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Created May 31, 2013 11:35
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The following is an exercise in refactoring to remove code duplication. The implementation language is Scala. How would you solve it?
object RefactorPuzzle {
case class IntRdr[+A](read: Int => A) {
def map[B](f: A => B): IntRdr[B] =
IntRdr(f compose read)
def flatMap[B](f: A => IntRdr[B]): IntRdr[B] =
IntRdr(n => f(read(n)).read(n))
}
object IntRdr {
def apply[A](a: A): IntRdr[A] =
IntRdr(_ => a)
}
// Return all the Some values, or None if not all are Some.
def runOptions[A](x: List[Option[A]]): Option[List[A]] =
x.foldRight[Option[List[A]]](Option(Nil))((a, b) => a.flatMap(aa => b.map(aa :: _)))
// Apply an Int to a list of int readers and return the list of return values.
def runIntRdrs[A](x: List[IntRdr[A]]): IntRdr[List[A]] =
x.foldRight[IntRdr[List[A]]](IntRdr(Nil))((a, b) => a.flatMap(aa => b.map(aa :: _)))
/*
========================
THE REFACTORING PUZZLE
========================
The two functions `runOptions` and `runIntRdrs` share a lot of duplicated code.
This is demonstrated below.
How to refactor this code to remove the code duplication?
*/
//------- ------------- ------- -----------
//def runOptions[A](x: List[Option[A]]): Option[List[A]] =
//def runIntRdrs[A](x: List[IntRdr[A]]): IntRdr[List[A]] =
// ------------ ----------- -------------------------------------------------
// x.foldRight[Option[List[A]]](Option(Nil))((a, b) => a.flatMap(aa => b.map(aa :: _)))
// x.foldRight[IntRdr[List[A]]](IntRdr(Nil))((a, b) => a.flatMap(aa => b.map(aa :: _)))
}
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