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DEF FN defines and names a function.
 
Syntax 1
  DEF FNname[(parameterlist)] = expression
 
Syntax 2
  DEF FNname[(parameterlist)]
     [statementblock]
  FNname = expression
     [statementblock]
  [EXIT DEF]
     [statementblock]
  END DEF
 
    ■ The name is a legal variable name, which is always prefixed with FN
      (for example: FNShort). The variable name (including the FN prefix)
      can be up to 40 characters long. The name can include an explicit
      type-declaration character to indicate the type of value returned.
      Names that are the same except for the type-declaration character
      are distinct names. For example, the following are names of three
      different DEF FN functions:
 
        FNString$
        FNString%
        FNString#
 
      To return a value defined by a DEF FN function, assign the value to
      the full function name:
 
        FNString$ = "No answer."
 
    ■ parameterlist is a list of variable names, separated by commas.
      A parameterlist has the following syntax:
 
        variable [AS type] [,variable [AS type]]...
 
          variable    Any valid BASIC variable name.
 
          AS type     type is INTEGER, LONG, SINGLE, DOUBLE, CURRENCY,
                      or STRING. You also can indicate a variable's type by
                      including a type-declaration character (%, &, !, #, @
                      or $) in the name.
 
 
    ■ In both versions of syntax, The argument expression is evaluated and
      the result is the function's value. In Syntax 1 (single-line syntax),
      expression is the entire body of the function and is limited to one
      logical line.
    ■ EXIT DEF immediately exits an executing DEF FN function. Program
      execution continues where the DEF FN function was invoked.
    ■ When the function is called, BASIC assigns the value of each
      argument to its corresponding parameter. Function arguments are
      passed by value. Functions defined by DEF FN do not accept arrays,
      records, or fixed-length strings as arguments.
    ■ When no expression is assigned to the name, the default return values
      are 0 for a numeric DEF FN function, and the null string ("") for a
      string DEF FN function.
 
Usage Notes
    ■ DEF FN must define a function before the function is used. If you
      call the function before it is defined by DEF FN, BASIC generates
      the error message, "Function not defined." DEF FN function
      definitions cannot appear inside other DEF FN definitions. In
      addition, functions defined by DEF FN cannot be recursive.
    ■ Functions defined by DEF FN can be used only in the module in
      which they are defined.
    ■ A DEF FN-defined function can share variables with the module-level
      code. Variables not in parameterlist are global--their values are
      shared with the module-level code. To keep a variable value local to
      a function definition, declare it in a STATIC statement.
    ■ DEF FN can return either numeric or string values. DEF FN returns a
      string value if name is a string-variable name, and a numeric value
      if name is a numeric-variable name. If you assign a numeric value to a
      string function name or assign a string value to a numeric function
      name, BASIC generates the error message, "Type mismatch."
    ■ If the function is numeric, DEF FNname returns a value with the
      precision specified by name. For example, if name specifies a
      double-precision variable, then the value returned by DEF FNname is
      double precision, regardless of the precision of expression.
    ■ Because BASIC may rearrange arithmetic expressions for greater
      efficiency, avoid using DEF FN functions that change program variables
      in expressions that may be reordered. The following example may give
      unpredictable results:
 
        DEF FNShort
           I=10
           FNShort=1
        END DEF
        I=1 : PRINT FNShort + I + I
 
      If BASIC reorders the expression so FNShort is called after
      calculating (I+I), the result is 3 rather than 21. You usually can
      avoid this problem by isolating the DEF FN function call:
 
        I = 1 : X = FNShort : PRINT X + I + I
 
      Embedding I/O operations in DEF FN-defined functions used in I/O
      statements, or embedding graphics operations in DEF FN-defined
      functions in graphics statements, may cause similar problems.