 0, even if there are previously recorded errors. It looks like FileMaker 15 is successfully setting $Error to empty until the first error, and successfully not appending new lines for error code 0. So FileMaker 15 isn't crashing. Great!

[the Otter]

Yes, thank you. That’s what I get for typing hurriedly. I have fixed my previous comment.