[Haskell-cafe] monad constraint + record update

[Brent Yorgey]

On Mon, Dec 22, 2008 at 06:19:07PM +0100, Peter Padawitz wrote:
> I'd like to define a monad Set for types in the class Eq. But how can the 
> arguments of Set be constrained when Set is defined as an instance of 
> Monad? instance Eq a => Monad Set where ... obviously cannot work.
>
> Is there a standard update function for fields in data types, something 
> that OO programmers do with assignments like obj.attr := value ?
>
> Peter
>

Note there is already a package on Hackage to do exactly this, rmonad.

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rmonad

-Brent