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Questions tagged [state-monad]

A monad allowing state information to be attached to calculations

A state monad allows a programmer to attach state information of any type to a calculation. Given any value type, the corresponding type in the state monad is a function which accepts a state, then outputs a new state along with a return value. Wikipedia has a brief overview.

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Understanding the state argument in the State Monad

I'm trying so hard to wrap my head around the State Monad, and I do not understand the following: Given the implementation of return and (>>=), when you say State $ \s ->...., where does s come from? I mean, when you start performing >>= ... >>=,…
Bercovici Adrian
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Monad transformer for inserts and total lookups on a Map?

I have a computation where I'm inserting values into a Map and then looking them up again. I know that I never use a key before inserting it, but using (!) freely makes me nervous anyway. I'm looking for a way to get a total lookup function that…
user11228628
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Combine ST and List monads in Haskell

Using the StateT monad transformer, I can create the type StateT s [] a, which is isomorphic to s -> [(a, s)]. Now I would prefer to use the STT monad transformer instead, as I would like to have multiple mutable variables of different types, and…
dremodaris
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Generating a unique value in Haskell do-notation

To generate x86 assembly code, I have defined a custom type called X86: data X86 a = X86 { code :: String, counter :: Integer, value :: (X86 a -> a) } This type is used in do-notation like the following. This makes it easy to write templates for…
Ryan
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How does `get` work in the CPS version of the State monad?

I am trying to understand continuation in general following this tutorial. However, I am having difficulties to understand following example in section 2.10: # let get () = shift (fun k -> fun state -> k state state) ;; get : unit => ’a =…
Jason Hu
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State monads: Transitioning from one state type to another

Let's say we have a stack of monads with a state monad transformer as the outer most transformer like this one: -- | SEWT: Composition of State . Except . Writer monad transformers in that -- order where Writer is the innermost transformer. -- the…
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When and Why do you use State Monads?

I can write a State Monad (in Scala/Java) and can pretty much follow the logic when I see others using it. I don't fully understand the problem it is solving. It is a monad wrapping the funciton S => (S,A). As such, when you nest functions that…
jordan3
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Stateful loop with different types of breaks

I am trying to convert the following stateful imperative code into Haskell. while (true) { while (get()) { if (put1()) { failImmediately(); } } if (put2()) { succeedImmediately(); } } Both the put1 and put2 read a state of…
jakubdaniel
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Is a state monad with two state variable types (in and out) still a monad?

Haskell's state monad State s a forces me to keep the same type of s during the whole do block. But since the state monad is really just a function, what if I define it as State i o a = State (i -> (o, a))?. The return and bind functions would look…
Juan
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Random number sequence un Haskell and State Monad, what am I doing wrong?

As part of my Haskell journey, I am implementing a raytracer and I need to be able to draw sequences of random numbers at several places in the code. Typically I would like to be able to get say 64 samples for each pixels and pixels are computed in…
overlii
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Difficulty with zoom and free monads

I am mucking around with free monads and lens, using the free monad to create my own version of the IO monad: data MyIO next = LogMsg String next | GetInput (String -> next) deriving (Functor) I am stacking this on top of a state monad…
Pubby
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How does the state monad work? (without code explanation)

For the last number of months I've been taking some spare time here and there to read up on Monads. I haven't worked with a functional language since my University days. So I don't really remember Haskell, and certainly have no idea about Scalaz.…
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Implementing recurrence relations on State monads (in Haskell or Scala)

I am working on a new implementation of the operators in http://www.thalesians.com/archive/public/academic/finance/papers/Zumbach_2000.pdf EDIT: clearer explanation here: https://www.olseninvest.com/customer/pdf/paper/001207-emaOfEma.pdf Briefly,…
experquisite
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Haskell - is state monad a sign of imperative thinking?

I am writing a simple game - Tetris. For the first time in my life I'm using functional programming for that goal, as a language I chose Haskell. However, I'm tainted with OOP and imperative thinking and scared of unconsciously applying this mindset…
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Searching for missing State Combinator for Lens

I currently have code that looks like this: do x <- use foo foo <~ runFoo x where foo is a Lens to a Foo field and runFoo :: MonadState m => Foo -> m Foo I think there should be a way to do this operation in one line, but I cannot find it. I…
John F. Miller
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