GCBASIC documentation
Syntax

    Function identifier  [( arg1 [ as Type ], arg2... argx) ] [As return_type]
        statements
        ...
        identifier = return_value
        ...
    End Function

About Functions

Functions are a special type of subroutine that can return a value.   This means that when the name of the function is used in the place of a variable, GCBASIC will call the function, get a value from it, and then put the value into the line of code in the place of the variable.

Functions are strict.  The function MUST be assigned to an appropiate variable or passed to another subroutine.  Calling a function with no assignment or returned value usage will raise an error condition.

Functions may have parameters - these are treated in exactly the same way as parameters for subroutines.   The only exception is that brackets are required around any parameters when calling a function.   The argument’s type is given by "As type" following the parameter. If a parameter in the declaration is given a default value, the parameter is optional. Array parameters are specified by following an identifier with an empty parenthesis.

Returning values : return_type specifies the data type returned by a function upon exit.  If no data type is specified, then the function will return the default data type which is a byte.   Functions return values by assigning the Function keyword or the function’s identifier to the desired return value, this method do not cause the function to exit, however.  

Exit Function keyword in a same function is unsupported when returning objects with constructors.  Since functions return values, function calls evaluate to expressions. Thus, function calls can be made wherever an expression is expected, like in Assignments or If statements.  Parentheses surrounding the argument list are required on function calls in expressions and even highly recommended if there are no arguments.

Using Functions

This program uses a function called AverageAD to take two analog readings, and then make a decision based on the average:

    'Select chip
    #chip 16F88, 20

    'Define ports
    #define LED PORTB.0
    #define Sensor AN0

    'Set port directions
    dir LED out
    dir PORTA.0 in

    'Main code
    Do
        Set PORTB.0 Off
        If AverageAD > 128 Then Set PORTB.0 On
        wait 10 ms
    Loop

    Function AverageAD
        'Get 2 readings, divide by 2, store in AverageAD
        'Note the cast, the result of ReadAD needs to be converted to
        'a word before adding, or the result may overflow.
        AverageAD = ([word]ReadAD(Sensor) + ReadAD(Sensor)) / 2
    end function

See Also Subroutines, Exit