RosettaCodeData/Task/Call-a-function/Bracmat/call-a-function-9.bracmat

( ( plus
  =   a b
    .     !arg:%?a ?b
        & !b:
        & '(.!arg+$a)
      | !a+!b
  )
& out$("1+2, not partial:" plus$(1 2))
& out$("1+2,     partial:" (plus$1)$2)
);