[Haskell-cafe] monad constraint + record update

Brent Yorgey byorgey at seas.upenn.edu
Tue Dec 23 14:28:29 EST 2008

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.



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