[Haskell-beginners] [Haskell-cafe] select :: [(Float, a)] -> a -- Weighted stochastic selection - help?

Nicolas Pouillard nicolas.pouillard at gmail.com
Sat Sep 6 06:33:41 EDT 2008

Excerpts from C.M.Brown's message of Sat Sep 06 12:04:10 +0200 2008:
> On Sat, 6 Sep 2008, Nicolas Pouillard wrote:
> > Excerpts from C.M.Brown's message of Fri Sep 05 22:12:05 +0200 2008:
> > > > Can you give an example? I don't see how that can be done using
> > > > Control.Monad.State(.Strict).State, unless invocations of put or modify are
> > > > considered side effects.
> > >
> > > Actually, yes, sorry; I do see your point. I guess it's just IO then.
> >
> > Technically, even the IO monad is pure, that's just the runtime-system
> > that consume your 'main' function that perform effects (and unsafeP...).
> But, sure the IO monad does have side-effects? I'm confused as to how it
> could be pure. Could you explain?
> > That's an important point to grasp about the way we do effects in a pure
> > language.
> >
> > Once we've understood that point one tend to be a little less precise and
> > consider IO as effect-full.
> I consider IO to be effect-full anyway - I can't see how it isn't!

In fact one consider the IO monad to be effect-full because we don't have a
runIO [1] function that is safe. So to be clear you go in the monad, but you
can't go out of it. In other terms you can make 'IO t' values but you can't
get values inside of it (without being yourself inside of it again).

For instance the State monad have a runState [2] function that allows you to
go out the of the monad, or in other terms to get the value inside. So the
State monad really is pure.

The ST monad is also pure and provides a pure running function runST [3].
What is interesting with the ST monad is that one don't choose the state
type, moreover one cannot access it either (no get and put functions).
Moreover by having this rank-2 type the runST function force the caller to
give a 'ST s a' computation that does not mix the state parameter 's'.

Internally one can see 'ST' to be defined by something like that:
  type ST s a = s -> (s, a)
So not far of the State monad.

The IO monad internally is defined as 'ST RealWorld a', what means that
'IO a' values are in fact RealWorld passing function.

An example:

  What is the side-effect of reducing 'putStrLn "Hello"'?

  Easy answer, there is no side-effect in a pure language.

  More precise answer:
    "Hello" :: String
    putStrLn :: String -> IO ()
    putStrLn "Hello" :: IO ()
  If one imprecisely expand IO:
    putStrLn "Hello" :: RealWorld -> (RealWorld, ())
  Thus an argument is still missing, so no effect

Is this clearer?

[1] hypothetical type: runIO :: IO a -> a
[2] runState :: State s a -> s -> (a, s)
[3] runST :: (forall s. ST s a) -> a

Nicolas Pouillard aka Ertai
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