Questions tagged [reader-monad]
75 questions
148
votes
4 answers
What is the purpose of the reader monad?
The reader monad is so complex and seems to be useless. In an imperative language like Java or C++, there is no equivalent concept for the reader monad, if I am not mistaken.
Can you give me a simple example and clear this up a little bit?
chipbk10
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33
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1 answer
Configuration data in Scala -- should I use the Reader monad?
How do I create a properly functional configurable object in Scala? I have watched Tony Morris' video on the Reader monad and I'm still unable to connect the dots.
I have a hard-coded list of Client objects:
class Client(name : String, age : Int){…
Larry OBrien
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19
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1 answer
Is it just a coincidence that Kleisli, ReaderT, and Reader are the same in Scalaz
In Scalaz
Kleisli[F, A, B] is a wrapper for A => F[B].
ReaderT[F, A, B] -- reader monad transformer -- is just an alias of Kleisli[F, A, B].
Reader[A, B] monad is a specialization of ReaderT with identity monad Id:
type Reader[A, B] =…
Michael
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13
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3 answers
Understanding the Reader monad
I'm reading PureScript by Example and got to the part introducing the Reader monad. The example goes like this:
createUser :: Reader Permissions (Maybe User)
createUser = do
permissions <- ask
if hasPermission "admin" permissions
then map…
kaqqao
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11
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1 answer
How to avoid stair-stepping with Monad Transformers in scala?
I have the following code that uses the Reader monad for configuration and also has to deal with IO[Option[String]] and I've ended up with code that stair-steps in my encode function.
How can I formulate a monad transformer for Reader and OptionT to…
cwmyers
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8
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2 answers
Does a Powerset-over-Reader monad exist?
The canonical 'Monad instance' for environment sharing plus nondeterminism is as follows (using pseudo-Haskell, since Haskell's Data.Set isn't, of course, monadic):
eta :: a -> r -> {a} -- '{a}' means the type of a set of a's
eta x = \r -> {x}
bind…
SEC
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2 answers
What can the Reader monad do that applicative functions cannot?
Having read http://learnyouahaskell.com/functors-applicative-functors-and-monoids#applicative-functors , I can provide an example of the use of functions as applicative functors:
Let's say res is a function of 4 arguments and fa, fb, fc, fd are all…
user4385532
7
votes
3 answers
One more time...can I have an example of state monad that does what I want?
I'm trying to understand the actual need for the reader and/or state monads. All the examples I've seen (including many on stackoverflow as I've hunted for suitable examples I can use and in various books and blog articles) are of the form (pseudo…
David
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2 answers
Is it possible to do the Reader Monad from Haskell in Clojure?
I've had a look at the algo.monads and fluokitten documentation. I've also read through monad blog entries by Jim Duey, Konrad Hinsen and Leonardo Borges.
The only reference I can find to the Reader Monad in Clojure is this google groups…
hawkeye
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7
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2 answers
Function composition hint
just looking for an explanation how does following composition work:
(=<<) . return
where
(=<<) :: (a -> m b) -> m a -> m b
return :: a -> m a
(.) :: (b -> c) -> (a -> b) -> a -> c
The final type:
GHCi> :t (=<<) . return
(=<<) . return ::…
David Unric
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6
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Is there any significant difference between StateT over Reader and ReaderT over State?
When I design my programming model I always have a dilemma which approach is better:
type MyMonad1 = StateT MyState (Reader Env)
type MyMonad2 = ReaderT Env (State MyState)
What are the benefits and trade offs between using one monad stack over…
radrow
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6
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1 answer
Modifying inner reader in a transformer stack
I'm pulling together code from a number of different places, and I'm trying to deal with the following:
Problem
I have a transformer stack with the following simplified type:
action :: m (ReaderT r IO) a
and I'm trying to use the action in the…
OllieB
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6
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2 answers
Haskell - depth for each node in binary tree using Reader monad
I wrote the following code. It is working and using the Reader monad.
Could you give me some hints about code style in Haskell ? Mainly, I mean monads -- I am newbie.
import Control.Monad.Reader
data Tree a = Node a (Tree a) (Tree a)
…
user6023611
6
votes
1 answer
Refactoring a Haskell function that uses the Reader monad
I've got some code that looks sort of like this, ignoring all the code that isn't relevant to my question:
import qualified Control.Monad.Reader as Reader
data FooEnv = FooEnv { bar :: Int -> Int }
type FooReader = Reader.Reader FooEnv
foo :: Int…
arussell84
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f, g, h :: Kleisli ((->) e) a b <=> f >>> (g &&& h) = (f >>> g) &&& (f >>> h)?
Edit: We will call an arrow p pure if exists such function f that: p = arr f.
I'm trying to get a better grasp of Arrows in Haskell, and I want to figure out when
f >>> (g &&& h) = (f >>> g) &&& (f >>> h), where f, g, h are arrows.
Obviously, it…
Zhiltsoff Igor
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