Processing IO values in haskell

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 ?

Style: map (\char -> [char]) ['a'..'d'] --> map (:[]) ['a'..'d'] — Landei
– Landei, Commented Jul 30, 2011 at 9:57

dave4420 · Accepted Answer · 2011-07-29 17:50:34Z

7

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.

edited Jul 29, 2011 at 17:50

dave4420

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answered Jul 29, 2011 at 17:11

Fred Foo

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3 Comments

fuz Over a year ago

@Heinrich Indeed. How about adding those typical questions to the tag wiki?

Max Komarychev Over a year ago

So, if I have any structure of type a which elements are read from standard input, file etc, i need to wrap it into the IO monad - somevar :: IO a and work with this values via liftM* functions ?

Fred Foo Over a year ago

@Max K.: I didn't dare suggest the use of liftM as I wasn't sure how advanced your Haskell skills are, but yes, that's the most elegant solution.

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