Problem with Scala Function and Call-by-Name

Question

I am trying to loop a the second parameter (exp) in this function that uses call by name parameters. The first 3 parameters are the index, boolean to stop loop, and increment function. I am getting an output with 10 '()'s when I am trying to loop "hello world" 10 times as seen in the test code. May I get some help with what is wrong here? Thanks

def forLoop(ival: => Int, f: (Int) => Boolean, g: (Int)=>Int)(exp: => Unit): Unit = {
  if(f(ival)==false) 
    return
  else {
    println(exp)
    forLoop(g(ival),f,g)(exp)
  }
}

def f(x: Int): Boolean = { x<10 }
def g(y: Int): Int = { y+1 }
val exp: Unit = "Hello World"
forLoop(0,f,g)("Hello World")

Also I think I formatted the parameter functions f and g when trying to use call by name but I am not sure. — gradstud
– gradstud, Commented May 29, 2020 at 0:15
I have changed it to return Unit and changed return true to just return in the base case. My bad. Sorry I am new — gradstud
– gradstud, Commented May 29, 2020 at 0:46

Mario Galic · Accepted Answer · 2020-05-29 00:31:25Z

2

The value "Hello World" is of type String however you are assigning it to Unit

val exp: Unit = "Hello World"

which compiler expands to

val exp: Unit = {
  "Hello World";
  ()
}

Note how () becomes the value of exp. Try changing the definition of exp to

val exp: String = "Hello World"

and second parameter list to

(exp: => String)

If you compile with compiler flag -Wvalue-discard, for example,

scala -Wvalue-discard -e 'val exp: Unit = "Hello World"'

you will get a warning

warning: discarded non-Unit value
val exp: Unit = "Hello World"
                ^

edited May 29, 2020 at 0:31

answered May 29, 2020 at 0:25

Mario Galic

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2 Comments

gradstud Over a year ago

I get the same result even when I declare val exp: String = "Hello World"

gradstud Over a year ago

Oh, my I am required to have the second parameter as Unit

jwvh · Accepted Answer · 2020-05-29 00:55:35Z

2

I think this meets your requirements.

def forLoop(ival: => Int, f: =>Int => Boolean, g: =>Int=>Int
           )(exp: => Unit): Unit =
  if (f(ival)) {
    exp
    forLoop(g(ival),f,g)(exp)
  }

def f(x: Int): Boolean = x<10
def g(y: Int): Int = y+1
def exp: Unit = println("Hello World")
forLoop(0,f,g)(exp)

answered May 29, 2020 at 0:55

jwvh

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