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Questions tagged [reader-monad]

75 questions
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How to inject dependencies through Scala Reader from Java code

Here is a dependency service: public class Service1 {} Scala code that uses it via reader: object TupleEx { type FailFast[A] = Either[List[String], A] type Env[A] = ReaderT[FailFast, Service1, A] import cats.syntax.applicative._ import…
Alexandr
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Preferring Reader monad over directly passing the environment as parameters

I was writing a basic CRUD app in Haskell, using libraries Servant and Opaleye. Servant to setup the API endpoints and Opaleye to store the data in DB. Let's say there's an endpoint GET /users which returns the list of all users from DB and another…
kishlaya
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How to see code of Monad Reader?

I am a beginner in Haskell, and I want to understand Reader Monad. I know how to use this monad. But I want to see the implementation of monad (particularly code of function "return"). How can I see this code?
Alex Aparin
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Recursively walking an AST using foldl and the Reader monad without boilerplate

I'm traversing an AST using simple pattern matching and the Reader monad. Elsewhere in my project I've defined a walk function for traversing the AST, which at its heart uses foldl to reduce the results of visiting each node in the tree into a…
Greg Hurrell
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The reader monad for dependency injection

I am a dev trying to bend my mind around the typelevel stack (Cats and Cats-Effect)after years of experience with less functional scala. I'm having a real trouble bending my head around the Reader monad. I can somewhat follow the examples here:…
Emil D
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Interpreting the `Reader` trait in "Simplicitly"

In "Simplicitly: foundations and applications of implicit function types", Odersky et al. briefly introduce the Reader monad, just to replace it with a superior alternative one paragraph later. This is what the given definition looks like: trait…
Andrey Tyukin
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Is it possible to implement this generic flip?

I would like to write an object with the type signature: genericFlip :: ( MonadReader (o (n c)) m , MonadReader a n , MonadReader b o ) => m (n (o c)) That is essentially a flip for monad readers. Now it is easy enough to write the…
Wheat Wizard
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Scala: for-comprehension with guard inside reader

Here is code example: type FailFast[A] = Either[List[String], A] import cats.instances.either._ def f1:ReaderT[FailFast, Map[String,String], Boolean] = ReaderT(_ => Right(true)) def f2:ReaderT[FailFast, Map[String,String], Boolean] =…
Alexandr
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Implementing the Reader monad (from Real World Haskell book)

newtype Reader e a = R { runReader :: e -> a } instance Monad (Reader e) where return a = R $ \_ -> a m >>= k = R $ \r -> runReader (k (runReader m r)) r I am having difficulty understanding these two snippets. I can tell that the first one…
金允雄Oliver
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Reader Monad - explanation of trivial case

I have been trying to get to grips with the reader monad and came across this tutorial. In it, the author presents this example: example2 :: String -> String example2 context = runReader (greet "James" >>= end) context where greet ::…
matt
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Reader Monad clarification

I am trying to make sense of the reader monad but can't seem to grasp what bind (>>=) does in this monad. Here's the implementation I am analyzing: newtype Reader e a = Reader { runReader :: (e -> a) } instance Monad (Reader e) where return a …
oskar132
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How to use the Reader Monad with (Int -> Int)?

I would like to learn, how to use the Reader Monad. Unfortunately only a small amount of example code is available I would like to create a Reader, where the environment and the retrived values are Int. I defined the type like this: type IntRead =…
Iter Ator
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Reader monad - how does it conform to Monad interface?

I'm learning category theory. I understand the concept of reader monad, it's even pretty easy to implement: case class Reader[DEP, A](g: DEP => A) { def apply(dep: DEP): A = g(dep) def map[B](f: A => B): Reader[DEP, B] = Reader(dep =>…
slnowak
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How is the below repository being threaded (passed) into each function call?

I was sure the below underscores are unit meaning function return value is ignored. (The below is taken from book functional and reactive domain modeling). If that's correct then I have a question about the below code: case class…
Jas
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Reader Monad in Purescript

I was playing with the Reader monad in Purescript and I encountered a weird behaviour. I don't know if it's because my lack of comprehension of this monad or if I'm missing something else. This is my code : type Level = Number type Doc = Reader…
ThomasC__
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