[Haskell-cafe] a problem defining a monad instance

Jason Dagit dagit at codersbase.com
Fri Nov 6 14:15:44 EST 2009

On Fri, Nov 6, 2009 at 10:19 AM, Neil Brown <nccb2 at kent.ac.uk> wrote:

>  Petr Pudlak wrote:
> The problem is the (Ord b) condition, which is required for the Map
> functions.  When I try to define the monad instance as
>  instance Monad Distrib where
>     return = dreturn
>     (>>=)  = dcompose
>  obviously, I get an error at (>>=):
>     Could not deduce (Ord b) from the context.
> Is there some way around? Either to somehow define the monad, or to
> achieve the same functionality without using Map, which requires Ord
> instances?
>  Not being allowed constraints on the variables for class methods is
> probably the problem I have most frequently run into recently in Haskell (is
> there any way to fix this, or does it open up a whole can of worms?).

I believe this paper is intended to propose a solution:

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