[Tutor] Visitor Pattern

[Sean 'Shaleh' Perry]

On Sunday 01 February 2004 18:05, Daniel Ehrenberg wrote:
> What is a visitor pattern? I've heard that it shows
> how good a language is, and that Lisp is really good
> because it can be done in two (cryptic) lines. How can
> a visitor pattern be implimented in Python?
>

It is from the original "Desgin Patterns" book which is one of the bibles of 
OO coding.

An excerpt:
<quote>

Intent

Represent an operation to be performed on the elements of an object structure.  
Visitor lets you define a new operation without changing the class of the 
elements on which it operates.

</quote>

The idea here is you have an object with "visits" another object and in some 
way interacts with it.

Common example:

for window in windowList:
   display.draw(window)

which in functional terms shows up as:

map(display.draw, windowList)

or in new terms

[display.draw(window) for window in windowList]

Visitor is a very, very common idiom in functional languages.