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package examples

/** This examples illustrates the use of Guarded
* Algebraic Data Types in Scala. For more information
* please refer to the Scala specification available
* from
object gadts extends Application { /* The syntax tree of a toy language */ abstract class Term[T] /* An integer literal */ case class Lit(x: Int) extends Term[Int] /* Successor of a number */ case class Succ(t: Term[Int]) extends Term[Int] /* Is 't' equal to zero? */ case class IsZero(t: Term[Int]) extends Term[Boolean] /* An 'if' expression. */ case class If[T](c: Term[Boolean], t1: Term[T], t2: Term[T]) extends Term[T] /** A type-safe eval function. The right-hand sides can
* make use of the fact that 'T' is a more precise type,
* constraint by the pattern type.
def eval[T](t: Term[T]): T = t match { case Lit(n) => n // the right hand side makes use of the fact // that T = Int and so it can use '+' case Succ(u) => eval(u) + 1 case IsZero(u) => eval(u) == 0 case If(c, u1, u2) => eval(if (eval(c)) u1 else u2) } println( eval(If(IsZero(Lit(1)), Lit(41), Succ(Lit(41))))) }

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