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?
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