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Renan Reis
@renanreismartins

Do you guys have a simple example?

The way the book creates the computation:

def simulateMachine(inputs: List[Input]): State[Machine, (Int, Int)] = for {
    _ <- sequence(inputs map (modify[Machine] _ compose update))
    s <- get
  } yield (s.coins, s.candies)
Fabio Labella
@SystemFw
which will result in an A
that's what State[S, A] means, "a sequence of transformation over an S, which will result in an A"
does this bit make sense?
I will get to simple examples soon, once the main concepts are clarified :)
Renan Reis
@renanreismartins

that's what State[S, A] means, "a sequence of transformation over an S, which will result in an A"

yes

Fabio Labella
@SystemFw
ok, so once you have built this sequence of transformations, you pass the initial state, and you get back the result A (and the final new state, which is also of type S)
does this also make sense?
this is what you do when you write mySequenceOfTransformationsWhichIsStateSA.run(initialState)
Renan Reis
@renanreismartins
Also does, looks like very much with Random number Generator, previously on the chapter. Till here i can follow you

this is what you do when you write mySequenceOfTransformationsWhichIsStateSA.run(initialState)

perfect :)

Luis Miguel Mejía Suárez
@BalmungSan
@renanreismartins btw, even if I know Fabio will be able to make it clear (because he always does). May I ask, what was your experience with Scala before that book?
It is quite complex and some people recommend learning the language + fp with it which IMHO it is really an overkill.
Fabio Labella
@SystemFw
right, so we can start by building the simplest State program, the program whose final result is the state itself
the name of this program is get, and its type is State[S, S], i.e. the sequence of transformations on S which will return S itself (which is what "getting the state" is)
fair enough?
Renan Reis
@renanreismartins

@BalmungSan I read:
Programming Scala - martin

did the coursera and...

https://alvinalexander.com/scala/functional-programming-simplified-book/

https://www.amazon.com/Functional-Programming-Java-functional-techniques/dp/1617292737 - this is very similar to the scala book, but in java and not sooooo deep, more into data structures than category theory / monads.

I had no problem in the Functional Scala till this chapter :/

Fabio Labella
@SystemFw
it's ok, this chapter is the first "real" one
don't get discouraged
Luis point is valid in general but I don't think it applies to you
it applies to people that still struggle heavily with the syntax and semantics of basics Scala, whereas in this case I feel like you are confused by the concepts and so started getting confused by scala as well (in the run question)
that being said, it's totally ok to do just part 1 of the Red Book, then go write code/read other books, then go back to Part 2 (and then Part 3)
Renan Reis
@renanreismartins
@BalmungSan thanks for checking man... let me know if you have any other content that you find relevant.
Fabio Labella
@SystemFw

he name of this program is get, and its type is State[S, S], i.e. the sequence of transformations on S which will return S itself (which is what "getting the state" is)

I can try and explain it all though, if you want. Does the above statement make sense?

Renan Reis
@renanreismartins
@SystemFw I'm getting there dont give up on me. Yes that makes sense too.
Fabio Labella
@SystemFw
ok cool so let's resume
Renan Reis
@renanreismartins
I dont know how to compose the transformations tho
Fabio Labella
@SystemFw
we need to build def get[S]: State[S, S], and the only way we can build State computations at this point is through the constructor, which case class State[S, A](run: S => (A, S)). In this case A == S, since we want our program get to be State[S, S]
we'll get there, but if you don't understand the primitives, it's harder to see the mechanics
Renan Reis
@renanreismartins

we need to build def get[S]: State[S, S], and the only way we can build State computations at this point is through the constructor, which case class State[S, A](run: S => (A, S)). In this case A == S, since we want our program get to be State[S, S]

im with you here too

Fabio Labella
@SystemFw
so to build get, we need to pass a function of type S => (S, S) to the constructor
which, when run, will take the input state, return it as the output, and leave it unmodified for the next state
Renan Reis
@renanreismartins
corrrect.
Fabio Labella
@SystemFw
so the final definition is this
def get[S]: State[S, S] = State(s => (s, s))
see if you can reconcile it with the description above
Renan Reis
@renanreismartins

get is a function that returns State[S, S]

and to construct State, we need to pass to its constructor a function that receives S and returns (A, S). Am I correct?

Fabio Labella
@SystemFw
so far yes, but I need you to understand the specific function that I passed to State, because that function defines get specifically

will take the input state, return it as the output, and leave it unmodified for the next state

that's the English description of what it does, you need to see that same description in the code

Luis Miguel Mejía Suárez
@BalmungSan
@renanreismartins yeah no worries, as Fabio said I just wanted to be sure you were not struggling with too many things.
If the book keeps getting hard I may recommend reading Scala with Cats first, which is basically a simplified (and maybe more practical?) version of the same material; I recommend it because that book is extremely clear.
Also, if you find it hard to under things like effects or monads those videos were the ones that made the click for me,
Renan Reis
@renanreismartins
@SystemFw I think I understood so far. You are binding that function to run. The function takes its parameter and return the tuple.
Fabio Labella
@SystemFw
yes, and the tuple leaves the state unmodified, and returns it as the output as well
Renan Reis
@renanreismartins
yes
Fabio Labella
@SystemFw
ok cool, so we have build the first state computation, which we can run
get[Int].run(3)._1 == 3
should be unsurprising
but make sure you understand because we are going to be build the first important transformation next
Rob Norris
@tpolecat
To me State is the key to the kingdom. More than anything else it’s what made FP click for me. So this is fun.
Fabio Labella
@SystemFw
(._1 is a method on tuples that returns the first element, I basically want to get the A out of State[S, A], by passing the initial state and applying all the transformations, in this case the only transform is leaving the state unmodified and returning it, i.e. get)
Renan Reis
@renanreismartins
@SystemFw just a second before we go further...
Fabio Labella
@SystemFw

To me State is the key to the kingdom.

yeah, although State per se isn't really used in real world FP, all the main concepts you need to work with real effects are there really (or most of them)

just a second before we go further...

sure