Get slot value of an object

Question

I need to retrieve a slot value (passing a slot name) from an instance which may contain other instances. Example:

(defclass MAINCONTROLLER (is-a USER)
    (slot uuid
        (type STRING))
    (slot param
        (type INSTANCE))
    (multislot zones
        (type INSTANCE))        
    (slot state
        (allowed-values a b c))
    (slot pump
        (allowed-values on off)))

(make-instance mainController of MAINCONTROLLER
    (uuid "myController123")
    (param [param-mainController])
    (zones [zone1] [zone2])                  
    (state a)                                       
    (pump on))

Slot named "param" contains an instance called [param-mainController].

CLIPS documentation suggests to retrieve a slot value with a send command with put- parameter. I tried to use a generic function to retrieve a parameter only by passing the slotname.

(defmessage-handler USER get-param (?param-name)
    (printout t "Slot value: " ?self:?param-name crlf))

But executing it I get:

(send [mainController] get-param state)

[MSGPASS2] No such instance mainController in function send.

FALSE

Some questions:

1) Do I need always to define a (create-accessor read) for every slot I need to read withsend command?

2) Could you please suggest some examples with best practices to retrieve a slot value from an instance?

Thank you, Nic

Answer 1

By default, get- and put- handlers are created for slots. To retrieve specific slots use (send <instance> get-<symbol>) outside of a class's message-handlers and ?self:<symbol> within. To retrieve a slot where the slot name is stored in a variable, use (send <instance> (sym-cat get- <variable>)). Using ?self:<variable> is invalid syntax for slot shorthand references.

CLIPS> (clear)
CLIPS> 
(defclass MAINCONTROLLER (is-a USER)
    (slot uuid
        (type STRING))
    (slot param
        (type INSTANCE))
    (multislot zones
        (type INSTANCE))        
    (slot state
        (allowed-values a b c))
    (slot pump
        (allowed-values on off)))
CLIPS> 
(defmessage-handler MAINCONTROLLER myprint ()
   (printout t ?self:state crlf))
CLIPS>    
(deffunction retrieve-slot (?ins ?slot-name)
   (printout t (send ?ins (sym-cat get- ?slot-name)) crlf))
CLIPS>        
(make-instance mainController of MAINCONTROLLER
    (uuid "myController123")
    (param [param-mainController])
    (zones [zone1] [zone2])                  
    (state a)                                       
    (pump on))
[mainController]
CLIPS> (send [mainController] get-state)
a
CLIPS> (send [mainController] myprint)
a
CLIPS> (retrieve-slot [mainController] state)
a
CLIPS>