Functions

Functions in XC=BASIC are essentially the same as Subroutines, with one important difference: functions must return a value. Everything else that is described on the previous page is true for functions and will not be repeated here.

Defining The Return Type

A function must always return a type, that is, the type of the value that it returns. You must define the return type in the function definition line using the AS keyword:

FUNCTION <fn_name> AS <type> (<arg> AS <type>)
  <statements>  
END FUNCTION

Calling a function

Functions can not be called using the CALL keyword, but rather they must be invoked as part of an expression. A function call is treated like an expression and its type is concluded from the function's return type. For example:

FUNCTION test AS LONG ()
  REM -- function body here
END FUNCTION

x = test()
REM -- The variable x will be implicitly defined as LONG
REM -- because the right hand side is a LONG type

Note

For the above reason, there are no void functions in XC=BASIC. Subroutines serve as void functions.

Returning a value

There are two ways to return a value from a function:

  • The QBASIC style, by assigning a value to the function name.
  • Using the RETURN keyword.

Both styles are accepted in XC=BASIC. Here are two examples to demonstrate each:

REM -- returning value the QBASIC style
FUNCTION test AS BYTE ()
  test = 42
END FUNCTION
PRINT test() : REM will output 42
REM -- returning value using the RETURN keyword
FUNCTION test AS BYTE ()
  RETURN 42
END FUNCTION
PRINT test() : REM will output 42

The above two codes are identical, but the RETURN keyword comes with a restriction: it immediately exits the function. There can be, however, situations where you want to continue executing the function even after specifying the return value, for example:

(TODO: provide example)

Note the use of the EXIT FUNCTION command that is used to early exit the function.

Warning

If the code exits from a function before any return value was specified, an undefined value will be returned.