Question

I’m writing a small program with IO actions in Haskell
here is

module StackQuestion where

import Data.Map (Map, insert, fromList)

type Name = String
type Value = String

readValue :: Name -> IO String
readValue name = do     putStrLn name
                        value <- getLine
                        return value

addPair :: Name -> Value -> Map Name Value -> Map Name Value
addPair = insert

names = map show [1..5]
values = map (\char -> [char]) ['a'..'d']
initialMap = fromList (zip names values)

As you can see I have some initial map with values, and function which adds a pair to map, functions which reads a value.

How I can get a clear String value from readValue and pass it to another function ?

Or should I change type Value = String to type Value = IO String and use map Map String (IO String) ?

And if I have Map String (IO String) how can i process this map, how I can get any value depening on data in IO containter (maybe some function func :: (a->b) -> IO a -> b)
For instance is there any way to compare IO String with the clear String ?

If I have function func I would have written

map :: Map String (IO String)
...
func (==) (map ! "key")

What is the strategy of working with IO values ?

Answer 1

How I can get a clear String value from readValue and pass it to another function?

You can’t; you’ll have to manipulate readValue‘s result while in the IO monad.

{- read value for name and store both in map -}
readAndStore :: Name -> Map Name Value -> IO (Map Name Value)
readAndStore name m  =  do value <- readValue name
                           return $ insert name value m

The return function takes the result of the insert and puts it back neatly into the IO monad. This code exemplifies a general pattern for manipulating values with ordinary functions inside IO.

Or should I change type Value = String to type Value = IO String

No; consider what that would mean. IO String means a computation with possible side-effects (IO) with String-typed result. You would be mapping names to computations. (That’s possible, but it’s not what you mean.)

The example I’ve shown above uses IO (Map Name Value) instead; i.e., a computation in the IO monad with Map-typed result.