The visitor pattern is essentially the same thing as Church encoding

Re:The visitor pattern is essentially the same thing as Chur... by mdoms on Thursday February 04, 2021 @01:12PM (#26028101)

So every shape now has to reference every other shape? If I want to add a new shape I not only have to define the shape function,

  def Triangle(a,b,c):
    return (lambda visitCircle, visitRectangle, visitTriangle : visitTriangle(a, b, c))

and update the area function, I also need to update every existing type like

  def Rectangle(x, y, w, h):
        return (lambda visitCircle, visitRectangle, visitTriangle : visitRectangle(x, y, w, h))

Am I understanding this correctly? How is this a win? What if I have a hundred different shapes, now every shape definition is just a list of a hundred parameters and hopefully I called the correct one in my copy&paste-fest.

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Re:The visitor pattern is essentially the same thing as Chur... by Twisol on Thursday February 04, 2021 @01:37PM (#26028372)

That's why we usually group all of these handlers into a single Visitor interface. Then you only add one class and one extra method to the Visitor, and all the other variants stay the same.

You're brushing up against the so-called "expression problem", though, where you can choose between adding types easily or adding algorithms easily, but not usually both.

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Re:The visitor pattern is essentially the same thing as Chur... by h_anna_h on Thursday February 04, 2021 @03:26PM (#26029394)

Here you go

  import math
  
  def Circle(x, y, r):
      return (lambda d : d['visitCircle'](x, y, r))
  
  def Rectangle(x, y, w, h):
      return (lambda d : d['visitRectangle'](x, y, w, h))
  
  def area(shape):
      return shape({
          'visitCircle':    lambda x, y, r    : math.pi * r * r,
          'visitRectangle': lambda x, y, w, h : w * h
      })
  
  exampleCircle = Circle(2, 1.4, 4.5)
  exampleRectangle = Rectangle(1.3, 3.1, 10.3, 7.7)
  
  print(area(exampleCircle))
  print(area(exampleRectangle))
  # 63.61725123519331
  # 79.31

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Re:The visitor pattern is essentially the same thing as Chur... by mdoms on Thursday February 04, 2021 @04:45PM (#26030159)

At this point I have to believe you functional programming guys are just taking the piss.

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Re:The visitor pattern is essentially the same thing as Chur... by h_anna_h on Thursday February 04, 2021 @06:45PM (#26031162)

What do you mean? This solves the issue that you mentioned. I am not really a "functional programming guy" myself.

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Re:The visitor pattern is essentially the same thing as Chur... by mdoms on Thursday February 04, 2021 @07:23PM (#26031457)

Yes it solves the problem while introducing a hard dependency from a shape to some stringly typed hashmap from who-knows-where.

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Re:The visitor pattern is essentially the same thing as Chur... by h_anna_h on Saturday February 06, 2021 @05:22AM (#26045596)

Not really sure what "stringly typed" means. This is just the effect of trying this trick in python.

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