You type an expression , and the interpreter responds by displaying the result of its evaluating that expression. One kind of primitive expression you might type is a number. The leftmost element in the list is called the operator , and the other elements are called operands . The value of a combination is obtained by applying the procedure specified by the operator to the arguments that are the values of the operands. The convention of placing the operator to the left of the operands is known as prefix notation , and it may be somewhat confusing at first because it departs significantly from the customary mathematical convention.
Prefix notation has several advantages, however.
No ambiguity can arise, because the operator is always the leftmost element and the entire combination is delimited by the parentheses. The resulting indentations display clearly the structure of the expression. Observe in particular that it is not necessary to explicitly instruct the interpreter to print the value of the expression.