Visitor pattern: Difference between revisions

=={{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>