Gregg Reynolds wrote:
However, consider:
getChar >>= \x -> getChar
An optimizer can see that the result of the first getChar is discarded and replace the entire expression with one getChar without changing the formal semantics.
Let's imagine that IO datatype is defined thus:
{-# LANGUAGE GADTs #-} {-# LANGUAGE NoImplicitPrelude #-}
import Prelude(Monad, Char) data IO a where GetChar :: IO Char Bind :: IO a -> (a -> IO b) -> IO b
getChar = GetChar (>>=) = Bind
It is perfectly possible to construct IO actions as values of this data type and execute them by some function evalIO :: IO -> Prelude.IO with the obvious definition. Now the question arises: do you think getChar >>= \x -> getChar would be optimized to getChar by compiler? If no, why would GHC want to do this optimization for standard IO?