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FUNCTION xxx (arg1 [,arg2, …]) [AS <type>}
Defines a callable function. This is the same as adding a new function to MMBasic while it is running your program.
'xxx' is the function name and it must meet the specifications for naming a variable.
The type of the function can be specified by using a type suffix (ie, xxx$) or by specifying the type using AS <type> at the end of the functions definition.
FUNCTION xxx (arg1, arg2) AS STRING
'arg1', 'arg2', etc are the arguments or parameters to the function (the brackets are always required, even if there are no arguments).
An array is specified by using empty brackets. ie, arg3().
The type of the argument can be specified by using a type suffix (ie, arg1$) or by specifying the type using AS <type> (ie, arg1 AS STRING).
The argument can also be another defined function or the same function if recursion is to be used (the recursion stack is limited to 50 nested calls).
To set the return value of the function you assign the value to the function's name. For example:
SQUARE = a * a
Every definition must have one END FUNCTION statement. When this is reached the function will return its value to the expression from which it was
called. The command EXIT FUNCTION can be used for an early exit.
You use the function by using its name and arguments in a program just as you would a normal MMBasic function.
When the function is called each argument in the caller is matched to the argument in the function definition. These arguments are available only inside the function.
Functions can be called with a variable number of arguments. Any omitted arguments in the function's list will be set to zero or a null string.
Arguments in the caller's list that are a variable and have the correct matching type will be passed by reference to the function.
This means that any changes to the corresponding argument in the function will also be copied to the caller's variable and therefore may be accessed after the function has ended.
Arrays are passed by specifying the array name with empty brackets (eg, arg()) and are always passed by reference and must be the correct type.
You must not jump into or out of a function using commands like GOTO, GOSUB, etc. Doing so will have undefined side effects including the possibility of ruining your day.
Last edited: 29 September, 2020