[Haskell] IORef sharing

Eli Ford eford at griptonite.com
Mon Oct 27 21:35:53 EDT 2008

> Rodney D Price wrote:
> Okay... However, when I use IO in a Haskell program,
> the response is usually pretty snappy.  It's not as
> if the Haskell runtime is hanging around, waiting for
> some time in the future when it might be appropriate
> to do IO.  It happens right now.  Yet the literature
> gives the impression that the IO monad in particular
> is a clever trick to preserve referential transparency
> by gathering up all the IO actions, but not necessarily
> actually *performing* them.  Then the Haskell runtime
> holds its nose and *performs* them when necessary.

It's really best to think of 'main' as returning an IO action. You can
call >>= all over the place, but if you never return the result up
through 'main', the resulting bound action will never be executed.
(Even then it might not be executed, if the runtime never passes through
that point in your program.)

Coming from an imperative background, I imagine that 'main' returns
pretty much immediately, and the runtime starts executing your action
right away.

To put it another way, 'main' is a constant -- it doesn't do work, so
much as it describes what you want the runtime to do.

Hope this helps!

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