Haskell IO - read from standard input directly to list

Tag: haskell

Haskell IO system is super hard to understand for me so i have question : How to read from standard input to list ? I know that there is function getLine :: IO String and interact. But i do not know how to convert the input to list so I can use it in this three functions :

powerset []     = [[]]
powerset (x:xs) = xss ++ map (x:) xss
                               where xss = powerset xs
main = print $ powerset([1,2])

import Control.Monad(filterM)
p = filterM(const[True,False])
main = p[1,2]

main = subsequences([1,2])

I want to be able to write 1 2 3 and pass this values to the function. Can you tell/show how to do it ?

Extra question

Haskell is full of magic so i was wondering if it possible to use input directly in the function like this :

main = subsequences(some input magic here)

Best How To :

You may write:

main = readLn >>= print . subsequences

You will need to nail down the type to be read, for example by having a monomorphic subsequences or by annotating readLn. In ghci:

Data.List> (readLn :: IO [Integer]) >>= print . subsequences
[1,2,3]
[[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]

(I typed in the first and second lines -- both followed by enter -- and the third line was the result.)

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