Procedures¶

This primer covers procedures including overloading, argument intents and dynamic dispatch.

A procedure groups computations that can be called from another part of the program. The procedure can be defined with zero or more “formal” arguments. Each formal argument can have a default value associated with it.

Formal arguments are supplied with values when the procedure is called. The arguments supplied at the call site are the “actual” arguments. If a name and = precede an actual argument, the actual is assigned to the formal argument with that name. Any remaining (unnamed) actual arguments are assigned to the remaining formal arguments in lexical order.

A procedure can return zero, one or more values (as a tuple). The return value type can be specified after the formal argument list. If no explicit return value type is supplied, the Chapel compiler infers the return value type.

Here is a procedure which takes an integer argument and returns an integer result. It computes the factorial of the argument.

proc factorial(x: int) : int
{
  if x < 0 then
    halt("factorial -- Sorry, this is not the gamma procedure!");

  return if x == 0 then 1 else x * factorial(x-1);
}

writeln("A simple procedure");
writeln("6! is ", factorial(6));
writeln();

Overloading Functions¶

Default integers in Chapel are 64-bits, so we may want to specify a version of factorial that operates on 32-bit integers to save space and potentially time (depending on the target architecture). This version also optimizes a bit, compressing the callstack by a factor of two by doing two multiplies.

This version “overloads” the previous version of factorial. Upon a call to factorial(), the compiler will choose the best fit.

proc factorial(x: int(32)) : int(32)
{
  if x < 1 then
    halt("factorial -- Invalid operand.");

  if x < 3 then return x;

  return x * (x-1) * factorial(x-2);
}

The argument type of this version must be different, so the two versions of factorial can be differentiated. If we pass in a (default) 64-bit integer value, we will get the 64-bit version.

writeln("Another simple procedure");
writeln("33! is ", factorial(33));

Whereas passing in a 32-bit integer will cause us to get the 32-bit version:

writeln("6! is ", factorial(6:int(32)));
writeln();

Overloading Operators¶

Procedure definitions allow you to overload operators, too. Here we define a new type, Point, and overload the definition of + to handle that type.

record Point { var x, y: real; }

Tell how to add two points together.

proc +(p1: Point, p2: Point)
{
  // Vector addition in 2-space.
  return new Point(p1.x + p2.x, p1.y + p2.y);
}

We can also overload the writeThis() routine called by writeln.

proc Point.writeThis(w) throws
{
  // Writes it out as a coordinate pair.
  w.write("(");
  w.write(this.x);
  w.write(", ");
  w.write(this.y);
  w.write(")");
}

writeln("Using operator overloading");
var down = new Point(10.0, 0.0);
var over = new Point(0.0, -5.0);
writeln("down + over = ", down + over);
writeln();

Details on Arguments¶

Here we define a class, Circle, and a function which creates a specific instance of it using a different style of argument definition than we have previously encountered.

class Circle {
  var center : Point;
  var radius : real;
}

Note that a default value for an argument can be provided, which will be used if a value for that argument is not specified in the call. Here, instead of specifying the type of x, y, and diameter, we provide them a default value of 0.0. Because we did not specify their type but did provide a default value, the type of these arguments is inferred to be the type of that value - in this case, it is real.

proc create_circle(x = 0.0, y = 0.0, diameter = 0.0)
{
  var result = new Circle();

  result.radius = diameter / 2;
  result.center.x = x;
  result.center.y = y;

  return result;
}

writeln("Using named arguments");

Using named actual arguments in the call can prevent confusion. Specifying that the first value provided should be used for the argument diameter allows us to define the arguments in any order. Additionally, we can take advantage of the default value for y by not specifying a value to use instead. Thus this call creates a circle at (2.0, 0.0) with a radius of 1.5.

var c = create_circle(diameter=3.0,2.0);

writeln(c);
writeln();

Procedures can also have arguments of indeterminate type: these are known as generic procedures.

proc unknownArg(x)
{
  writeln(x);
  if x.type == int then
    writeln("I see you've passed me an integer!");
  else if x.type == string {
    writeln("I liked that last variable so much, I'll write it again!");
    writeln(x);
  }
}
var intArg = 5;
var strArg = "Greetings, procedure unknownArg!";
var boolArg = false;
writeln("Using generic arguments");
unknownArg(intArg);
unknownArg(strArg);
unknownArg(boolArg);
writeln();

Argument Intents¶

Normal (default) intent means that a formal argument cannot be modified in the body of a procedure. To allow changing the formal (but not the actual), use the in intent.

config param useSleep = true; // Set at compile time, used to speed up testing
use Time;

proc countDown(in n : uint = 10) : void
{
  while n > 0
  {
    writeln(n, " ...");
    if useSleep then sleep(1);
    n -= 1;
  }
  writeln("Blastoff!");
}

writeln("Using the \"in\" intent");
var s = 5 : uint;
countDown(s);
writeln("s is still ", s);  // 5
writeln();

The inout intent will write back the final value of a formal parameter when the procedure exits.

proc countDownToZero(inout n : uint = 10) : void
{
  while n > 0
  {
    writeln(n, " ...");
    if useSleep then sleep(1);
    n -= 1;
  }
  writeln("Boink?");
}

writeln("Using the \"inout\" intent");
var t = 3 : uint;
countDownToZero(t);
writeln("t is now ", t);    // 0
writeln();

Similar to the inout intent, the ref intent causes the value of the actual to change depending on the function. However, while the inout copies the argument in upon entering the function and copies the new value out upon exiting, using a ref intent causes any updates to the formal to immediately affect the call site.

proc countDownToZeroToo(ref n : uint = 10) : void
{
  while n > 0
  {
    writeln(n, " ...");
    if useSleep then sleep(1);
    n -= 1;
  }
  writeln("Flippity boop");
}

writeln("Using the \"ref\" intent");
var bip = 3 : uint;
countDownToZeroToo(bip);
writeln("bip is now ", bip);        // 0
writeln();

The out intent causes the actual argument to be ignored when the procedure starts. The actual is assigned the value of the corresponding formal when the routine exits.

This arctan routine puts the result in the argument with out intent and returns the number of iterations it needed to converge.

atan x = x - x^3/3 + x^5/5 + sum_3^inf (-1)^i x^(2i+1)/(2i+1).

This actually converges very slowly for x close to 1 in absolute value. So we set the error limit to be 3 significant digits.

proc atan(x : real, out result : real)
{
  result = 0.0;
  var count = 0;
  var lastresult = 0.0;
  for i in 1.. by 2
  {
    var twoIP1 = 2 * count + 1;
    var term = x ** twoIP1 / twoIP1;
    result += if count % 2 == 0 then term else -term;
    count += 1;
    if abs(result - lastresult) < 1.0e-3 then break;
    lastresult = result;
  }
  return count;
}

writeln("Using the \"out\" intent");
var theta : real;
var n = atan(1.0, theta);
writeln("Computed Pi as about ", 4.0 * theta, " in ", n, " iterations.");
writeln();

A procedure can take a variable number of arguments – of indeterminate type. It is expanded like a generic procedure, with the required number of arguments having types which match the actual arguments.

Note: see the Variadic Arguments primer for further information on procedures with a variable number of arguments

proc writeList(xs ...?k) {
  var first = true;
  for x in xs {
    if first then first = false; else write(" ");
    write(x);
  }
  writeln();
}

writeln("Using variable argument lists.");
writeList(1, "red", 8.72, 1..4);
writeln();