scala/scala

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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"

and you can see that there are primitives like get, which build simple functions directly

and combinators like map, which build more complex functions based on previous State computations

so that whole state computations (in this case a), end up having a more complex function which represents the sequence of transformations

and when you are done building that sequence of transformations, you can apply it to various initial states, by passing them to the run functions, and get the result out

does that make sense?

Renan Reis

@renanreismartins

yes, it does

State.map(get[Int])(_ + 1).run(1) in our case, because we do not use infix

Fabio Labella

@SystemFw

yeah

you can follow the same reasoning to understand pure, flatMap and put

the rest happens purely in combinator land

e.g. modify is get.flatMap(s => put(f(s))

Renan Reis

@renanreismartins

damn, I understood it.

Fabio Labella

@SystemFw

:+1:

Luis Miguel Mejía Suárez

@BalmungSan

btw, even if I know Fabio will be able to make it clear (because he always does)

told ya

Renan Reis

@renanreismartins

DAMN! @SystemFw thank you so much, first for the insanely well done explanation. Makes me feel it is really possible to understand deeply the concepts. Second for the enormous patience and time to go step by step, without it would be impossible to understand fully. Very much appreciated.

Fabio Labella

@SystemFw

you're welcome

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