Language Definition – User Defined Functions

The first type of functions being supported are the classic BASIC user defined or single statement functions.  These functions are defined with a DEF statement and whose identifier name starts with FN.  They may contain multiple arguments or no arguments.  The entire function must be defined on one statement (line), though multiple user functions can be defined on the same line separated by colons.  Two examples are:

    DEF FNHypot(X,Y)=SQR(X*X+Y*Y)
    DEF FNLength=LEN(First$+Last$)

Using FN for these functions means that there can be no other variables, arrays, functions or subroutines that start with FN.  These user functions may also have data type (the default type is double precision).  The user function arguments are considered local variables to the function and are not related to variables of the same name outside the function.  For the example above, X and Y are local to FNHypot(), but X and Y outside of the function will not be affected when FNHypot() is called.  The variables in FNLength are not arguments and are therefore not local.  Any variables used in a function not listed as arguments are regular variables.  These functions may also call other functions, but there needs to be check to make sure the function does not call itself.