How to use Functions in StreamSelector component?

edit

I want to check a field (a map type) is empty or not, then I choose to use isEmptyMap() function as ${!isEmptyMap(record:value('/hitRuleSets'))} , but when I click check button, the sdc compains the following error:

SELECTOR_03 - Invalid condition '${!isEmptyMap(record:value('/hitRuleSets'))}': com.streamsets.pipeline.api.el.ELEvalException: CMN_0104 - Error evaluating expression ${!isEmptyMap(record:value('/hitRuleSets'))}: javax.servlet.jsp.el.ELException: No function is mapped to the name "isEmptyMap"

How to use Functions in StreamSelector component?