How to get normal value from IO action in Haskell

Question

I have the following function:

get :: Chars -> IO Chars get cs = do     char <- getChar     let (dats, idx) = (curData cs, curIndex cs)     let (x,y:xs) = splitAt idx dats     let replacement = x ++ (ord char) : xs     return $ Chars replacement idx 

and I'd like to get a Chars value out of it, not an IO action. I have no idea how to do this, or if it is even possible.

The Chars type is basically just a container:

data Chars = Chars {                curData  :: [Int],                curIndex :: Int                -- etc.              } 

The specifics aren't that important, I just want to know if there's a way for this function to return a Chars instead of an IO Chars.

If not, how do I pass this as an argument to a function that takes a Chars? I'm kind of new to Haskell I/O, but I don't think I want all of my functions that take Chars as arguments to instead have to take IO Chars as arguments, and then extract and repackage them. It seems unnecessary.

Thanks!

Answer 1

You can't, because that would violate referential transparency.

IO in Haskell is made this way exactly to distinguish between actions whose result and effects may vary depending on the interaction with the environment/user and pure functions whose results are not going to change when you call them with the same input parameters.

In order to pass the result to a pure function taking a Chars in input you have to call your IO action into another IO action, bind the result with the <- operator to a variable and pass it to your pure function. Pseudocode example:

myPureFunction :: Chars -> ...  otherAction :: Chars -> IO () otherAction cs = do   myChars <- get cs   let pureResult = myPureFunction myChars   ... 

If you're new to IO in haskell, you may wish to have a look at the Input and Output chapters in Learn You a Haskell for a Great Good! and Real World Haskell.

There is actually a way to simply get a pure value out of an IO action, but in your case you shouldn't do it, as you're interacting with the environment: the unsafe way is ok only when you can guarantee you're not violating referential transparency.

Answer 2

It's impossible (I lie, there is an extremely unsafe way to cheat your way out of it).

The point is that if any I/O is performed, the behaviour and result of your programme may not depend only on explicit arguments to the used functions, thus that must be declared in the type by having it IO something.

You use the result of an IO a action in a pure function by binding the result in main (or something called from main) and then applying the pure function, binding the result in a let,

cs ::Chars cs = undefined  main = do   chars <- get cs   let result = pureFunction chars   print result 

or, if the function you want to apply to chars has type Chars -> IO b

main = do     chars <- get cs     doSomething chars