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object Main {
trait Ensure[a] {
def ensure(postcondition: a => Boolean): a
}
def require[a](precondition: => Boolean)(command: => a): Ensure[a] =
if (precondition)
new Ensure[a] {
def ensure(postcondition: a => Boolean): a = {
val result = command;
if (postcondition(result)) result
else sys.error("Assertion error")
}
}
else
sys.error("Assertion error");
def arb[a](s: List[a]) =
require (! s.isEmpty) {
s.head
} ensure (result => s contains result);
def main(args: Array[String]) = {
val s = List(1, 2);
Console.println(arb(s))
}
}
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