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
8
votes
4 answers
Guess My Number, a monadic headache
To test my skills in Haskell, I decided I wanted to implement the very first game you find in Land of Lisp / Realm of Racket. The "Guess My Number" game. The game relies on mutable state to run, as it constantly has to update upper and lower bounds…
Electric Coffee
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7
votes
3 answers
Is print in Haskell a pure function?
Is print in Haskell a pure function; why or why not? I'm thinking it's not, because it does not always return the same value as pure functions should.
bubu
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7
votes
2 answers
Idiomatic Haskell syntax without do-blocks or accumulating parentheses?
I'm fairly new to Haskell and have been trying to find a way to pass multiple IO-tainted values to a function to deal with a C library. Most people seem to use the <- operator inside a do block, like this:
g x y = x ++ y
interactiveConcat1 = do {x…
user4861515
7
votes
4 answers
Refactoring “staircasing” with case of `Maybe` values in `IO` code
The following function f attempts to read an Int twice by using an IO (Maybe Int) function twice, but “short-circuits” execution after successfully reading one Int:
readInt :: IO (Maybe Int)
f :: IO (Maybe Int)
f = do
n1 <- readInt
case n1 of
…
beta
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7
votes
2 answers
How do I use map over a list with do notation - ie avoid type `IO ()' with type `[IO ()]'?
So this question is about Monads more generally (in particuar for Fay), but my example uses the IO monad.
I have a function where the input is a list of strings and I would like to print each string one by one. So here was my idea:
funct ::…
aled1027
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6
votes
1 answer
Is there any way to lift content from IO to other container without run?
According to the cats official document: https://typelevel.org/cats-effect/typeclasses/liftio.html, if we want lift something from IO to other container, you should implement LiftIO trait, but the example explicitly run unsafeRunXXX method to fetch…
user3593261
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6
votes
2 answers
What is the IO Haskell Monad equivalent in Scala standard API?
I know there are some almost identical implementations in Scalaz of IO operators like putStrLn :: String -> IO () and getLine :: IO String but I mean about Scala standard API why there aren't such equivalents?
I know Scala is not a pure language as…
salc2
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6
votes
1 answer
How to construct a no-op IO() expression in Haskell?
Here's an excerpt of a domain-specific file-IO function I'm writing:
let
cp :: FilePath -> IO ()
cp "." = putStr "" -- OUCH!
cp ".." = putStr "" -- CRIKEY!
cp fname = custom logic here...
in mapM_ cp filepaths
I understand mapM_…
metaleap
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6
votes
2 answers
In what sense is IO monad special (if at all)?
After diving into monads I understand that they are a general concept to allow chaining computations inside some context (failing, non-determinism, state, etc) and there is no magic behind them.
Still IO monad feels even if not magic, but special.…
Konstantin Milyutin
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6
votes
1 answer
Dynamically generate Tasty `TestTree` from the file system
I have written a file parser using the Parsec library. I would like to write a high-level unit test using the Tasty testing framework to ensure that the parser correctly parses some given files.
I have three well formatted files in the following…
recursion.ninja
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5
votes
3 answers
Why is there difference between throw and throwIO?
I am trying to get a firm grasp of exceptions, so that I can improve my conditional loop implementation. To this end, I am staging various experiments, throwing stuff and seeing what gets caught.
This one surprises me to no end:
% cat X.hs
module…
Ignat Insarov
- 4,660
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5
votes
1 answer
Why isn't this this applicative statement being lazily evaluated, and how can I understand why?
abc :: IO (Int)
abc = do
print "abc"
pure $ 10
xyz :: IO (Int)
xyz = undefined
main :: IO ()
main = do
x <- (((+) <$> abc <*> abc) <* xyz)
print x
Why in the above is xyz being evaluated? I would assume due to Haskell's lazy nature it…
Chris Stryczynski
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5
votes
2 answers
Mocking of BlazeClientBuilder[IO] to return mock client[IO]
I am using the BlazeClientBuilder[IO].resource method to get Client[IO]. Now, I want to mock the client for unit testing but cannot figure out how to do so. Is there a good way of mocking this and how would I do that?
class ExternalCall(val…
Kumar Waghmode
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5
votes
1 answer
Haskell: [IO ()] to IO ()
Haskell wiki has the following question:
https://en.wikibooks.org/wiki/Haskell/Higher-order_functions
for :: a -> (a -> Bool) -> (a -> a) -> (a -> IO ()) -> IO ()
for i p f job = -- ???
I was able to come up with the following…
coder_bro
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5
votes
1 answer
How can I use an `IO String` inside a Spock request handler?
I have the following function that produces a random string of characters in multiples of 1024:
import System.Random
rchars :: Int -> [IO Char]
rchars n = map (\_ -> randomRIO ('a', 'z')) [n | n <- [0..n]] -- a wasteful "iteration"-like func
rstr…
Alex
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