(operator operand ...)
A procedure call is written by simply enclosing in parentheses expressions for the procedure to be called (the operator) and the arguments to be passed to it (the operands). The operator and operand expressions are evaluated and the resulting procedure is passed the resulting arguments. See Lambda Expressions, for a more complete description of this.
Another name for the procedure call expression is combination. This word is more specific in that it always refers to the expression; “procedure call” sometimes refers to the process of calling a procedure.
Unlike some other dialects of Lisp, Scheme always evaluates the operator expression and the operand expressions with the same evaluation rules, and the order of evaluation is unspecified.
(+ 3 4) => 7 ((if #f = *) 3 4) => 12
A number of procedures are available as the values of variables in the
initial environment; for example, the addition and multiplication
procedures in the above examples are the values of the variables
*. New procedures are created by evaluating
If the operator is a syntactic keyword, then the expression is not treated as a procedure call: it is a special form.