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".
2010/12/15 Stephen Tetley
Doesn't mapBanana work if you give it this signature:
mapBanana :: (Something -> Something) -> Banana -> Banana mapBanana f (Banana b) = Banana (f b)
Possibly you are wanting a polymorphic Banana?
newtype Banana a = Banana a
mapBanana :: (a -> b) -> Banana a -> Banana b mapBanana f (Banana x) = Banana (f x)
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