[Haskell-cafe] Re: evaluation semantics of bind
barsoap at web.de
Thu Feb 5 12:59:11 EST 2009
Gregg Reynolds <dev at mobileink.com> wrote:
> getChar >>= \x -> getChar
> An optimizer can see that the result of the first getChar is discarded
It isn't discarded. The first getChar results in a value of type IO
Char, always and ever. Whether or not the Char inside the IO Char gets
evaluated or not is another question... >>= could not care less about
what you do to the values that are _inside_ the monad.
If you look at the type
(>>=) :: (Monad m) => m a -> (a -> m b) -> m b
, you will find it hard to do much of significance to "a" and/or "b"
Whether or not a monad behaves strictly or lazyily wrt. to "m" depends
on the definition of the functions that come with it: IO happens to be
strict,  to be lazy. This is nothing that messes with semantics,
though, it celebrates them.
 Well, you can pass them to id, seq or par, and replace them with
(c) this sig last receiving data processing entity. Inspect headers
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