What would be the best way to do this? unsafePerformIO? Template Haskell? Something else? I have never used either of those so I don't know many of the details of using them. Note that the program...

It's widely understood that unsafePerformIO is not type safe. This is typically demonstrated by using it to implement unsafeCoerce: box :: IORef a box = unsafePerformIO (newIORef undefined) {-# NOI...

I was wandering in the Restricted Section of the Haskell Library and found these two vile spells: {- System.IO.Unsafe -} unsafeDupablePerformIO :: IO a -> a unsafeDupablePerformIO (IO m) = case ...

I often find this pattern in Haskell code: options :: MVar OptionRecord options = unsafePerformIO $ newEmptyMVar ... doSomething :: Foo -> Bar doSomething = unsafePerformIO $ do opt <- re...

Haskell is generally referenced as an example of a purely functional language. How can this be justified given the existence of System.IO.Unsafe.unsafePerformIO ? Edit: I thought with "purely func...

I am creating a Haskell application that generates a random number on an infinite loop (only when requested by a client). However, I should only use pure functions for that purpose. Is it safe to w...

In this pseudocode block: atomically $ do if valueInLocalStorage key then readValueFromLocalStorage key else do value <- unsafeIOToSTM $ fetchValueFromDatabase key writeValueToLocalStorage ...

I'm creating an FFI module to a library in C which wants a 1-time, non-reentrant function to be called before anything else is. This call is idempotent, but stateful, so I could just call it in eve...

To get acquainted with unsafePerformIO (how to use it and when to use it), I've implemented a module for generating unique values. Here's what I have: module Unique (newUnique) where import Data...

I have been reading about unsafePerformIO lately, and I would like to ask you something. I'm OK with the fact that a real language should be able to interact with the external environment, so unsaf...

I have a data type which contains an IORef as an important element. This means there is not a clean way to make it a member of the show type class. This is not too bad as I have a print function in...

I am trying to get a Haskell function to show whenever it is applied by adding a call to "putStrLn": isPrime2 1 = False isPrime2 n = do putStrLn n null (filter (==0) (map (mod n) (filter isPrim...

I wrote a function in haskell that takes a few parameters like Word32, String (ignore currying) and outputs IO Word32. Now, this is a function in the true sense: for the same inputs, the output wil...

I found some sample code, and changed it a little counter = unsafePerform $ newIORef 0 newNode _ = unsafePerformIO $ do i <- readIORef counter writeIORef counter (i+1) return i Which ret...

There has been some talk at work about making it a department-wide policy of prohibiting the use of unsafePerformIO and its ilk. Personally, I don't really mind as I've always maintained that if I ...