Chapter 2, Exercise 3: Implement partial1?

[Harit Himanshu]

Hello

I am learning Functional Programming and Scala both for the very first time. I encountered the exercise 3 which says

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C

EXERCISE 3 (hard): Implement partial1 and write down a concrete usage of it. There is only one possible implementation that compiles.

I am not sure what approach I need to take, can someone please help?

Thanks

+ Harit 

[para...@gmail.com]

Hi Harit,

It might bend your mind a bit if you are just starting out with FP, but you'll get a hang of it.

Let's start with the function definition:

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C 

We need to return a function that takes a B and returns a C. Let's type it out

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C  = {

  // return a function that takes a B and returns a C

  (b: B) => /* Somehow get a value of type C here */

}

How can we get a value of type C? To help us out, we can use the function f that takes (A,B) and returns a C.

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C  = {

  // return a function that takes a B and returns a C

  (b: B) => f(a, b)

}

This can be shortened to 

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C = f(a, _)

But if you find the last definition hard to comprehend, stick to the previous one till you get more comfortable with Scala.

Thanks

Param

Parambir Singh about.me/parambir

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[Harit Himanshu]

Thanks Param

I tried to use it as  

class ex03 {
  def partial1[A,B,C](a:A, f:(A,B) => C): B => C = f(a,_)
}

and make it run as  

class ex03Test extends UnitSpec {

  behavior of "partial1"
  it should "return 6" in {
    val e = new ex03
    val multiplyByTwo = e.partial1(2, (a:Int, b:Int) => a * b)
    multiplyByTwo(3) mustBe 6
  }
}

Thanks

On Sun, Jul 5, 2015 at 5:45 PM, Parambir Singh <para...@gmail.com> wrote:

Hi Harit,

It might bend your mind a bit if you are just starting out with FP, but you'll get a hang of it.

Let's start with the function definition:

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C 

We need to return a function that takes a B and returns a C. Let's type it out

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C  = {

  // return a function that takes a B and returns a C

  (b: B) => /* Somehow get a value of type C here */

}

How can we get a value of type C? To help us out, we can use the function f that takes (A,B) and returns a C.

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C  = {

  // return a function that takes a B and returns a C

  (b: B) => f(a, b)

}

This can be shortened to 

def partial1[A,B,C](a:A, f:(A,B) => C):B=>C = f(a, _)

But if you find the last definition hard to comprehend, stick to the previous one till you get more comfortable with Scala.

Thanks

Param

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