scala/scala

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Fabio Labella

@SystemFw

don't worry about the implementation yet, just see if you can work out the signature, given the description above

and let me know if you get stuck

Renan Reis

@renanreismartins

signature is fine, I was thinking of the implementation

def map[B](f: A => B): State[S, B]

Fabio Labella

@SystemFw

I prefer to write the signature as def map[A, B]: State[S, A] => (A => B) => State[S, B]

I.e. show the input state as well

the real one will look like yours because it's inside State to allow to be called infix

but yeah basically they are equivalent

so now we need to implement it

Renan Reis

@renanreismartins

yes, but the infix ones gives me the impression we are working with the obj and then confuses me a bit. Do you mind if i reason about the one you wrote?

I understood it already actually.

Fabio Labella

@SystemFw

yeah that's why I use the non infix one to explain :)

but when I write example I cheat and write get[Int].map.. etc

scala 3 will have infix functions and solve this annoying pedagogical trap

anyway, sounds like you understood the signature, and understood how to use it as well

Renan Reis

@renanreismartins

I will put that on the companion object instead of the case class,

yes

Fabio Labella

@SystemFw

the problem is that using get in that shape is annoying, and the one in the class is better. Understanding the one in the object is better though yeah

anyway, we just need to implement it now

we will try to follow type driven programming

it's a bit verbose in chat, but I'll try

also if a moderator prefers this convo to be moved to DM, please let me know (Seth basically)

ok so

def map[A, B]: State[S, A] => (A => B) => State[S, B]

we always start from the end, i.e. State[S, B]

how do we create a State[S, B]? at this point the only thing we have that type checks, it's the constructor

Renan Reis

@renanreismartins

im implementing it, one moment.

I think more people can benefit from such explanation.

Fabio Labella

@SystemFw

ah ok I'll let you try

Renan Reis

@renanreismartins

I'm stuck inside a monad lol. how can I get A?

Fabio Labella

@SystemFw

ok let's follow the process I started instead

which is type-driven programming

how do we create a State[S, B]? at this point the only thing we have that type checks, it's the constructor

so that's where we got to

the constructor takes an S => (S, B)

so we have the first step

def map[A, B](fa: State[S, A])(f:  A => B) : State[S, B] = State[S, B] { s: S =>
  // more work to do here
  def result: (S, B) = ???
  result
}

make sense so far?

this method is very useful in general btw

(we are getting at the same point you got to I think, but I think that showing the method will also show how to get unstuck)

Renan Reis

@renanreismartins

we need to take A and apply F

Fabio Labella

@SystemFw

ok, so we have another step

if I was in an editor, I'll type it properly with a helpful layout, but it's not tenable here unfortunately

Renan Reis

@renanreismartins

val a: (A, S) = fa.run(s)

Fabio Labella

@SystemFw

ah ok, you did get it, nice

can you see if you can complete the whole thing now?

Renan Reis

@renanreismartins

def map[S, A, B](fa: State[S, A])(f: A => B): State[S, B] = State[S, B] { s: S =>
    val (a, s2) = fa.run(s)
    val b: B = f(a)
    (b, s2)
  }

Fabio Labella

@SystemFw

cool

so one great thing to point out here

if you look at this example

val a = get[Int].map(_ + 1).map(_.toString)

a.run(3)._1 // "4"
a.run(7)._1 // "8"

even though we haven't looked at State in its entirety yet (pure, flatMap, put, modify etc)

we have already answered your initial question

which was "who builds the run function"

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