Monadic interface to IO in pure functional languages, especially Haskell.
Monadic interface to IO in pure functional languages, especially Haskell.
Questions tagged [io-monad]
194 questions
0
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1 answer
Monad transformers with IO and Maybe
I am trying to stack up IO and Maybe monads but either I don't understand monad transformers well enough or this is not possible using transformers. Can some one help me understand this?
f :: String -> Maybe String
main :: IO ()
main = do
input…
Bilentor
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3 answers
Is h a handle or a lambda function (or both)?
I am looking at a simple IO program from the Haskell Wikibook. The construction presented on that page works just fine, but I'm trying to understand "how".
The writeChar function below takes a filepath (as a string) and a character, and it writes…
mttpgn
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2 answers
parMapN that finishes even when one program encounters an error
Using parMapN, multiple IOs can be executed in parallel, like this:
import cats.implicits._
import cats.effect.{ContextShift, IO}
import scala.concurrent.ExecutionContext
implicit val cs: ContextShift[IO] =…
Florian Baierl
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1 answer
How do I load a GIF-image from computer in Haskell?
I want to have a GIF-image looping in the background of a game (For example with Gloss Juicy or module Codec.Picture.Gif)
Managed to work with a BMP image like this:
image :: IO Picture
image = loadBMP "image.bmp"
But now I want to do the same…
blobfish
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- 3
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1 answer
Purpose of re-boxing pure value for GHCI
Can someone explain to me why when using ghci I have to "re-box" the value returned by a monad?
listAction=listDirectory "D:\\"
lenAction=listAction>>=length
Error:
Couldn't match type `Int' with `IO b0'
Expected type: [FilePath] -> IO…
Bercovici Adrian
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Passing a randomly generated list as a parameter in Haskell
I am new to Haskell and really having trouble with the whole IO thing.
I am trying to find out how long it takes to traverse a list in haskell. I wanted to generate a list of random numbers and pass it as a parameter to a function so that I can…
AR17
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1 answer
Control Monad (when or the likes) and non IO() return types
I am trying to use the "when" function in my program because I want to have an action preformed if something is true and not if its not true.
I could also accomplish the same thing by using the Maybe monad but that will clutter the rest of my…
John Brown
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1 answer
Haskell: Turtle: Managing Shell type
This a working snippet:
import Turtle
...
groom :: FilePath -> IO ()
groom src = do
view (ls src)
...
I can see a list of paths on the console. Actually I'd like to have something like [FilePath] for use, e.g.:
treeCount :: FilePath ->…
Alexey Orlov
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1 answer
How to display Response from an HTTP GET Request in front-end using Scotty?
I'm trying out Scotty for the first time and I can't seem to get past making my GET request. The Response is returned as type
IO (Response bytestring-0.10.8.1:Data.ByteString.Lazy.Internal.ByteString)
I know I need to convert it to a type that…
Kahlil Abreu
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0 answers
Difference in how IO works in FP vs Non FP languages
Let's take the simple case here (pseudo code)
def readnonfp(): String = {
nonMonadicIO.readFile("somefile")
}
def readfp(): IO[String] = {
monadicIO.readFile("somefile")
}
Now this will be maybe composed like…
ffff
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0
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1 answer
IO action nested in other monads not executing
I have a
foobar :: IO (ParseResult [(String,String)])
ParseResult is a monad defined here: https://hackage.haskell.org/package/haskell-src-exts-1.13.5/docs/Language-Haskell-Exts-Parser.html#t:ParseResult
I want to take those strings and write them…
LambdaScientist
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1 answer
Example of while loop in Haskell using monads
I want to write a loop in haskell using monads but I am having a hard time understanding the concept.
Could someone provide me with one simple example of a while loop while some conditions is satisfied that involves IO action? I don't want an…
domoremath
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1 answer
Haskell Errors Binding a CSV File to a Handle
So I was writing a small toybox of utility functions for CSV files and throughout testing the functions I'd been binding the file by hand with
table' <- parseCSVFromFile filepath
but (from Text.CSV)
parseCSVFromFile :: FilePath -> IO (Either…
Tshimanga
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1 answer
Haskell & Scrypt: How to get the encrypted hash
*Sorry for the basic question, just started learning Haskell.
I am trying to write a function that will get a string and return an encrypted hash.
What I have come up with for the moment is:
encrypt :: ByteString -> ByteString
encrypt = do
x <-…
user6307701
0
votes
1 answer
Combining and splitting assignment in Haskell IO do block
I /think/ I have a similar misunderstanding of the language in two places involving how variable assignment works in do blocks, involving the IO monad. Could you help me understand (1) is it the same misunderstanding, (2) how to clear it up (in an…
Mittenchops
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