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

There is very little magic around IO and ST monads, much less than most people believes.

The dreaded IO type is just a newtype defined in GHC.Prim:

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

First of all, as can be seen above the argument of IO constructor is not the same as the argument of return. You can get a better idea by looking at a naive implementation of State monad:

newtype State s a = State (s -> (s, a))

Secondly, IO is an abstract type: it's an intentional decision not to export the constructor so you can neither construct IO nor pattern match it. This allows Haskell to enforce referential transparency and other useful properties even in presence of input-output.