<f:layout name="Test" />
{namespace rf=DigiComp\FluidRenderFunctions\ViewHelpers}
<f:section name="Content"><f:spaceless>
    <rf:registerRenderFunction as="testFunc">
        {subject.name} is cool
    </rf:registerRenderFunction>
    <f:form.select options="{testEntities -> rf:applyRenderFunction(function: 'testFunc')}" />
</f:spaceless></f:section>