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The parameters are a possibly empty, comma-separated list of variable names. Optional All parameters can be assigned a default value.  More details can be found in Using Optional and Named Parameters. 

Zero or more declarations can be included to prepare data for the action block. 

The action block is the same as the action block in a rule. Any action, including a user-defined action, can be used in the action block. Actions can be simple or compound. A simple action is a valid action block. For example, the following defines an action called send_warning using the event:send action:

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The variable send_warning only has meaning in an action context within a rule. Because user-defined actions are first-class values (i.e., they can be returned as the result of executing an expression), they can be passed into functions or other user-defined actions and returned as the result from a function. You can thus write recursive actions.

User defined actions may also return values. The returned value may be a string, map, array etc. Consider the following from the pico-engine version 0.43.0:

    deleteChild = defaction(pico_name){
      ent_children = children(){"children"}
      child_collection = ent_children.collect(function(child){
                                              (child{"name"} ==  pico_name) => "to_delete" | "dont_delete"
                                            })
      child_to_delete = child_collection{"to_delete"}.head()

      every {
        engine:removePico(child_to_delete{"id"})
      }
      returns
      {
        "updated_children": child_collection{"dont_delete"},
        "child": child_to_delete
      }
    }

To retrieve the returned value (in this case a map), bind it to a new name using "setting":

deleteChild(<give pico name>) setting(returnValue)