The Visitor Pattern is an OO Design Pattern that makes it easy to add new operations to a set of related classes without having to modify each class to add the new method. It is somewhat related to pattern matching and algebraic types. General discussions at [L1 ] and [L2 ].
Here is a simple example in Tcl to demonstrate the idea. The example here will be a simple OO tree representing integer arithmetic expressions (similar to expr, but much simpler). First, we define the nodes that represent each class, and give each a visit method:
package require XOTcl 1.5
namespace import xotcl::*
# A dummy Term class as superclass
Class Term
# Integers
Class Int -superclass Term
Int method init val { my set val $val }
Int method visit v { $v visitInt [my set val] }
# Addition
Class Add -superclass Term
Add method init {a b} { my set a $a; my set b $b }
Add method visit v {
my instvar a b
$v visitAdd [$a visit $v] [$b visit $v]
}
# Multiplication
Class Mul -superclass Term
Mul method init {a b} { my set a $a; my set b $b }
Mul method visit v {
my instvar a b
$v visitMul [$a visit $v] [$b visit $v]
}
# Create an example: t1 = (12 + (3 * 4))
Add t1 [Int new 12] [Mul new [Int new 3] [Int new 4]]We can now use these visit methods to define as many operations as we want over our complex data-type:
# Pretty printer
Object pprint
pprint proc visitInt val { return $val }
pprint proc visitAdd {a b} { return "($a + $b)" }
pprint proc visitMul {a b} { return "($a * $b)" }
# Simple interpreter/calculator
Object calc
calc proc visitInt val { return $val }
calc proc visitAdd {a b} { expr {$a + $b} }
calc proc visitMul {a b} { expr {$a * $b} }
# Example:
puts "[t1 visit pprint] = [t1 visit calc]"
#=> (12 + (3 * 4)) = 24See also A lambda calculus interpreter with arithmetic for an alternative to the visitor pattern.
Lars H: I still can't help but feeling this is just data is code, but with extra indirection (sort of turning itself inside out at every step) to make it fit into an OO framework. Which is natural, I suppose, if OO is your primary mechanism for structuring stuff… However, Tcl generally favours functional idioms over OO-idioms.