I think what you're looking for are monoid actions: http://hackage.haskell.org/package/monoid-extras-0.2.2.0/docs/Data-Monoid-Action.html

And the laws the typeclass should satisfy:

foo bar = id

foo f . foo g = foo (f `mappend` g)

The identity law is optional, in whihc case your 'e' would be a semigroup without a default 'bar'

On 21 April 2014 11:46, Tobias Brandt <tob.brandt@googlemail.com> wrote:

Your class looks like Unfoldable from Data.Collections. foo is insert and bar is empty. The other three methods can be defined in terms of insert and empty.

On 21 April 2014 07:09, Bardur Arantsson <spam@scientician.net> wrote:

Hi all,

I was wondering if anyone out there knows if this type class has a name?

  class Foo a e where
     foo :: e -> a -> a
     bar :: a

(I also have a fundep a -> e, but that's not essential.)

Essentially the usage is:

   You have a sequence of "events" e_i and a starting value
   "bar" and can use the "foo" function to apply all events
   to that starting value

      foo e_n $ foo e_{n-1} $ ... $ foo e_0 $ bar

   and thus get the final value of the a induced by the
   events e_i.

(I've included the "bar" starting value in the type class, but I suppose
it could be supplied by a Default instance.)

Regards,

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe