69 lines
2.0 KiB
Scala
69 lines
2.0 KiB
Scala
package examples.expressions
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class Ref[a](var elem:a) {}
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abstract class Lang {
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trait Visitor {
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def caseNum(n: int): unit
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}
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abstract class Exp {
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def visit(v: visitor): unit
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}
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type visitor <: Visitor
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class Num(n: int) extends Exp {
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def visit(v: visitor): unit = v.caseNum(n)
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}
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class Eval(result: Ref[int]) requires visitor extends Visitor {
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def caseNum(n: int) = result.elem = n
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}
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}
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abstract class Lang2 extends Lang {
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trait Visitor2 extends Visitor {
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def casePlus(left: Exp, right: Exp): unit
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}
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type visitor <: Visitor2
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class Plus(l: Exp, r: Exp) extends Exp {
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def visit(v: visitor): unit = v.casePlus(l, r)
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}
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// class Eval2(result: Ref[int]): visitor extends Eval(result) with Visitor2 {
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class Eval2(result: Ref[int]) requires visitor extends Eval(result) with Visitor2 {
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def casePlus(l: Exp, r: Exp) =
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result.elem = { l.visit(this); result.elem } + { r.visit(this); result.elem }
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}
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class Show2(result: Ref[String]) requires visitor extends Visitor2 {
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def caseNum(n: int) = result.elem = n.toString()
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def casePlus(l: Exp, r: Exp) =
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result.elem =
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"(" + { l.visit(this); result.elem } +
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"+" + { r.visit(this); result.elem } + ")"
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}
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}
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object Main {
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def main(args: Array[String]): unit = {
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//val l1 = new Lang { type visitor = Visitor } // not yet implemented
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object l1 extends Lang { type visitor = Visitor } // workaround
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val e1: l1.Exp = new l1.Num(42)
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val iref = new Ref(0)
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Console.println("eval: " + { e1.visit(new l1.Eval(iref)); iref.elem })
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//val l2 = new Lang2 { type visitor = Visitor2 } // not yet implemented
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object l2 extends Lang2 { type visitor = Visitor2 } // workaround
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val e2: l2.Exp = new l2.Plus(new l2.Num(5), new l2.Num(37))
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val sref = new Ref("")
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Console.println("eval: " + { e2.visit(new l2.Eval2(iref)); iref.elem })
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Console.println("show: " + { e2.visit(new l2.Show2(sref)); sref.elem })
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e1.visit(new l1.Eval(iref))
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e2.visit(new l2.Show2(sref))
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}
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}
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