[Haskell-cafe] Problems with nested Monads

Job Vranish jvranish at gmail.com
Fri Jul 10 17:25:55 EDT 2009

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
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 <ekmett at gmail.com> 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 <jvranish at gmail.com> 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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