Only generate one warning about shapes

[hadley]

ggplot(data = mpg) + 
  geom_point(mapping = aes(x = displ, y = hwy, shape = class))

not two!

[hadley]

In fact you'll get one warning for each layer + one for the legend (guide_train.legend):

ggplot(mpg, aes(x = displ, y = hwy, shape = class)) + 
  geom_point() + 
  geom_point()

Maybe DiscreteScale$map() needs to memoise the call to palette()?

[thomasp85]

You want to use the memoise package or brew up some solution inside the ScaleDiscrete class?

[hadley]

I think do something internal - i.e. have a field that stores the palette. But I'm not sure what happens if n changes, or even if that's possible. Also I don't remember exactly the semantics of scales - I think that would also need a tweak to the clone method.

Let's chat about this on our call so I can make sure you have my vague thoughts on the problem before you start work on it.

[hadley]

I think we need a cache_palette() method that records both the palette and the number of levels. If the number is different in a future call, warn, and re-generate. That will at least allow us to see if and when n differs.