haskell - Do statement under a where clause

Question

I'm trying to convert IO [String] to [String] with <- binding; however, I need to use a do block to do that under a where statement, but Haskell complains about the indentation all the time. Here is the code:

decompEventBlocks :: IO [String] -> IO [[String]]
decompEventBlocks words
 | words' /= [] = block : (decompEventBlocks . drop $ (length block) words')
 | otherwise = []
  where 
   do
    words' <- words
    let block = (takeWhile (/="END") words')

What is the reason for that ? And how can we use do block in a where statement ? Moreover, is there any chance that we can have some statements before the guards ?

Answer 1

You cannot convert for IO String to a String.

What you can do, however, is bind the contents of IO String to a 'variable', but that will still result in the whole computation being embedded inside IO.

foo = do
   x <- baz -- here baz is the IO String
   let x' = doStuff x
   return x' -- embeds the String inside IO, as otherwise the computation would result in IO ()

To answer your question

foo x = baz x -- x here is your 'IO String'
  where
    baz x = do
      x' <- x
      return $ doStuff x'