objc:send

(objc:send receiver "selector:with:" arg1 arg2)

Sends a message. The receiver is an object, a class, or a class name; nil answers nil. The arguments and the result are marshalled by the selector's own type encoding, read from the runtime: an object parameter takes an object, nil or a string (sent as an NSString); a selector parameter takes its name; BOOL takes t/nil; a struct such as NSRect takes a list of numbers and comes back as one. A selector the receiver does not respond to, a wrong argument count or an ill-typed argument is an error. Every send runs on the main thread. Part of the macOS-only objc package -- the interpreter (java -jar, or the rontolisp native binary) and a compiled .class / .jar, never a .wasm; on a machine without the runtime it signals an error. See the macOS GUI guide.

CL-USER> (objc:send (objc:string "hello world") "length")
11
CL-USER> (objc:send (objc:string "hello world") "rangeOfString:" "world")
(6 5)
CL-USER> (objc:send "NSNumber" "numberWithDouble:" 2.5)
#<objc __NSCFNumber>
CL-USER> (objc:send (objc:send "NSNumber" "numberWithDouble:" 2.5) "doubleValue")
2.5

The guide's marshalling table lists every declared type and its Lisp form.