On Wed, 2009-03-25 at 15:32 +0000, Simon Marlow wrote:
Jonathan Cast wrote:
On Wed, 2009-03-25 at 15:09 +0000, Simon Marlow wrote:
the ordering that the state monad expects (and I can never remember which way around they are in Control.Monad.State).
Really? I found it obvious once I figured out it how simple it made (>>=). With the order from Control.Monad.State (with constructors ignored):
a >>= f = \ s -> case s a of (x, s') -> f x s'
Reversing the order of the components of the result gives you
a >>= f = \ s -> case s a of (s', x) -> f x s'
which just looks weird.
It might look weird to you, but that's the way that GHC's IO and ST monads do it. It looks perfectly natural to me!
Right. Consider this an argument for fixing IO/ST(/STM?) to conform to the self-evidently correct ordering of Control.Monad.State :)
(and you have the a and s the wrong way around in 'case s a', BTW).
Um, yes. /Mea culpa/.
Try doing it with mapAccumL, which is arguably the right abstraction, but has the components the other way around.
Define
swap (a, b) = (b, a)
ew, that's far too crude. I think you mean
swap = uncurry $ flip (,)
Ah, yes. jcc