Since version 2.2

Syntax:

fun <fun_name><sigil> [(<arg1>, <arg2>, <argn> ...)]
  [<statements>]
  return <expression>
endfun

The FUN … ENDFUN directives are used for defining a user function. Functions are similar to procedures with the exception that:

• They must return a value.
• They can be called from within an expression.

The name of the function and the name of a parameter can't start with a command or directive name. For example, you can't define a function named repeatnumber as REPEAT is a command. Likewise, you can't name a parameter funparameter1 as FUN is a directive.

Define your functions /before/ calling them, preferably in the beginning of your source file, like in the example bellow:

fun sum_of_integers(a, b)
  return a + b
endfun

To define a function of a type other than integer as well as defining parameters other than integers, use sigils:

fun sum_of_bytes!(a!, b!)
  return a! + b!
endfun  

It is valid to have more than one RETURN statements in a function:

fun max(a, b)
  if a > b then
    return a
  else
    return b
  endif
endfun

Note there are no such things as void functions in XC=BASIC and the following will not work:

fun dont_do_this(x)
  rem -- no return statement
endfun

If you want a routine that does not return a value, use Procedures instead.

You can omit the parameter list, but bear in mind that you still have to write the empty parentheses:

fun no_params()
  ...
endfun

You can call a function from within an expression by using the function name, sigil and the argument list in parentheses, for example:

let x! = my_function!(a, b!) * 2

Note that in the example above my_function! is a function that returns a byte and accepts an integer and a byte as arguments.

Important note: you must pass exactly the same amount of parameters and the parameter types must match those in the function definition.

While procedures do support direct recursion, functions don't.