How to throw an error (at generation-time) in the default branch of a template switch?

I want to make sure that my template switch is only called with nodes it can handle. If it is used with an unrecognized concept, I would like code generation to fail with an error. Is there a way to throw generation-time errors from templates?

Note: The workaround I have been using is to generate invalid output that will cause an error down the line, for example:

default: <T  "unknown command: " + $COPY_SRC$[null]  T>

which will then generate an error like

textgen error: 'No textgen for Draw.structure.Fill' in [rightExpression] Fill null[847086916112855242] in Draw.sandbox@0

when Fill is a concept not recognized by the switch.

Solution

Instead of default that has a consequence you can define a "DISMISS TOP RULE" and then report the error there:

template switch operationsSwitch extends <none>                              

parameters                                                                   
<< ... >>                                                                    

  null-input message: <none>                                                 

  cases:                                                                     

        ...                                                     


  default: DISMISS TOP RULE error : operation  not supported by generator

This will then present an error in the messages window and the user/developer can click the message to get to the input node and the switch that failed to match from there.