On Wednesday 15 December 2010 23:15:51, Tim Baumgartner wrote:
I'll provide more details. Banana is actually a (shopping) cart:
newtype Cart = Cart (Map.Map Product Amount)
mapCart f (Cart m) = Cart (f m)
I was too lazy to write down the signature of mapCart, since the type of the cart content might evolve.
Now I have some state: type Shopping a = State Cart a
and I update it: buy :: Amount -> Product -> Shopping () buy amount product = modify . mapCart $ Map.insertWith (+) product amount
and the last line is the place where I use mapCart. This works as expected but I wanted to be sure that I do it "the right way".
Yes, it's the right way. You should keep an eye on modify though, it may be too lazy and build large thunks in the state if you do it a lot.