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flatMap implementation in Scala


flatMap implementation in Scala

By : user2956042
Date : November 22 2020, 10:48 AM
wish helps you When you call flatMap and pass the function A => Option[B], flatMap calls map and passes the same function, but the B in flatMap is not same B as in map. For example, if you pass some
code :
Int => Option[String]


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How to turn this for comprehension into a flatMap implementation

How to turn this for comprehension into a flatMap implementation


By : user2692149
Date : March 29 2020, 07:55 AM
This might help you You can use the -Xprint:parser command line option of scalac to see the desugared version:
Source Code
code :
object Main {
  trait Operation

  trait Parser[A] {
    def map[B](fn: A => B): Parser[B] = ???
    def flatMap[B](fn: A => Parser[B]): Parser[B] = ???
  }

  object Parser {
    def natural: Parser[Int] = ???
    def space: Parser[String] = ???
    def list[A](p: Parser[A]): Parser[A] = ???
    def operation: Parser[Operation] = ???
  }

  def operationParser: Parser[(Operation, Int, Int)] = {
    for {
      n1 <- Parser.natural
      _ <- Parser.list(Parser.space)
      op <- Parser.operation
      _ <- Parser.list(Parser.space)
      n2 <- Parser.natural
    } yield (op, n1, n2)
  }
}
$ scalac -Xprint:parser Main.scala 
[[syntax trees at end of                    parser]] // Main.scala
package <empty> {
  object Main extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    abstract trait Operation extends scala.AnyRef;
    abstract trait Parser[A] extends scala.AnyRef {
      def $init$() = {
        ()
      };
      def map[B](fn: _root_.scala.Function1[A, B]): Parser[B] = $qmark$qmark$qmark;
      def flatMap[B](fn: _root_.scala.Function1[A, Parser[B]]): Parser[B] = $qmark$qmark$qmark
    };
    object Parser extends scala.AnyRef {
      def <init>() = {
        super.<init>();
        ()
      };
      def natural: Parser[Int] = $qmark$qmark$qmark;
      def space: Parser[String] = $qmark$qmark$qmark;
      def list[A](p: Parser[A]): Parser[A] = $qmark$qmark$qmark;
      def operation: Parser[Operation] = $qmark$qmark$qmark
    };
    def operationParser: Parser[scala.Tuple3[Operation, Int, Int]] = Parser.natural.flatMap(((n1) => Parser.list(Parser.space).flatMap(((_) => Parser.operation.flatMap(((op) => Parser.list(Parser.space).flatMap(((_) => Parser.natural.map(((n2) => scala.Tuple3(op, n1, n2)))))))))))
  }
}
def operationParser: Parser[scala.Tuple3[Operation, Int, Int]] =
  Parser.natural.flatMap(((n1) =>
    Parser.list(Parser.space).flatMap(((_) =>
      Parser.operation.flatMap(((op) =>
        Parser.list(Parser.space).flatMap(((_) =>
          Parser.natural.map(((n2) =>
            scala.Tuple3(op, n1, n2)))))))))))
$ scalac -Xshow-phases
    phase name  id  description
    ----------  --  -----------
        parser   1  parse source into ASTs, perform simple desugaring
         namer   2  resolve names, attach symbols to named trees
packageobjects   3  load package objects
         typer   4  the meat and potatoes: type the trees
        patmat   5  translate match expressions
superaccessors   6  add super accessors in traits and nested classes
    extmethods   7  add extension methods for inline classes
       pickler   8  serialize symbol tables
     refchecks   9  reference/override checking, translate nested objects
       uncurry  10  uncurry, translate function values to anonymous classes
     tailcalls  11  replace tail calls by jumps
    specialize  12  @specialized-driven class and method specialization
 explicitouter  13  this refs to outer pointers
       erasure  14  erase types, add interfaces for traits
   posterasure  15  clean up erased inline classes
      lazyvals  16  allocate bitmaps, translate lazy vals into lazified defs
    lambdalift  17  move nested functions to top level
  constructors  18  move field definitions into constructors
       flatten  19  eliminate inner classes
         mixin  20  mixin composition
       cleanup  21  platform-specific cleanups, generate reflective calls
    delambdafy  22  remove lambdas
         icode  23  generate portable intermediate code
           jvm  24  generate JVM bytecode
      terminal  25  the last phase during a compilation run
Why does scala.util.Success.flatMap (Try.flatMap) use try-catch again after applying its argument?

Why does scala.util.Success.flatMap (Try.flatMap) use try-catch again after applying its argument?


By : user3187208
Date : March 29 2020, 07:55 AM
I wish did fix the issue. If I understand your question correctly, you are wondering why you need the (potentially) double try/catch, well f could throw an exception before returning the Try:
code :
scala> import scala.util.Try
import scala.util.Try

scala>   val someTry = Try(1)
someTry: scala.util.Try[Int] = Success(1)

scala>   someTry.flatMap(theInt => {
     |     throw new Exception("")
     |     Try(2)
     |   })
res1: scala.util.Try[Int] = Failure(java.lang.Exception: )
Scala : Map and Flatmap on RDD

Scala : Map and Flatmap on RDD


By : Krish
Date : March 29 2020, 07:55 AM
With these it helps I have an RDD with this structure
code :
import java.time.LocalDate
import java.time.format.DateTimeFormatter
import java.time.Period    


val df1 = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.S")

val futureDate = LocalDate.parse("2100-01-01 00:00:00.0", df1)

val yourRequiredRdd = yourRdd
  .map({
    case (t, list) => {
      val list1 = list.map({
        case (id, dateStr, id2) => (id, LocalDate.parse(dateStr, df1), id2) 
      })

      val oldestDate = list1
        .filter({ case (id, date, id2) => id2.equals("C101") })
        .map(_._2)
        .foldLeft(futureDate)((oldestDate, date) => {
          val period = Period.between(oldestDate, date)
          if (!period.isNegative()) oldestDate else date
        })

      val newList = list1
        .map({
          case (id, date, "C101") => {
            val periodFromOldestDate = Period.between(oldestDate, date)
            val extraNumber = if (periodFromOldestDate.getYears() >= 2) 2 else 1
            (id, date, "C101", extraNumber)
          }
          case (id, date, id2) => {
            (id, date, id2, 1)
          }
        })

      (t, newList)
    }
  })
  .flatMap({
    case ((pid, name), list) => list.map({
      case (id, date, code, level) => (id, name, code, pid, date, level)
    })
  })
Implementation of flatMap() for State transition

Implementation of flatMap() for State transition


By : LucianoTutoriales
Date : March 29 2020, 07:55 AM
wish helps you Remember that we're calling f and g inside a new anonymous function, as indicated by the line
code :
rng => { ... }
Scala: Implementation of flatMap using foldRight

Scala: Implementation of flatMap using foldRight


By : Imtiyaz Kbizsoft
Date : March 29 2020, 07:55 AM
wish of those help First, just ignore the constraints and think about the flatMap function in this case. You have a List[A] and a function f: A => List[B]. Normally, if you just do a map on the list and apply the f function, you'll get back a List[List[B]], right? So to get a List[B], what would you do? You would foldRight on the List[List[B]] to get back a List[B] by just appending all elements in the List[List[B]]. So the code will look somewhat like this:
code :
def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
    val tmp = xs.map(f) // List[List[B]]
    tmp.foldRight(List[B]())((outCurr, outAcc) => outCurr ++ outAcc)
}
scala> def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
 |     val tmp = xs.map(f) // List[List[B]]
 |     tmp.foldRight(List[B]())((outCurr, outAcc) => outCurr ++ outAcc)
 | }
flatMap: [A, B](xs: List[A])(f: A => List[B])List[B]

scala> flatMap(List(1, 2, 3))(i => List(i, 2*i, 3*i))
res0: List[Int] = List(1, 2, 3, 2, 4, 6, 3, 6, 9)

scala> List(1,2,3).flatMap(i => List(i, 2*i, 3*i))
res1: List[Int] = List(1, 2, 3, 2, 4, 6, 3, 6, 9)
def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
    val tmp = xs.foldRight(List[List[B]]())((curr, acc) => f(curr) :: acc) // List[List[B]]
    tmp.foldRight(List[B]())((outCurr, outAcc) => outCurr ++ outAcc)
}
scala> def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
     |         val tmp = xs.foldRight(List[List[B]]())((curr, acc) => f(curr) :: acc) // List[List[B]]
     |         tmp.foldRight(List[B]())((outCurr, outAcc) => outCurr ++ outAcc)
     |     }
flatMap: [A, B](xs: List[A])(f: A => List[B])List[B]

scala> flatMap(List(1, 2, 3))(i => List(i, 2*i, 3*i))
res3: List[Int] = List(1, 2, 3, 2, 4, 6, 3, 6, 9)
def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
    xs.foldRight(List[B]()) { (curr, acc) => // Note: acc is List[B]
        val tmp2 = f(curr) // List[B]
        tmp2 ++ acc
    }
}
scala> def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
     |     xs.foldRight(List[B]()) { (curr, acc) => // Note: acc is List[B]
     |         val tmp2 = f(curr) // List[B]
     |         tmp2 ++ acc
     |     }
     | }
flatMap: [A, B](xs: List[A])(f: A => List[B])List[B]

scala> flatMap(List(1, 2, 3))(i => List(i, 2*i, 3*i))
res4: List[Int] = List(1, 2, 3, 2, 4, 6, 3, 6, 9)
def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = {
    xs.foldRight(List[B]()) { (curr, acc) => // Note: acc is List[B]
        val tmp2 = f(curr) // List[B]
        tmp2.foldRight(acc)((inCurr, inAcc) => inCurr :: inAcc)
    }
}
def flatMap[A, B](xs: List[A])(f: A => List[B]): List[B] = 
   xs.foldRight(List[B]())((curr, acc) =>
   f(curr).foldRight(acc)((inCurr, inAcc) => inCurr :: inAcc))
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