Visitor pattern: Difference between revisions
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</syntaxhighlight>{{out}} Same as Phix entry.
=={{header|Nim}}==
This is a translation of the Wikipedia C# example.
Note that Nim has no notion of “class” but only object types which allow simple inheritance. But it provides a way to define methods with dynamic dispatch and allows procedure overloading. So the translation of the C# example is easy.
<syntaxhighlight lang="Nim">import std/strutils
type
ExpressionPrintingVisitor = object
Expression = ref object of RootObj
Literal = ref object of Expression
value: float
Addition = ref object of Expression
left, right: Expression
# Expression procedures and methods.
method accept(e: Expression; v: ExpressionPrintingVisitor) {.base.} =
raise newException(CatchableError, "Method without implementation override")
method getValue(e: Expression): float {.base.} =
raise newException(CatchableError, "Method without implementation override")
# ExpressionPrintingVisitor procedures.
proc printLiteral(v: ExpressionPrintingVisitor; literal: Literal) =
echo literal.value
proc printAddition(v: ExpressionPrintingVisitor; addition: Addition) =
let leftValue = addition.left.getValue()
let rightValue = addition.right.getValue()
let sum = addition.getValue()
echo "$1 + $2 = $3".format(leftValue, rightValue, sum)
# Literal procedure and methods.
proc newLiteral(value: float): Literal =
Literal(value: value)
method accept(lit: Literal; v: ExpressionPrintingVisitor) =
v.printLiteral(lit)
method getValue(lit: Literal): float = lit.value
# Addition procedure and methods.
proc newAddition(left, right: Expression): Addition =
Addition(left: left, right: right)
method accept(a: Addition; v: ExpressionPrintingVisitor) =
a.left.accept(v)
a.right.accept(v)
v.printAddition(a)
method getValue(a: Addition): float =
a.left.getValue() + a.right.getValue()
proc main() =
# Emulate 1 + 2 + 3.
let e = newAddition(
newAddition(newLiteral(1), newLiteral(2)),
newLiteral(3))
var printingVisitor: ExpressionPrintingVisitor
e.accept(printingVisitor)
main()
</syntaxhighlight>
{{out}}
<pre>1.0
2.0
1.0 + 2.0 = 3.0
3.0
3.0 + 3.0 = 6.0</pre>
=={{header|Phix}}==
Line 175 ⟶ 255:
Visiting body.
Visiting engine.
Visiting car.
Kicking my front left wheel.
Kicking my front right wheel.
Kicking my back left wheel.
Kicking my back right wheel.
Moving my body.
Starting my engine.
Starting my car.
</pre>
=={{header|Python}}==
This is based on the [[wp:Visitor_pattern|Wikipedia]] Python example, but uses structural pattern matching instead of multiple visit methods.
<syntaxhighlight lang="python">
"""An example of the visitor pattern using structural pattern matching.
Requires Python >= 3.10.
"""
from __future__ import annotations
from abc import ABC
from abc import abstractmethod
class CarElement(ABC):
def accept(self, visitor: CarElementVisitor) -> None:
visitor.visit(self)
class CarElementVisitor(ABC):
@abstractmethod
def visit(self, car_element: CarElement) -> None:
"""Override this in `CarElementVisitor` subclasses."""
class Body(CarElement):
"""Car body."""
class Engine(CarElement):
"""Car engine."""
class Wheel(CarElement):
"""Car wheel"""
def __init__(self, name: str) -> None:
self.name = name
class Car(CarElement):
def __init__(self) -> None:
self.elements: list[CarElement] = [
Wheel("front left"),
Wheel("front right"),
Wheel("back left"),
Wheel("back right"),
Body(),
Engine(),
]
def accept(self, visitor: CarElementVisitor) -> None:
for element in self.elements:
visitor.visit(element)
super().accept(visitor)
class CarElementDoVisitor(CarElementVisitor):
def visit(self, car_element: CarElement) -> None:
match car_element:
case Body():
print("Moving my body.")
case Car():
print("Starting my car.")
case Wheel() as wheel:
print(f"Kicking my {wheel.name} wheel.")
case Engine():
print("Starting my engine.")
class CarElementPrintVisitor(CarElementVisitor):
def visit(self, car_element: CarElement) -> None:
match car_element:
case Body():
print("Visiting body.")
case Car():
print("Visiting car.")
case Wheel() as wheel:
print(f"Visiting my {wheel.name} wheel.")
case Engine():
print("Visiting my engine.")
if __name__ == "__main__":
car = Car()
car.accept(CarElementPrintVisitor())
car.accept(CarElementDoVisitor())
</syntaxhighlight>
{{out}}
<pre>
Visiting my front left wheel.
Visiting my front right wheel.
Visiting my back left wheel.
Visiting my back right wheel.
Visiting body.
Visiting my engine.
Visiting car.
Kicking my front left wheel.
Line 279 ⟶ 467:
===Translation of C# example===
As is often the case in practice, the following example departs somewhat from the typical operation of the pattern described above. There is no abstract Visitor class - only a concrete Visitor class - and the 'visit' methods are called something else.
<syntaxhighlight lang="
construct new(){}
Line 352 ⟶ 540:
{{libheader|Wren-str}}
Note that Wren is dynamically typed and can only overload methods based on arity and not on argument type. In the following example, rather than having separate methods for each element type, we instead have a single 'visit' method which tests the type of the argument at run time and takes the appropriate action.
<syntaxhighlight lang="
// abstract class
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