Why can't I use IO constructor

Question

Why can\'t I do this:

import Data.Char

getBool = do
  c <- getChar
  if c == \'t\' 
    then IO True 
    else IO False

instead of using

Answer 1

You can use IO instead of return. But not such easy. And you also need to import some inner modules.

Let's look at source of Control.Monad:

instance  Monad IO  where
    {-# INLINE return #-}
    {-# INLINE (>>)   #-}
    {-# INLINE (>>=)  #-}
    m >> k    = m >>= \ _ -> k
    return    = returnIO
    (>>=)     = bindIO
    fail s    = failIO s

returnIO :: a -> IO a
returnIO x = IO $ \ s -> (# s, x #)

But even to use IO instead of return, you need to import GHC.Types(IO(..)):

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

After this, you can write IO $ \ s -> (# s, True #) (IO is a State) instead of return True:

Solution:

{-# LANGUAGE UnboxedTuples #-}  -- for unboxed tuples (# a, b #)
{-# LANGUAGE TupleSections #-}  -- then (,b) == \a -> (a, b)
import GHC.Types (IO (..))
import Data.Char

getBool = do
  c <- getChar
  if c == 't' 
    then IO (# , True #)
    else IO (# , False #)