A function-identifier is a syntactically correct combination of character-strings and separators that reference a function. The data item represented by a function is uniquely identified by a function-name with its arguments, if any. A function-identifier may include a reference-modifier.
Format
FUNCTION function-name-1 [({argument-1}...)]
Syntax rules
argument-1 must be an identifier, a literal, or an arithmetic expression. Specific rules governing the number, class, and category of argument-1 are given in the definition of each function (see chapter "Intrinsic functions").
A function-identifier which references an alphanumeric or national function may be specified anywhere in the general formats that an identifier is permitted and where the rules associated with the general formats do not specifically prohibit reference to functions, except as follows:
as a receiving operand of any statement,
where the rules associated with the general formats require the data item being referenced to have particular characteristics (such as class and category, usage, size, sign, and permissible values) and the evaluation of the function according to its definition and the particular arguments specified would not have these characteristics.
A function-identifier which references an integer or numeric function may be used only in an arithmetic expression.
General rules
The class and other characteristics of the function being referenced are determined by the function definition.
At the time reference is made to a function, its arguments are evaluated individually in the order specified in the list of arguments, from left to right. An argument being evaluated may itself be a function-identifier or may be an expression containing function-identifiers. There is no restriction preventing the function referenced in evaluating an argument from being the same function as that for which the argument is specified.