Questions tagged [reader-monad]
75 questions
5
votes
1 answer
Reader and MonadReader
Why is there a Reader monad and a MonadReader monad in Control.Monad.Reader? The package documentation talks about the Reader monad and then launches into the MonadReader documentation directly with no explanation. What is the difference between…
andro
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5
votes
1 answer
I know how to use it but I don't understand exactly how it does it (Reader monad)
Consider the following code (with obvious parts left out)
main = do
let s = "123456";
let len = runReader calculateContentLength s
putStrLn $ "Original 's' length: " ++ (show len)
calculateContentLength :: Reader String…
David
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4
votes
1 answer
Confusing ReaderT definition
As an exercise I've been reimplementing some common monads and their corresponding transformers; here are some of the types I've defined:
newtype Writer w a = Writer { runWriter :: (w, a) }
newtype WriterT w m a = WriterT { runWriterT :: m…
Giacomo Cavalieri
- 375
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4
votes
1 answer
Composition of Readers for Dependency Injection in Scala
Here is a simple service example, whose methods return reader:
trait Service1_1{
def s1f1:Reader[Map[String,Int],Int] =
Reader(_("name"))
def s1f2:Reader[Map[String,Int],Int] =
Reader(_("age"))
}
Here is a service-consumer, that…
Alexandr
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4
votes
2 answers
Why to define the constructor parameter of Reader as a function?
When learning the Reader Monad, I find that it is defined as:
newtype Reader r a = Reader { runReader :: r -> a }
instance Monad (Reader r) where
return a = Reader $ \_ -> a
m >>= k = Reader $ \r -> runReader (k (runReader m r)) r
I want to…
hliu
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4
votes
2 answers
Scalaz: how does `scalaz.syntax.applicative._` works its magic
This question is related to this one, where I was trying to understand how to use the reader monad in Scala.
In the answer the autor uses the following code for getting an instance of ReaderInt[String]:
import scalaz.syntax.applicative._
val…
Damian Nadales
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4
votes
1 answer
Reader monad in Scala: return, local, and sequence
I'm using the Reader monad in Scala as provided by the scalaz library. I'm familiar with this monad as defined in Haskell. The problem is that I cannot find the functions equivalent to return, local, and sequence (among others).
Currently I use…
Damian Nadales
- 4,907
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3
votes
1 answer
The Reader Monad in Kotlin with arrow-kt
I working on a medium-sized Kotlin project where I need to thread configuration information read from file through many nested calls of pure functions. This seems to be an obvious case for the Reader monad. However, I have not figured out how to…
Ulrich Schuster
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3
votes
1 answer
Reader monad - reader vs asks function difference?
There is a asks function for reader monad, which defined exactly as reader function, why it exists as a separate function with a definition the same as a reader? why not always use reader?
class Monad m => MonadReader r m | m -> r where
-- |…
Evg
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3
votes
1 answer
What is "a function that you call" and what is "a function that call you"?
I'm trying to understand what is Haskell Reader monad, but I struggle with this part in the book:
The “read-only” nature of the type argument r means that you can swap in a different type or value of r for functions that you call, but not for…
kirill fedorov
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3
votes
2 answers
How to wrap monadic action in IO
I am trying to treat a ReaderT X IO monad as IO to achieve the following:
-- this is the monad I defined:
type Game = ReaderT State IO …
Markus Rother
- 414
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3
votes
0 answers
Scala: Dependency Injection via Reader vs parameter list
There are some options to inject dependencies in FP. I want to compare here only the two:
Injection via parameter list
Injection via Reader
The 2nd case is more composable and less verbose, in case I invoke several methods with dependencies. It…
Alexandr
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3
votes
2 answers
Create ReaderT[F, D, A] from F[A]
type MapReaderOrOption[A] = ReaderT[Option, Map[String,String], A]
I can create it from ReaderT.apply:
def f:MapReaderOrOption[Int] = ReaderT(_ => Option(10))
From A type via type enrichment and pure method:
import cats.Applicative
import…
Alexandr
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3
votes
1 answer
Scala-cats: reader-composition
import cats.data.ReaderT
import cats.instances.either._
trait Service1
trait Service2
case class Cats(name:String)
type FailFast[A] = Either[List[String], A]
type Env = (Service1, Service2, Cats)
type ReaderEnvFF[A] = ReaderT[FailFast, Env,…
Alexandr
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3
votes
1 answer
Scala: Dependency Injection via Reader and compatibility
When we implement DI via Reader, we make a dependency a part of our method signature. Assume we have (without implementations):
trait Service1 { def f1:Int = ??? }
trait Service2 { def f2:Reader[Service1, Int] = ??? }
type Env= (Service1,…
Alexandr
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