4.9 Function/procedure

Sub-programs can be written as functions or procedures. The only difference between functions and procedures is that functions must return a value while procedures must not return a value. The type of a sub-program which returns a value using the return~<expression> statement becomes func. If return is not used, or is used without an expression, the type becomes proc. The type of the sub-program therefore need not be declared. It is an error to use a procedure in an expression or pass a procedure as an argument to another sub-program where a function should have been passed.

A function or procedure declaration begins with a variable name followed by an argument list. The argument list is enclosed by a round bracket pair ('(' and ')'). A '()' specifies an empty argument list. The function body is in enclosed between the '{' and '}' brackets. E.g.

   >/* An example of a function declaration */
   >f() { return sin(PI/2); }
   >/* An example of a procedure declaration */ 
   >p() {print "Value of f() = ",f(),"\n";}
   >f()
              1.00000
   >p() 
   Value of f() =    1.00000

A sub-program can be passed as an argument to another sub-program. An argument corresponding to a sub-program can be specified using the func (for a function) or proc (for a procedure) directive. E.g.

   >f(x) { return sin(x); }
   >p(func fa,x) {print "The value of f(",x%5.2f,") =",fa(x),"\n";}
   >p(f,10)
   The value of f(10.00) =  -0.54402

All symbols (variables, functions, procedures) used in the sub-program code must be either global variables declared before the sub-program declaration or must be one of the argument list. Temporary variables, the scope of which is within the sub-program only, can be declared using the auto directive. E.g.

   >f(x) { return sin(x); }
   >p(func fa,x)
     {
       auto t;
       t=fa(x);
       print "The value of f(",x%5.2f,") =",t,"\n";
     }
   >p(f,10)
   The value of f(10.00) =  -0.54402