I think it's best understood if you work backwards from the print calls
1. print is fairly straightforward, prints to stdout what is inputted
2. def area(shape) returns the result of shape call with the two lambda parameters representing the area of a circle as the first parameter, and the second representing the area of a rectangle.
3. def Circle(x,y,r) & def Rectangle(x, y, w, h) are functions which return a lambda that both accepts two parameters, visitCircle and visitRectangle. visitCircle corresponds to the lambda x, y, r : math.pi * r * r in def area(shape). visitRectangle corresponds to the lambda x, y, w, h : w *h in def area(shape). So the def Circle(x, y, r) returns a lambda that takes as input these two formulas, but chooses the formula for calculating a circle's area, whereas def Rectangle(x, y, w, h) takes as input the same two formulas, but chooses the formula for calculating a rectangle's area.
This is basically what the blog post explains, though without some Haskell knowledge it is probably hard to understand. (Sorry if I explain something you already know)
The basic idea is that types can be “recreated” in an untyped way with only functions. It is also important to note the distinction between sum types and product types. A sum type is basically a (disjunct) set of types, for example Shape is a sum type on Rectangles and Circles (in this case only these things are considered the available shapes and every shape is either one or another). A product type is more straight-forward, basically any struct/class with multiple fields is an example, for example Circle is a product type with a radius and an x and y coord of its centerpoint.
For example here the two-ary methods represents the two types:
return (lambda visitCircle, visitRectangle : visitCircle(x, y, r))
Is going to represent circles, by awaiting two functions that work as constructors, and dispatching to the first one. Rectangle is similarly defined, only dispatching to the second function.
The part that corresponds to pattern matching/the second dispatch is basically where we would like to use our sum-types (either a circle or a rectangle). It provides both implementation in the form of a pair of functions, the first one corresponds to the circle’s area-definition the second to the rectangle’s.
The “magic” happens at calling area(exampleSomething). The area function will return as seen a pair of functions, basically every possible implementation on the possible types. Now that pair of functions will go to either a Circle ‘constructor’ function or a Rectangle one, and as we defined those, it will use up respectively the first or the second function in the pair based on its “type”. Also note that the correct arity of parameters will be passed to the selected function.
The good explanations you received lack a fundamental definition that will help those without formal CS education or some experience in FP to understand that code :
A lambda is a function that take two arguments:
- a list of arguments A
- a block of code C
And return an anonymous function having the arguments in A and C as a body.
In python lambda is an expression and the syntax is :
#A is the part before the : and C the part after
lambda a1 a2 a3 : a1 + a2 + a3
