Enhancing the Hoc Language

Enhancing the hoc language is in fact very easy. First, you need access to YODA's source code. Then, you should write your new hoc builtin-function according a certain template. Suppose you want to add a new function, which gets two parameters, adds them, and returns the result (a slow way of saying a+b...). Your implementation could look like:

/*>--------------------------------------------------------------------------*/
double add(int i, Datum *p)
/*
 * add its two parameters
 *
 * input: i: number of parameters actually passed
 *        p: the arguments   
 *
 * return: sum of argument 1 and argument 2
 *         -1: wrong number of arguments (not too clean...)
 */
/*<--------------------------------------------------------------------------*/
{
if (i != 2)
   {
   cerr << ((i<2) ? "too few arguments\n" : "too many arguments\n");
   return -1;
   }

return p[0].val + p[1].val;
}

Parameters are passed as an array of type Datum, which is a union to hold all types of objects from within the user-task. Its definition is:

union Datum
{                               /* interpreter stack type */
Symbol  *sym;
char    *str;
double  val;
};

For user applications, only the str and val components are relevant. whenever an arithmetic value (e.g. a variable or expression) is passed, val is used to access the actual value. If the function is called with a string argument (remember: there are only string literals inside hoc), the str component is used.

To make this function part of the built-in set of hoc functions, you have to add it to the structure builtins_va[] in init.cc:

...
extern double add(int i, Datum *p);  /* declare your function */
...
static struct
{               /* Built-ins with varargs */
char    *name;
double  (*func)(int, Datum *);
} builtins_va[] =
{
{   "printf", print},
{   "histo", histo},
{   "hfill", hfill},
...
{   "update", update},
{   "add",    add},        /* <- your new function in the list */
{   0,        0  }
};

Compile, and check out your new built-in with a simple hoc file like:

proc init()
     {
     c = add(1.234, 4.321);
     printf("sum is : %f\n", c);
     }
...