Actually, you can make a joinInner for the State monad. However, it
does not allow the inner function (h) to change the state, because how
state is threaded through a monad N is different for each N.
i :: (Monad n) => (a -> State s b) -> (b -> n c) -> (c -> State s d) -
> (a -> State s (n d))
i f g h = joinInnerState . liftM (liftM h . g) . f
joinInnerState :: Monad n => State s (n (State s a)) -> State s (n a)
joinInnerState (State g) = State $ joinInnerAsReader . g
where
joinInnerAsReader (n, s) = (liftM (fst . ($ s) . runState) n, s)
joinInner is the only one of the 3 that works, because the outer M
gives you an initial state to work with.
Sjoerd
On Jul 10, 2009, at 11:25 PM, Job Vranish wrote:
Yeah, I think the problem with my case is that while M is a specific
monad (essentially StateT), N can be an arbitrary monad, which I
think destroys my changes of making a valid joinInner/joinOuter/
distribute.
Maybe someday Haskell will infer valid joinInner/joinOuter for
simple cases :D
Thanks for you help. I'll definitely have to see if I can find that
paper.
- Job Vranish
On Fri, Jul 10, 2009 at 3:09 PM, Edward Kmett <[email protected]>
wrote:
The problem you have is that monad composition isn't defined in
general. You would need some form of distributive law either for
your monads in general, or for your particular monads wrapped around
this particular kind of value.
What I would look for is a function of the form of one of:
distribute :: N (M a) -> M (N a)
joinInner :: M (N (M a)) -> M (N a)
joinOuter :: N (M (N a)) -> M (N a)
that holds for your partiular monads M and N.
IIRC Mark P. Jones wrote a paper or a lib back around '93 that used
these forms of distributive laws to derive monads from the
composition of a monad and a pointed endofunctor.
-Edward Kmett
On Fri, Jul 10, 2009 at 11:34 AM, Job Vranish <[email protected]>
wrote:
I'm trying to make a function that uses another monadic function
inside a preexisting monad, and I'm having trouble.
Basically my problem boils down to this. I have three monadic
functions with the following types:
f :: A -> M B
g :: B -> N C
h :: C -> M D
(M and N are in the monad class)
I want a function i where
i :: A -> M (N D)
the best I can come up with is:
i :: A -> M (N (M D))
i a = liftM (liftM h) =<< (return . g) (f a)
I'm starting to feel pretty sure that what I'm going for is
impossible. Is this the case?
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--
Sjoerd Visscher
[email protected]
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