• function is specified in terms of its name, parameters, and code block.
  • name -- used for calling it
  • parameters -- used for specifying values
  • block -- code that is executed when called

function main(args ...) { statements ... return value; }

• invocation
  • functions are called using function call syntax f() or f(1)
  • inside the function parameters are bound to the values given in the call

examples

Suppose we create houses using explicit polygons as follows:

const h = polygon([-1, -1], [1, -1], [1, 1], [0, 2], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-2],-[-1,-1));
return h;

we can now want write it in a more reusable manner. We can place the house construction in a function and name it house as follows:

function house () { 
  return polygon([-1, -1], [1, -1], [1, 1], [0, 2], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-2],-[-1,-1)); 
}
return house();

where we split it into a house function and a house call. Now suppose that we want to make it even more reusable by making the roof height about the square body of the house be a parameter.

We can now add that as a parameter and refer to it inside the house code block:

function house (roofHeight) { 
  return polygon([-1, -1], [1, -1], [1, 1], [0, 1 + roofHeight], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-1-+-roofHeight],-[-1,-1)); 
}
return house(2);

which is the same as where the body of house is copied and run with the parameter roofHeight set to 2:

var rootHeight = 2;
return polygon([-1, -1], [1, -1], [1, 1], [0, 1 + roofHeight], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-1-+-roofHeight],-[-1,-1)); 

Now if I had two calls to house as follows:

var h1 = house(1);
var h2 = house(2);
return union(h1, h2);

it would execute in the same as the following code:

const rootHeight = 1;
const h1 = polygon([-1, -1], [1, -1], [1, 1], [0, 1 + roofHeight], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-1-+-roofHeight],-[-1,-1));
const rootHeight = 2;
const h2 = polygon([-1, -1], [1, -1], [1, 1], [0, 1 + roofHeight], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-1-+-roofHeight],-[-1,-1));
return union(h1, h2);

we can further decompose it and have functions call functions:

function roof () { 
  return polygon([-1, -1], [1, -1], [0, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[0,-1)); 
}
function house () { 
  return union(square(), roof().translate([0, 2, 0])); 
}
return house();

Local Functions

Functions can be written at top level or within other functions:

function house () { 
  return polygon([-1, -1], [1, -1], [1, 1], [0, 2], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-2],-[-1,-1)); 
}
function main () { 
  return house(); 
}

or

function main () { 
  function house () { 
    return polygon([-1, -1], [1, -1], [1, 1], [0, 2], [-1, 1](/jackbackrack/cons-beginners-course/wiki/-1,--1],-[1,--1],-[1,-1],-[0,-2],-[-1,-1)); 
  }
  return house(); 
}

homework

• build a human function with torso, arm, leg, head local functions
  • first write functions without parameters
  • next write functions with at least one dimension parameter such as length
• write a container function
  • where container is a hollow cube with one side open
  • takes dimensions and thickness as parameters