I have a powerset implementation that I'm trying to run here: http://rextester.com/runcode. I'm still getting this error and can't figure out how to make it right. I'm trying to read as much as possible about IO in haskell but it is super hard for me.

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

Answer:

the problem is `main = ...`

`main`

should have type `IO ()`

but you give an expression with type `[[Integer]]`

(as the compiler tells you) - so as I think you want to output the result to the console I think you are looking for `print`

this works for me:

```
import Control.Monad (filterM)
powerset = filterM (const [True, False])
main =
print $ powerset[1,2]
```

having said this you should add the top-level signatures:

```
module Main where
import Control.Monad (filterM)
powerset :: [a] -> [[a]]
powerset = filterM (const [True, False])
main :: IO ()
main =
print $ (powerset [1,2] :: [[Int]])
```

the usual way to get input from **stdin** is `getLine :: IO String`

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